On a slightly tangential topic, what is the state of the “global warming will cause a permanent el nino” theory
[Response: While I wouldn't quite characterize this in terms of a "permanent El Nino", a reasonably up-to-date discussion of possible climate change influences on ENSO is provided in our previous article El Nino and Global Warming --mike]
and what are the implications for Atlantic hurricanes?
[Response: There is also some discussion of this point in the RC article linked above. --mike]
Is it likely that a useful proxy will ever be found for hurricanes?
[Response: It's definitely a tricky problem, but there is some progress on local landfalling hurricanes - overwash in coastal ponds in Massachusetts, tree ring isotopes in Georgia etc. Open ocean proxies don't have the resolution, and so a complete record will never happen - the hope is that if landfalling data can be significantly extended it might give reasonable long term statistics - but right now my understanding is that this is all very preliminary. Any comments to contrary welcome! - gavin]
Klotzbach 2006 uses the same dataset as Webster et al. 2005 (note typo in your post, not 2006) and finds no trends in numbers of Cat 4-5 or ACE other than in the Atlantic.
So shouldn’t Kossin et al. actually reinforce Klotzbach 1986 and suggest that any global increase in storm intensity (or numbers of intense storms) necessarily occurred before 1986 (or 1983)?
Whether that increase is due to data or climate is a separate issue, but isn’t there multiple lines of evidence now that there are no global trends since the early- to mid- 1980s, whereas in the Atlantc there has been?
At least the argument that it’s simply the North Atlantic Oscillation that’s responsible for the increased intensity trend in the Atlantic has apparently been discarded; at least it isn’t mentioned at Prometheus:
“The paper does find the Atlantic to be more active over the past 23 years. No one in the world has ever questioned whether or not the Atlantic has been more active over the past 3 decades. Any assertion that the Atlantic has become more active is hardly “fresh evidence.” This is not news.
That assertion is false for a number of reasons; for example see this article in science 2001:
Stanley B. Goldenberg, Christopher W. Landsea, Alberto M. Mestas-Nunez, William M. Gray
suggested that the hurricane data was too spotty to support the trend:
One may ask whether the increase in activity since 1995 is due to anthropogenic global warming. The historical multidecadal-scale variability in Atlantic hurricane activity is much greater than what would be “expected” from a gradual temperature increase attributed to global warming. There have been various studies investigating the potential effect of long-term global warming on the number and strength of Atlantic-basin hurricanes. The results are inconclusive. Some studies document an increase in activity while others suggest a decrease. Tropical North Atlantic SST has exhibited a warming trend of ~0.3°C over the last 100 years; whereas Atlantic hurricane activity has not exhibited trendlike variability, but rather distinct multidecadal cycles as documented here and elsewhere. The extreme activity in 1995 has been attributed in part to the record-warm temperatures in the North Atlantic. The possibility exists that the unprecedented activity since 1995 is the result of a combination of the multidecadal-scale changes in Atlantic SSTs (and vertical shear) along with the additional increase in SSTs resulting from the long-term warming trend. It is, however, equally possible that the current active period (1995-2000) only appears more active than the previous active period (1926-1970) due to the better observational network now in place. During the previous active period, only 1966-1970 had continual satellite coverage. Further study is essential to separate any actual increase from an apparent one due to more complete observations.
So, progress has indeed been made. Now the question is whether the hypothesis that this is all due to the North Atlantic Oscillation can be discarded or not – would that be yes, or is the jury still out?
Note that even more recently, the following perspective in science was published:
“Science 28 July 2006
Can We Detect Trends in Extreme Tropical Cyclones?
Christopher W. Landsea, Bruce A. Harper, Karl Hoarau, John A. Knaff.
Subjective measurements and variable procedures make existing tropical cyclone databases insufficiently reliable to detect trends in the frequency of extreme cyclones.”
It’s rather strange that Prometheus is just sweeping this whole issue under the rug. Does that indicate acceptance of a global-warming related trend in hurricane intensity in the North Atlantic?
The scientific debate on what has occurred in the Atlantic is alive and well. Some scientists assert that the 1995-2006 period is more active in the Atlantic than any time in the past 50-100 years (e.g., Webster, Holland, Emanuel), and others argue that it is not (Goldenberg, Landsea, Gray). The current state of the peer-reviewed literature provides support for both perspectives (see WMO consensus statement). E.g., see the exchange between Emanuel and Landsea in Nature December 2005:
I thank Dr. Trenberth for his comments. I’ll respond here, and I’ll apologize in advance for the length of my response. It may be longer than the article itself…
The algorithm does in fact “allow for different structures or size of storms” and it is entirely justifiable to expect it to perform reasonably well outside of the Atlantic. We state that
“The EOFs contain information about hurricane eye temperature (when an eye is present), the height of the convective eyewall clouds, and the average radial structure of cloudiness around the storm [cf. Kossin et al., 2007], and these factors are correlated with hurricane intensity. For example, warmer eye temperature and higher eyewall clouds (indicated by colder cloudtop Tb) are strongly related to greater intensity. This is the foundation of the Dvorak Enhanced Infrared (EIR) technique [Dvorak, 1984], which is utilized by all tropical forecast offices in every ocean basin to estimate hurricane intensity with geostationary infrared imagery. In the Dvorak EIR technique, eye Tb and cloud-top Tb are directly related to intensity with a look-up table. The EOFs represent these temperatures but also contain additional information about radial structure such as eye size and radial extent of the cold cloud-tops above the eyewall. This additional information is also related to intensity [Kossin et al., 2007].”
We also state that
“The relationship between Tb (represented by the PCs) and intensity is analogous to the basis of the Dvorak technique, and latitude has been shown to modify this relationship [e.g., Kossin and Velden, 2004] as well as affect radial structure and size [Kossin et al., 2007].”
The relationship between these EOFs and storm size and structure is also very clearly documented in Mueller, et al. (2006) and Kossin et al. (2007). Both papers are in WAF. These are recent papers and Dr. Trenberth is probably unaware of them. Yes, the EOFs are fixed by construction, but their weighted linear combinations do describe size, and the PCs then describe size variability. This is a fundamental concept of course. It is also been documented that storm size is actually not that well correlated with intensity (compare Hurricanes Charlie and Katrina for example; same intensity, vastly different sizes).
Furthermore, the Dvorak technique (used in all basins) actually makes little to no distinction regarding storm size. For example, when an eye is present, the raw Dvorak estimates depend only on two temperature measurements: eye and eyewall. Size makes no difference whatsoever in these cases.
As to the basin-dependence, it seems that Dr. Trenberth is unfamiliar with the Dvorak technique. The intensity in the best track, as deduced by the Dvorak technique, is estimated in an analogous manner in all basins. We very clearly state in the paper that the relationship between the Tb field and intensity is fundamentally the same (within a fixed bias that does not affect the normalized values or the trend). We state:
“The fundamental relationships between infrared imagery (as well as the other predictors) and intensity do not change among the various ocean basins, and the Dvorak technique is applied in much the same way everywhere. There are differences in the method for converting raw Dvorak technique output into intensity estimates, but these differences can only create a temporally constant bias and this will not affect the trend analyses shown in the next section.”
This is a statement of fact that any hurricane expert would be aware of and it would not be debated. Dr. Trenberth can consult with any hurricane forecaster (i.e. the creators of the best track) to confirm this. The Dvorak technique also does not depend on ENSO or basin size, etc. and nor should our algorithm. The goal was to create homogeneous Dvorak-type (i.e. IR-based) intensity estimates to test against trends in the existing heterogeneous Dvorak-based records. The increased activity in the Northwest Pacific, tropopause height, land-sea configurations, basin size, ENSO, and the role of easterly waves that Dr. Trenberth elaborates on are not at all relevant in this context. Ultimately, it seems that Dr. Trenberth is questioning the tenets of the Dvorak technique itself, but it’s not our goal to convince anyone of the validity of a method that has been applied globally for 30 years by every forecast office on every continent (see Velden et al. 2006). It would also be inappropriate for us to give a history of the Dvorak technique in our paper, and we have a reasonable expectation that an interested reader will consult the references we offer. I strongly encourage Dr. Trenberth to do this.
Dr. Trenberth’s remark about the EOFs in the Western North Pacific being different than the EOFs from the Atlantic is a good one. This was looked at, and should have been mentioned. The EOFs are essentially identical. I’ve put the figure on my website.
As I mentioned above, the different linear combinations of the EOFs are responsive to size, and the PCs, which are the predictors in the algorithm, control these combinations. A large storm in the Atlantic, like Gilbert (1988) or Floyd (1999) or Katrina (2005) is very similar to a large storm in the W. Pacific. On average, the storms in the W. Pacific are larger, but that doesn’t mean that the EOFs could not be combined appropriately to represent this. The Atlantic has large storms too.
As a final point about our algorithm performing reasonably well outside of the Atlantic, note that the algorithm performed remarkably well in the Eastern Pacific.
As to point 3), this is simply false and I don’t know what Dr. Trenberth is basing this somewhat inflammatory comment on. The first draft of our paper considered mean kinetic energy (MKE) instead of PDI. The results were the same for both MKE and PDI (or ACE for that matter). In fact, most presentations I’ve given lately have used the MKE results because I could make all the same points using my old slides. We originally looked at MKE because we thought that an intensive variable based on intensity would be more useful than an extensive variable like PDI, which is strongly dependent on the number and duration of storms. Ultimately we had a lot of comments that there was no need to introduce a new variable into the mix and if we could show the same thing with PDI, which also helps to more directly address Emanuel (2005), then that might be the better route. Again, the results don’t change so I don’t see this as a big deal.
As to the differences between the start versus the end of the time series, I think Dr. Trenberth is missing a key point to this analysis. There may be biases, and in fact we expect biases when we apply the algorithm to other basins. Just as raw Dvorak “T-numbers” are converted to winds differently in the West Pacific versus the Atlantic, our algorithm is expected to have a systematic bias. The point is that the bias is not time-dependent. We are testing for trends, not making contemporaneous absolute comparisons. This type of confusion is what motivated us to standardize all the variables. A systematic bias affecting the absolute values is ok. The consistency over time is the important goal.
Dr. Trenberth is incorrect to say that the estimates should be better in the latter parts of the records than the beginning. First, all basins from the JTWC best track are measured almost entirely with the Dvorak technique. There is no ground truth available to “recalibrate” the forecasters. The Dvorak technique is a subjective technique and it has in fact been hypothesized that forecasters have been overestimating intensity in recent years because of the higher quality/resolution satellite data (Harper and Callaghan 2005). So this is quite opposite from Dr. Trenberth’s claim that accuracy should improve with time. Furthermore, Dr. Trenberth is also apparently unaware (although it was stated clearly in the paper) that the W. Pacific had aircraft reconnaissance until 1987 at which point they were forced to rely entirely on Dvorak satellite estimates of intensity. So we would expect the accuracy to be greatest in the earliest part of the record (1983-1987). We state:
“Since aircraft reconnaissance into storms was routine in the Northwest Pacific during the earlier period of our record 1983-1987, the best track intensities during this period are likely to be more accurate than the later period 1988-2004 after the termination of reconnaissance. This indicates a systematic over-estimation of intensity in the later period of the JTWC record when compared to the UW/NCDC data.”
Dr. Trenberth, I am puzzled by how forcefully you are dismissing our results while you apparently have virtually no knowledge of how intensity estimates are formed or how best track records are constructed. You seem to know very little about many of the things that a hurricane researcher would consider very fundamental. There’s no crime in this of course, but you’re taking an adversarial tack on these things and I think that’s unreasonable.
As a general parting note, I’d like to respond to the comment in the introductory section of this website:
“However, rather than this study being taken for what it is – a preliminary and useful attempt to make homogeneous a part of the data (1983 to 2005) – it is unfortunately being treated as if it was the definitive last word. We’ve often made the point that single papers are not generally the breakthroughs that are sometimes implied in press releases or commentary sites and this case is a good example of that.”
I’ve been greatly concerned about this for quite some time, as public statements made by other scientists suggested that our work would really get to the bottom of a number of questions. These were generous and supportive statements, but I knew that people were overstating our work. In addition to using the phrase “as a first step” in a number of places, my concern led me to insert the following into our paper:
“…the method applied here, while far from a panacea, is a reasonable first cut at the problem. Future analyses will hopefully expand on our work.”
I for one have no illusions regarding the importance of this work, but I think it is indeed a reasonable first cut at the problem and I think we’ve presented the results objectively and transparently.
Thanks for your comments. I think you have hit a number of things on the mark. I just want to offer a couple of responses (don’t worry, this will be much shorter than my last message)
Some of your comments regarding basin differences, ENSO, etc were addressed in my comment to Dr. Trenberth. Those differences should not matter when considering best track intensity estimates. Also the comparisons of how close the series are at the beginning versus the end is not relevant since you can translate either curve up or down as you like. We only care about the trends here and not absolute contemporaneous comparisons.
I will have to disagree with your claim that “the East Pacific regime in terms of dynamical and thermodynamical conditions is very close to the North Atlantic conditions.” I’d like to put this forth on the Tropical Storm listserv to get more feedback, but I would say that Eastern Pacific storms are very different from Atlantic storms. The SST structure is completely different, as is the shear. Storms intensify and weaken very differently there. There is lots of very cold water and stratus decks in the area that kill storms within hours if they stray too close. There is a pronounced ITCZ in the neighborhood. I would guess that the static stability is different there too. Basically all the things storms care about are different between the basins. So with all of this being said, the fact the our algorithm performs well in the E. Pacific as well as the Atlantic can’t be chaulked up to the storms and environment being the same.
Let me suggest a different possibility for why the algorithm finds bigger disparities everywhere except the ATL and EPAC: The Atlantic and E. Pacific best tracks are constructed by NHC forecasters who are the best trained hurricane forecasters in the world, and they stay at their jobs for decades. Their interest in best track quality is exceedingly high. The best track from the other basins are constructed at JTWC by temporarily assigned military personnel, many of whom have no experience whatsoever with hurricanes. They are cycled through every couple years and never are allowed to get very good at hurricane forecasting. It’s not their career. This is why the JTWC best track comes with such a strong written warning regarding the quality of the intensity estimates. The creation of the best track at JTWC is also a very low priority item (ask anyone who works there). They do it when they can and they don’t really have the manpower. That’s why the 2005 JTWC best track was only made available about a month ago, while the 2006 NHC best track has already been available for months. Yes, this is all anecdotal, but it does offer an alternative reason for why we found differences in the JTWC basins and not in the NHC basins.
If you look at the diagram (Fig 4) in Webster et al. plotting NCAT45 vs time, the appropriate diagram to consider is NCAT45/N (the fractional number of NCAT45). This is because in the latter part of the time series there is a decrease in the total number of tropical cyclones, largely owing to a large multidecadal cyclone in the WPAC (which comprises 40% of the global tropical cyclones), see Fig 3 in Webster et al. The NCAT45/N curve in Fig 4 does show more of a linear trend (albeit with some zigs and zags).
The main issue is that the 35 year period used by Webster is marginally on the short side for doing a global trend. While averaging among all the basins does average out much of the basin-specific multidecadal cycles, owing to the large number of WPAC tropical cyclones, we still see a signal from the Pacific multidecadal cycle in the global average. Going to shorter periods like 20 years really doesn’t make much sense (physically or statistically) in trying to decipher a trend.
We all appreciate the extra effort you made to comment here, it is such a help when the authors spend some time here to answer questions.
I still suspect that the higher SST values and much stronger and larger tropical cyclones particularly in the Western Pacific are in a different region of “parameter space” that wasn’t included in your training data set. While size isn’t included in the Dvorak scheme, it does reflect some differences in dynamics that may influence intensity in more subtle ways that may not be captured by your objective analysis scheme. Do you have any thoughts on how you might convince us “skeptics” on this in terms of a further analysis that might be done?
With regards to the Dvorak scheme, I was astonished by the Wu article in the December 06 EOS. I view the differences in these 3 analyses of the Western Pacific tropical cyclone intensity to be “error bars” on the s Dvorak scheme, largely associated with subjectivity of the analyst. Pretty large error bars indeed.
I have one other question. I understand why you only went back to 1983, but based on your expert judgment and the analysis you have done, how far back in time would you expect the NATL and EPAC data to be reliable? 1977? 1973? Certainly prior to 1970, we the the kerfuffle between landsea and emanuel regarding Landsea’s 1993 intensity correction to sort out.
By the way, I view your confirmation of the intensity data since 1983 in the NATL and EPAC to be a huge help in trying to sort this whole thing out.
What most people seem to forget here, is that on one hand there are fundamental thermodynamical arguments which demand that hurricane intensity increase over time, while on the other hand, there are these two big chunks of ice sitting in both polar regions, which will counteract the warming process in their own special way. These two different effects will compete for many centuries to come.
Re 14: Greenhouse heating would be a secondary or even tertiary source of the energy of tropical–regardless of where they occur. Personally, I think that the level of effort being directed at teasing our such a signature or disproving it is overblown. Increased tropical storm intensity will likely be way down on the list of serious impacts of climate change. Much higher on the list will be the melting of those two big chunks of ice you refer to, and the subsequent release of more greenhouse gases trapped in permafrost. The signature of anthropogenic climate change is strongest there, just where the models predict we’ll see it. Looking for signatures in tropical storms, while an interesting diversion, is nonetheless a diversion from strengthening the case for anthropogenic causation and the need to do something about it. I do not intend by this to denigrate the very good work by Jim Kossin or others. It is important research. It just isn’t central to the argument for anthropogenic causation of climate change.
Ray, the uncertainties in the tropical cyclone data preclude the cylones from being used as a “smoking gun” for gobal warming. The importance of the hurricane-global warming link in the overall global warming debate is more psychological/emotional. For the first time (after Katrina and the Emanuel/Webster papers), the public sensed that one degree warming could potentially have some serious consequences, if one degree would make future Katrinas more likely. So in terms of near term impacts of global warming, the risk of increased hurricane activity is pretty high on the list of things that the public is worried about.
What gets me is how contrarians use the hurricane issue (like “scientists cannot say Katrina was caused by GW,” or “there’s no increased trend in the Pacific”) to imply that GW has been disproved. Increased hurricanes are only ONE possible effect of GW, and even if (for the sake of argument) they were found even in the future not at all to be linked to GW, that would in no way disprove GW.
We reg folks only have time for news headlines (not sci articles), so the man-in-the-street reads, “Katrina not caused by GW,” & thinks “GW has been disproved & I can rest easy,” while his wife reads in the Drs office, “GW increasing Hurricane intensity” (a reporter read “warming oceans increasing hurricane intensity” and got confused, and I don’t fault him for it, bec he/she’s very much like that person-in-the-street). And the couple is so busy fighting about GW that they fail to buy those CF or LED bulbs, or even turn off lights not in use.
I’m trying to think like a denialist: since GW is not happening, then all its supposed effects have to be shown to be bogus as well, or due to other factors. These effects are serious problems for them. It’s tiring just to watch contrarians jump through all their inelegant hoops and jump at very thin threads (what next, after cosmic rays?).
OTOH, for GW believers, if there were lack of evidence for increased hurricanes it would be a total NON-PROBLEM. Since there are 2 types of energy, kinetic and heat, we might expect that some of the increasing GW heat would translate into increased hurricanes & storms…or maybe it’ll be doled out in bits and pieces that no one hardly notices…or maybe it’ll just mainly stay in a heat form and not get kinetic on us. Weather is a lot more chaotic than climate, and there are many many factors impacting it, including Sally in Cincinnati singing, “Rain, rain go away.”
We have no trouble at all understanding how internal COMBUSTION engines in our cars translate into driving over to the beach, so this heat-to-motion (GW to storms) should be a no-brainer. Eventually, if not right now, we would expect to see increase in storm intensity and perhaps frequency in a globally warming world, all things being equal…which they are not, since even weathermen can’t well predict next week’s weather due to some butterfly flapping it’s wings in Japan gumming up the wind system.
The important point here is that GW is going to entail many types of serious harms, one of which will likely be increased storms…if not now, then most likley later. And we reduce our GHGs like maniacs now, not that we think it’ll reduce Africa’s current drought, but that it will help in the future.
An important point in the article, I felt, was: “the very real and dangerous increases in recent Atlantic hurricane activity will no doubt continue to provide a heightened sense of purpose to research addressing how hurricane behavior might change in our changing climate…”
Comment by Lynn Vincentnathan — 2 Mar 2007 @ 10:35 AM
The cold stored in polar ice is small compared to the solar forcings related to global warming. It melts without having much cooling effect, and in short order there is net warming because of the reduced albedo of wet snow vs. dry snow and bare rock vs. snow cover.
Re 16. Thanks for the response, Judith. I agree that hurricanes seem to be what the public responds to. However, we know that the public in general does a poor job of accurately perceiving risk. What is more, I think that the denialist camp has exploited the lack of a clear answer on tropical storms and the public’s seeming obsession with it to obfuscate the very clear evidence that does exist for climate change and the anthropogenic causation thereof. How many times have we heard denialists cite the mild Atlantic hurricane season in 2006 as “proof” that we’re all just a bunch of alarmists? This is just turnabout for the irresponsible chatter that blamed Katrina and every other hurricane on climate change in 2005.
The fact that the general public is ignorant of the nature of scientific evidence and scientific consensus means that we have to be very careful in what we link to climate change.
Re: 17. Lynn, you may be giving too much credit to the denialist camp. Some are contrarians–bent on showing how much smarter they are than all the smart people. Some are sheep, merely spouting the party line without any understanding of the science. And some really don’t care, figuring, probably correctly, that if they can make enough money, they can insulate themselves against any adverse effects of climate change. I saw this type when I lived in Africa a lot. They were more than happy to leave their country in squalid conditions as long as they had money to withdraw from Swiss bank accounts that they could use to insulate themselves from the squalor.
Judith Curry wrote: “So in terms of near term impacts of global warming, the risk of increased hurricane activity is pretty high on the list of things that the public is worried about.”
I think that worry is appropriate, given the massive damage inflicted by a single hurricane — Katrina — which even the world’s wealthiest nation has had, and continues to have, great difficulty dealing with. In the US alone, multiple major cities in addition to New Orleans are extremely vulnerable to such storms. If things had gone slightly differently, hurricane Rita’s impact on Houston could have caused a disaster even worse than Katrina’s impact on New Orleans.
I would suggest that another near term impact that the public ought to be worried about is drought. An extreme, prolonged drought affecting North America’s prime agricultural regions would be devastating to the entire world, and unlike sea level rise which is so often discussed as a threat from global warming, a “mega-drought”, like a mega-hurricane, could occur suddenly in any given year, without warning.
I absolutely agree with you that storms in W. Pacific are different than Atlantic storms. And it’s not just SST. The monsoon trough there affects storms in a variety of ways unique to the region. (as an aside, Emanuel 2000 showed that WPAC storms and ATL storms evolve almost identically when intensity is normalized by local potential intensity). But this is not necessarily relevant to what we did. The Dvorak technique is applied the same way there and all that’s changed between ATL and WPAC is the pressure-wind relationship (by the way, the Dvorak technique was also trained in the WPAC). When an eye exists, which is certainly the case for the mature storms that Webster et al(2005) and Emanuel (2005) were most concerned with, the intensity is based on two temperatures; warmest pixel in the eye and temperature of coldest ring around the eye. The “Dvorak rules” that are used to convert raw T-numbers to a CI-number (Dvorak’s final output) have nothing to do with what basin it’s being applied in.
I’ve just revisited Dvorak (1984) and can’t find any aspect to the Dvorak technique that reflects any basin-dependent differences in dynamics. Can you tell me specifically what you’re referring to? Different wind-pressure relationships aren’t relevant to our work, since this wouldn’t introduce a trend, and again, trends were all we were after.
It seems that we are converging on the need to create a better Dvorak technique, and I think this is a worthy topic, but our goal was to compare with the existing Dvorak estimates, warts and all. That’s what the best track used by Webster etal, Emanuel, and almost everyone else, is based on, so I’m not sure why anyone would expect our algorithm to take all these other local environmental factors into account. The cross-validation in our paper also suggests that the Dvorak estimates are more accurate than ours, but not that much. I was really surprised by this considering the simplicity of our algorithm.
To answer your question about convincing skeptics, our algorithm could be tested against intensity estimates from early recon data in the WPAC. It would require finding the data, but I haven’t made such an attempt. Do you have a grad student who enjoys a good hunt? But again, if the Dvorak technique is applied in the same way in both basins, and strong storms are measured simply by IR brightness temperatures, (and all the previous work used these Dvorak estimates to measure trends), then I don’t see why it is so hard to believe that our algorithm could be applied to both basins.
As we mentioned in the article, the folks at NCDC are working to extend the data back to about 1977 or so. The first geostationary satellite SMS-1 was launched in 1971 (if memory serves). I think that the GOES series started in 1973. There were polar orbiters in the 1960′s. My colleagues at NCDC have described how difficult it is to collect this historical data, but they are working on it. It’s a very unfortunate constraint and even in the best of circumstances, I don’t see how this type of satellite reanalysis will really get to the bottom of the pre-1970 Atlantic correction factor debate. What our analysis does do is confirm that the explicitly stated data issues that come with the JTWC best track do have a temporal dependence in them, and as it happens, this bias introduces an upward trend in metrics based on intensity.
Re 21: Something we do agree on. I think drought is a much more likely, demonstrable and potentially much more devastating impact than increased hurricane intensity. So to, are the more intense rainfall events likely to result from climate change–and note that these can occur in the same places as droughts. The impacts I am most concerned about are those that strike at the very infrastructure of civilization–particularly our agriculture–in a world with over 9 billion people in it.
It seems there are two debates here. One concerns the science, Jim Kossin et al. paper. Other the “hype” (RC article) or the “mixing of science, politics and the media” (Judith Curry et al 2006, BAMS).
Concerning science, I recall the first main sub-hypothese in Curry 2006 : “the frequency of the most intense hurricanes is increasing globally”. In the light of Kossin 2007 (or previous critics on data accuracy like Landsea 2006), we can just say that there’s no consensus for the moment and that the statement still needs to be confirmed, before any assumption on tropical SST increase / intensity and GW / tropical SST increase. As Jim puts it nicely and modestly (many thanks to him for participating), “future analyses will hopefully expand on our work.” Longer trends will be useful for progress in our undestanding. I’m interested by Jim’s answer to Judith’s question in #13: “how far back in time would you expect the NATL and EPAC data to be reliable?”
Concerning the hype, hurricane activity seems a classical example of strenghts and weakenesses of popular sensibilization to GW, and its potental counterproductive effects. As Judith observed in #16, the emotional impact of extreme events like Katrina has been sometimes perceived as a good occasion for incriminating GHGs and changing our mind about them. But if a more rational exam concludes that we don’t know the real culprit (maybe GHGs, maybe not), it could well lead to the opposite conclusions among laymen (science is definitely full of uncertainties, medias always exaggerate, all that hype around GW is politically biased, etc.). An that’s true IMO for many points unsufficiently documented, but highly mediatized because of their psychological impact (rate of sea-level rise, effects on biodiversity, tipping points in carbon cyles, global frequency of extreme events, etc.)
> how difficult it is to collect this historical data … a very unfortunate constraint
Aside: a topic (elsewhere, sometime) on this would be welcome. Got data on paper? digitizing and distributed, volunteer proofreading are working for Project Gutenberg. If data’s on old tape or 12″ CP/M floppy disks …
I think the folks working on this question (changes in tropical storm frequency and intensity) are not seeing the forest for the trees.
Most TS losses occur from the storm surge, the water pushed over the land, or inland flooding caused by huge amounts of rainfall often generated by lesser tropical storms hung up over coastlines. Neither of these appear to be particulary linked to storm intensity or storm frequency.
Jennifer Irish (one of Andrew Dessler’s collegues though a similar entry onto his blog generated no discussion) at Texas A&M University recently gave a talk on analyses that showed a weak correlation between storm intensity and surge height or area. For example, Katrina had the highest recorded surge height but was barely a category 3 when it hit, versus both Camille (a 5) and Andrew (a 5). Other category 3 storms have had a third the surge height as Katrina. Rather she found storm size was a more important factor. Katrina, Wilma, Rita, all three were large storms that owed (according to what came out of the tropical storm prediction center) their rapid increase in size to high sea surface temps (Gulf loop currents).
It seems to me that determining the effect AGW has on the generation of large storms (versus total number or intensity) would be a more fruitful line of research.
Also, I’d think modeling storm size would be easier than storm intensity for the same reasons predicting average global temperature is easier to predict than next week’s weather. Storm intensity appears to be subject to rapid changes caused by chaotic preturbations.
I’ve seen it said these can’t be used to conclude much about warming, but — we know the stratosphere is cooling; we know the heat engine works; we know tornado-forming conditions reach high enough to divide the jet stream — isn’t that going to increase the temperature difference? Or is the stratosphere not warming at the level of the jet stream yet? Mostly curious whether hurricanes also poke up enough to split the jet stream wind.
This is probably one of the best threads ever on a science blog. It is very encouraging, educational, and enlightening for non-experts to have esteemed scientists like Drs. Curry, Kossin, and Trenberth discussing their observations and hypotheses about this important topic for all to see, rather than simply leaving it to a conference setting where scientists are the only ones present.
Congratulations all for your efforts and thank you for sharing this with the rest of us!
RE: #9 – As a confessed East Pacific TC geek, I was also quite shocked by Dr. Curry’s assertion regarding basin similarity. Another interesting thing is the exceedingly low occurrence of recurvature for storms that do not transition to typhoons and continue into WPAC. Again, sheer, stratus and cold water are key factors. And by way of comparison, we really do have cold water out here in a way that you will not encounter in say, Iberia or NW Africa litorals. The California Current is quite strong, and, our upwelling is fierce, clear down to Los Cabos. You have to pass Los Cabos into the Sea of Cortez to get into the truly warm water, or of course, go further south, to where the current truly leaves the shore.
It seems to me that it is a mistake to overemphasize the metrics regarding hurricane intensity in the past 30 years.
It was pointed out previously at RC that the case for AGW does not rest with the hockey-stick, but with the theoretical argument that (a) CO2 is increasing due to burning of fossil fuels, and (b) more CO2 in atmosphere means warming temperatures.
Moreover, as each hurricane season is relatively short, it is especially susceptible to the caprices of “weather”, i.e., a season may be strong or weak due to a particular weather pattern that is just a few weeks long and that does not reflect any long-term trend whatever. Moreover, the number of data points (seasons) is still relatively small. And, as increase in ocean temperature lags increase in atmospheric temp., even if no AGW signal is yet visible in the hurricane data, this may not mean very much in terms of the future impact of AGW on hurricane intensity.
So are there good theoretical reasons to expect stronger hurricanes due to higher SSTs? I would certainly think that there are, given that high SSTs are routinely used (by AGW-denier Bill Gray, et al) to predict the intensity of individual seasons. But maybe there are other factors I am not aware of, as a layperson.
RE “The IPCC says there is insufficient evidence to determine whether there is any trend in tornadoes.” That may be from a scientific view. From a concerned-person-view, I tend to consider nearly every weird weather event (logically expected as something GW might cause) as impacted by GW, until proven otherwise. Back in 1992 during Hurricane Andrew, I thought, yep, global warming….when will they ever learn, when will they every learn. And those tornadoes this week…
We live in a globally warming world. The burden of proof should be on the skeptics to prove GW is NOT impacting such&such. I know this is not science or nonsense to scientists, but it makes sense to me.
So if anyone has proof at, say, .05, that GW is not presently increasing storm intensity, I might even consider believing them. (And that might be hard to do, since we might be in a trend right now that doesn’t show up clearly in the data for another 10 or 20 years. So in 2039, the definitive study will come out: “We’ve been in a whopper of a GW-caused increased storm trend for four decades, but, of course, back in 2007 they couldn’t have known that.)
In any case, I won’t stop reducing my GHGs.
Comment by Lynn Vincentnathan — 2 Mar 2007 @ 4:58 PM
The cold stored in polar ice is small
I wasn’t aware that cold was ‘stored’. Anyways, clearly I am referring to the ability of meltwater to interfere with ocean currents, in their own special way, which is clearly yet to be determined. The ice sheets and the oceans are the sleeping giants here, and how their interactions play out as the ice sheets melt and the oceans overturn will determine the course of evolution of the warming which we will inevitably experience. We have just embarked on the great see saw ride in the last 30 years or so, clearly we have a lot to learn. In addition to an international polar project, we need an international ocean project, the continents already clearly being overrun with humans.
Re: 31. Lynn, your post reminds me of the quip that stated that economists had managed to predict 10 of the last 4 recessions.
Ultimately, the thing is that we cannot know with certainty whether any single weather event was influenced by climate change. And the variability of weather is such that you could find “evidence” for any trend you wanted if you looked at a short enough sequence.
Ultimately, climate change poses risks–that is a certain probability of a certain adverse event. We know how to deal with risks–you allocate effort (read $$) in proportion to the risk. So we should already be shelling out a significant wad of cash to try to mitigate risk, and as our certainty grows, so should the funds allocated. The thing is that the public’s perception of risk is distorted, and they tend to over-react to catastrophic but improbable risks that are imminent while discounting very real risks in the “distant” (read >5 years) future. It is probably our evolutionary heritage that makes us look for the rare poisonous snake while getting lost on the trail and starving to death.
Thanks for everyone’s interest in our work. As a co-author, I am somewhat disturbed by overall response of the media, and to a certain extent of some of the scientific community to our work. In this respect, I agree, to a certain extent, with Gavin’s assessment of our work (of course, I think it’s a little more convincing that Gavin suggests, but I’m a co-author, what do you expect?).
An essential component of scientific method is the continual testing and verification of hypotheses put forth within our respective communities. This ensures that the result of a given test is not contaminated by the particular circumstances (in our case, a particular data set) under which that test is performed. Hence, we conduct the same tests as Emanuel (2005) and Webster (2005), only using a data set that is more appropriate for analyzing climatic signals in hurricane activity. We find that a major result of these papers – that hurricane activity is increasing in most ocean basins – is not reproducible, due probably to the quality of the data that was originally used to establish this claim. This result is independent of the fact that we’re looking at a short time period – the two data sets clearly diverge. Interestingly, we find very good agreement in the Atlantic and East Pacific, as explained by Kossin, above.
Are our results a definitive statement that global warming is “honing in on the Atlantic” (as depicted in the media)? Of course not! We’re looking at trends in a 23 year time series – we’re not delusional. Do the results shoot down the hypothesis that hurricane intensity may increase in the future? Again, of course not. BUT, the results do cast serious doubt on the quality of data used to establish this claim (the “best track” data). Hence, I feel our work is an important contribution to the ever-evolving and growing body of evidence that ultimately either disproves a scientific hypothesis, or contributes to its acceptance as a theory. We are very careful to cast our results in that light.
On a contrary note, I worry that Gavin may have missed a key point of our research when stating (in the introduction above) that our technique “doesn’t work as well in the other basins”. The implication is that the method should not be trusted outside the Atlantic and East Pacific basins due to a discrepancy between the best track data and the UW / NCDC analysis. Another, more likely interpretation is that the inhomogeneities in the methods by which the “best track” data are constructed lead to serious errors in long term climatic signals in that data set in the first place. Rather than casting doubt on the UW/ NCDC results, the results indicate that we should seriously question the validity of climatic signals in the best track data outside of the Atlantic or Eastern Pacific. Thus, our paper sheds a lot of light on previous claims of a strong global warming / hurricane link that use the best track data. And, it highlights a very interesting scientific question – why is it that hurricane activity is increasing in the Atlantic but nowhere else, even though SST is increasing everywhere?
As a parting note, I’ll again thank everyone for their interest, and for the ongoing debate. Regardless of our particular qualms with specific research results, we’re all in this for one purpose – to make sure we get this right!
- Dan Vimont
[Response: Dan, thanks for the comment. This is a little outside my field and so I was not trying to pass judgment on your study. In general though, I've learnt that where there are discrepencies, there are often interesting problems to be looked into (on all sides), and that it's best not to prejudge what future research will find. I recall the 'satellite/surface' discrepancy or the ice age tropical temperature issue - much was said that proved rather premature. I'm trying and avoid that! -gavin]
Speaking of papers…
“…the Senate Republican Policy Committee that (Sen. Kay Bailey Hutchison) chairs issued a 10-page document titled, “Global Warming: The Settled Versus the Unsettled Science.” Beyond three general areas of scientific agreement, the Republican document explains “there is considerable uncertainty” about man-made climate change.”
“The IPCC Summary notes that there is evidence for increased hurricane intensity in the North Atlantic, which is correlated with higher sea surface temperatures. It adds this substantial cautionary note, however: that “multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity.” It also states that even though there is some evidence of increased intensity, “There is no clear trend in
the annual numbers of tropical cyclones.”28
In November 2006, the World Meteorological Organization (WMO) held a two week International Workshop on Tropical Cyclones (or hurricanes) where leading hurricane researchers wrote a Statement on Tropical Cyclones and Climate Change. The Statement says that recent research has found “evidence for a substantial increase in the power of tropical cyclones in the West Pacific and Atlantic basins during the last 50 years.” It notes, however, that some scientists argue that the trend may not be real, and instead is due to improved monitoring of strong tropical cyclones in recent years.
With regard to changes in hurricane activity, the statement says that the Atlantic basin has the most reliable historical hurricane records. The causes of multi-decadal variability in major hurricane activity in this basin is still being debated. Some scientists argue that Atlantic hurricane activity fluctuates naturally. The Statement notes one study that disputes this, arguing that variability is due to a combination of changes in solar activity, volcanic and manmade aerosols, and greenhouse gases.
Does Human Activity Contribute to Changes in Hurricane Intensity?
The IPCC Summary assesses the likelihood of increases in intense hurricanes as “likely [better than 66 percent chance] in some regions since 1970,” and states that the likelihood of a human contribution is “more likely than not,” though in the footnote it also notes that the “magnitude of anthropogenic [human] contributions [was] not assessed.”
The WMO Statement notes that, “Though there is evidence both for and against the existence of a detectable anthropogenic signal [i.e., human component] in the tropical cyclone climate record to date, no firm conclusion can be made on this point.” Part of the difficulty in identifying the human component, if any is, as the IPCC Summary itself notes, that tropical cyclone activity varies naturally over the decades. “This variability makes detecting any long-term trends in tropical cyclone activity difficult,” according to the WMO statement. The statement also notes that, “The recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions.”
Finally, the Statement notes that, “The scientific debate…is not as to whether global warming can cause a trend in tropical cyclone intensities. The more relevant question is how large a change: a relatively small one several decades into the future or large changes occurring today? Currently, published theory and numerical modeling results suggests the former.”29
Climate change continues to be a controversial scientific issue. The IPCC process is a major effort to provide a comprehensive overview of the state of climate science, though as is evident even from the Summary – which, due to the necessity for brevity, tends to downplay uncertainties – there are still many difficult obstacles and uncertainties to overcome in determining the influence of human activities on the climate.”
There are three areas upon which there is general agreement. The average temperature has risen, atmospheric concentrations of greenhouse gas have risen, and the latter has probably contributed to the former. The important question is whether changes will be relatively benign or catastrophic. This is still an open question. It would be prudent for policymakers to continue to closely monitor the science.”
This is it? All we have to do is monitor the science?
What gets me is how alarmists use the hurricane issue (like “scientists say Katrina was caused by GW,” or “there’s increased trend in the Atlantic”) to imply that GW has been proved. Increased hurricanes are only ONE possible effect of GW, and even if (for the sake of argument) they were found even in the future to be linked to warming, that would in no way prove GW.
We reg folks only have time for news headlines (not sci articles), so the man-in-the-street reads, “Katrina not caused by GW,” & thinks “GW has been disproved & I can rest easy,” while his wife reads in the Drs office, “GW increasing Hurricane intensity” (a reporter read “warming oceans increasing hurricane intensity” and got confused, and I don’t fault him for it, bec he/she’s very much like that person-in-the-street). And the couple is so busy fighting about GW that they fail to buy those CF or LED bulbs, or even turn off lights not in use.
I’m trying to think like an alarmist: since GW is happening, then all its supposed effects have to be shown as well, or not due to other factors. These absent effects are serious problems for them. It’s tiring just to watch alarmists jump through all their inelegant hoops and jump at very thin threads (what next, after polar ice caps aren’t melting?).
OTOH, for GW skeptics, if there were evidence for increased hurricanes it would be a total NON-PROBLEM. Since there are 2 types of energy, kinetic and heat, we might expect that some of the increasing solar warming heat would translate into increased hurricanes & storms…or maybe it’ll be doled out in bits and pieces that no one hardly notices…or maybe it’ll just mainly stay in a heat form and not get kinetic on us. Weather is a lot more chaotic than climate, and there are many many factors impacting it, including Sally in Cincinnati singing, “Rain, rain go away.”
We have no trouble at all understanding how internal COMBUSTION engines in our cars translate into driving over to the beach, so this heat-to-motion (solar warming to storms) should be a no-brainer. Eventually, if not right now, we would expect to see increase in storm intensity and perhaps frequency in a globally warming world, all things being equal…which they are not, since even weathermen can’t well predict next week’s weather due to some butterfly flapping it’s wings in Japan gumming up the wind system.
The important point here is that solar warming is going to entail many types of serious harms, one of which will likely be increased storms…if not now, then most likley later. And we reduce our GHGs like maniacs now, not that we think it’ll reduce Africa’s current drought, but that it will help in the future.
An important point in the article, I felt, was: “the very real and dangerous increases in recent Atlantic hurricane activity will no doubt continue to provide a heightened sense of purpose to research addressing how hurricane behavior might change in our changing climate…” so “give us more money!!!”.
RE#36 I do. And it comes from Emanuel I believe, which is to say the Pacific and Indian Oceans are already warmer, thus this is an opening in the natural system that needs to catch up given the rising global mean temperature. If my nonclimate background is off, I’m confident someone with more expertise will adjust this accordingly.
Given that this issue is still generating much disagreement, are that aspects that everyone can agree on? In particular, would anyone question that the observed trends in sea surface temperature in all basins can be attributed to anthropogenic global warming? Hurricanes are not the best place to look for a global warming signal, but SSTs and surface ocean heat content are.
(Ref: http://www.sciencemag.org/cgi/content/abstract/1123560v1 )
It seems that the hurricane issue is of greater interest than SSTs for the reasons stated in the comments. However, just looking at the isolated case of Katrina, if Katrina hadn’t encountered a deep warm pool of water in the Gulf of Mexico, it wouldn’t have intensified over the Gulf (historically rare behavior for hurricanes to do that). This brings in another variable, the depth of the warm layer, as well as the SST issue. In the absence of counteracting factors, it’s hard to argue that hurricanes won’t have a higher probability of intensifying along the southeastern United States. One could also argue that there is a chance that ‘permanent El Nino conditions’ could shift wind patterns in the Atlantic basin making in less probable that hurricanes will develop vertical structure – but that’s a bit too convenient to be reassuring. It is the kind of argument that contrarians will sieze upon, however.
The practical issue is what matters to people who live in these areas, however – and I think most climate scientists would recommend reinforcing levees and not building in flood plains, and it seems that the insurance industry is taking the same approach. Whether or not a trend in hurricane intensity has been detected is still apparently open to question – but the trend in SSTs seems unquestionable.
As a member of the non-scientific “skeptic” camp (but not a denialist), I find this particular topic and most other topics introduced on this site to be extremely valuable in forming my thoughts on the entire GW/human impact debate. Thanks for providing this resource (I even can use it at times to help with my insomnia while trying to follow some of the more technical aspects of the comments..again, a great public service).
All joking aside, this thread highlights a trend I see in much of the RC blogs……drifting over into commentary/politics/hype versus sticking to science. Why is Katrina used as an example of hurricane intensity, when it was documented as only a Category 3 hurricane (albeit with a large storm surge)? If you take New Orleans out of the equation, Katrina would only be a footnote other than to those directly affected along the coastline of MS/LA. The larger question is the extreme risk (both in human and economic terms) that the exponential growth of the human footprint along the US Southern Coastline has created. Nobody cares if a Category 5 hurricane hits undeveloped areas..it doesn’t get ratings! What research is being done on the growing human risk along the various coastlines of the world, both from Hurricanes and (God forbid) rising sea-levels?
Also, one final dig. Unless Global Warming has only been a problem since January 2001 (date of a certain inauguration), please keep the political commentary out of postings…it’s beneath this fine effort.
Re 38. Wahoo, Katrina is significant because prior to landfall it was a really big storm. It was a very large storm and it had (at the time) the 4th deepest low pressure ever measured. Its near coincidence with Wilma and Rita, which were even deeper raised eyebrows. So for climate folks, yes, we do care about a Cat 5 in an uninhabited area, because it is another piece in a trend.
As to the political sniping, I agree that it is unprofitable. However, I think you have to understand the level of frustration in the scientific community and in the climate community in particular with the tendency in the current administration to “manage information”. To some extent, there is an inevitable conflict between politicians, who thrive on being able to manage and spin information, and scientists, who require free exchange of information to do their jobs. However, by any measure, this administration has been heavy handed in “editing” scietific reports. While I think that this tendency goes all the way to the top, I think the worst offenders have been ham-handed political appointees, who have made things much worse than they had to be. However, I will take your admonition to heart and make an effort to avoid political bashing. The debate is not between liberal and conservative, but between science and anti-science.
Re: 37. Vince, if you wish to “think like an alarmist” might I suggest looking at the evidence. The question of whether climate change is influencing hurricane strength is still open and debated within the scientific community. The question of whether humans are changing climate is settled. Do not confuse the two issues.
Comment by Lynn Vincentnathan — 3 Mar 2007 @ 10:14 AM
#33, thanks, Ray. I get your point. I think I was trying to argue the prudence perspective r/t risk. So, while “we cannot know with certainty whether any single weather event was influenced by climate change,” we cannot know if it wasn’t. And prudence (one of those Godly virtues, like mortification) would require us to mitigate GW, even without high certainty that it is happening, or that it is impacting severe weather events. Of course, in prudent ways, like all the measures that do not harm us (some even helping us).
And policy-makers, you’d think, would not be able to argue against that position…unless they are solely focused on helping those few fossil fuel companies that refuse to diversity into alternative energy. And even those companies could benefit by becoming more energy/resource efficient/conservative.
Comment by Lynn Vincentnathan — 3 Mar 2007 @ 12:03 PM
Some of my comments were posted perhaps a bit out of context. I welcome the response from Jim Kossin and I wish to emphasize that I was not dismissing their results at all. On the contrary I welcome them as a useful, but incomplete, contribution to the science of tropical cyclones and climate change. The development of the new dataset is a very good first step also but much more can and must be done. It takes resources and commitments from funding agencies, and I have been working to that end for some time (in paricular through my role as Chair of the WCRP Observations and Assimilation Panel) by calling for extra efforts in reprocessing satellite observations and reanalyses.
Jim Kossin may be right, in part, that my criticism of their work applies also to the Dvorak technique, but it is not at all obvious to me that the latter applied to the new data set would give the same results as in their recent study. It is very bothersome that there is no convergence of results over time, and maybe this calls current practices into question, but those aspects should be resolved.
Some of the big picture things are being missed here. Basically with global warming there is an expectation of increases in tropical storm activity. Please note that the IPCC SPM statement very carefully focussed on activity and not just intensity. Such activity may extend to individual thunderstorms, but it is certainly expected also for tropical cyclones owing to their relationship with the ocean. Only the organized TCs take significant heat out of the ocean ameliorating global warming of the oceans. Then the increased activity can be manifested in several ways. These could be increases in numbers, intensity, size, and duration. We have some data on numbers and duration but not the other quantities. Currently intensity is a spot value and not integrated over the size and duration of the storm. Size is not kept as part of best track data. There are published listings of many possible metrics that can be retrieved and kept and the need is to go back and reprocess the data to recover a much more comprehensive set.
The physical expectation is that TCs are apt to become more intense and bigger with warming. But because one big storm takes much more heat out of the ocean than two smaller storms, the expectation is that numbers may decline. Duration could vary: a more intense storm may peter out more quickly by having used up the available energy in the immediate vicinity (from convergence in the atmosphere as air spirals in, to surface moisture from evaporation in the strong winds) unless it moves into a new environment. The track of the storms seems to be largely a crap shoot depending mostly on the weather situation at the time. I know of no arguments about landfalling vs nonlandfalling storm trends that make sense.
It seems that many agree that the environment in each TC basin is different. The tropopause differs substantially (relates directly to potential intensity) and the sea surface temperature patterns and land configurations vary greatly. The sooner we can come to grips with the issues and set about compiling the requisite datasets and analyzing them, the better off we will be. The Kossin et al studies are good first steps, but just first steps.
[Response: Kevin is right to point out that we may not have provided the full context for interpreting the various quotes that we provided, including those attributed to him. That's our fault, and we're thankful to him for taking the extra time in his busy schedule to come in and provide a fuller context for the remarks. -mike]
Comment by Kevin Trenberth — 3 Mar 2007 @ 12:22 PM
Ray says in #19 (as have others) …”How many times have we heard denialists cite the mild Atlantic hurricane season in 2006 as “proof” that we’re all just a bunch of alarmists? This is just turnabout for the irresponsible chatter that blamed Katrina and every other hurricane on climate change in 2005…”
As a skeptic (I’m certain the Holocaust took place) you’re absolutely right, the nil hurricane season of 2006 does not disprove AGW. But it’s logical many would claim that in response to all the cheering for AGW proof from the 2005 hurricanes, which seemed like a solid good point at the time but now has to be called irresponsible as part of the back-peddling.
[Response: 2006 was an average year, not 'nil', and if you look back at the pages of RealClimate on the issue you will not see any 'cheering'. Indeed, you will see plenty of statements pointing out the fallacy of taking one event or season as proof of anything. It is easy (and a little lazy) to only rail against the stupidest or most extreme position on the 'other side' of an issue. Serious discussion only occurs when people address the more considered statements. - gavin]
[Response: In fact, 2006 was slightly above average w/ 10 total named storms, and substantially above average for an El Nino year (for which the typical number of named storms is closer to 7). Part of the explanation is that SSTs over the main development region were anomalously warm, favoring development even in the face of an unfavorable shear environment (associated with the incipient El Nino during Fall '06). - mike]
It’s worth noting how the scientific discussion of the hurricane issue has differed from the media discussion. Right during and after the 2005 hurricane season the most common media explanation was that hurricane activity had a 20- to 30- year cyclical behavior that was dependent on the North Atlantic Oscillation. (see C. Landsea on this at http://www.aoml.noaa.gov/hrd/Landsea/20thCenturyHurricanes.html ) The data to support that theory thus seems scantier than the data that supports an increasing trend in hurricane intensity, yet the NAO lnk was widely repeated on CNN withou much questioning. Similarly, there was an immediate media rush to blame this winter’s anomalous record warmth on a mild El Nino event, though that explanation is surely only partial… and there was also record warmth in Moscow. It would be nice to see reporters covering the scientific discussion more carefully, in other words.
These two topics are related in that the current ENSO forecast ( http://www.bom.gov.au/climate/enso/ ) shows a higher chance of La Nina conditions this fall. Note to reporters: a scientist’s willingness to make predictions of the future is an indication of the current level of understanding of the science; for example Hansen et al predicted that Pinatubo’s eruption in 1991 would produce a significant aerosol cooling effect, and they were right; but would anyone be willing to predict that La Nina (assuming conditions set in) will result in a record hurricane season this fall?
Gavin, I appreciate most of what you’re saying and as I mentioned in my earlier comments I certainly don’t consider our findings to be the last word, and I encourage others to expand on our work. But you are in fact passing judgment by saying:
“While the technique works well in the North Atlantic (picking up almost all of the storms seen in the standard data), it doesn’t work as well in other basins – possibly because the characteristics of tropical cyclones are not universal, or because the coarse early remote sensing data are still not sufficient.”
This statement is not actually justified. First, the technique works exceedingly well in the Eastern Pacific as clearly documented in our paper (also see my comment #9 above). Second, as Dan Vimont correctly points out, it is the fact that the agreement is not as good in the other basins that is the key finding here, it’s not a suggestion that our results are somehow questionable. There is no reason to believe that our method would not capture the salient features in basins besides the Atlantic, and this can be confirmed by any hurricane specialist including those who operationally apply the Dvorak technique and/or participate in constructing the best track. An alternate hypothesis for the basin differences is offered in my comment #9.
It seems that most of the people who have commented formally here on our paper are not actually in a particularly good position to comment on whether our algorithm, or any IR-based method such as the Dvorak technique itself, is applicable in all ocean basins. While you and the others who have commented (somewhat negatively) on the validity of our results are certainly experts in your fields, I doubt that any of this group would claim any expertise in the procedures and applications of the Dvorak technique or the operational protocols of the forecast offices that construct the best track data that most of the recent studies have been based on. Yet, this is where the all of the main objections seem to be coming from. This is a clear disconnect that in all fairness should be considered before stating that our algorithm “doesn’t work well in other basins”. Unfortunately, these words are the first thing people see when they visit your blog and they are in fact not well-founded.
Could our results be proven wrong by subsequent studies? Absolutely. Am I sure that they are giving us the right answer? No. Are there warts on our algorithm? Yes. Are there warts on the algorithms used to estimate the intensities that form the best track records. Yes. We’re all estimating intensity from IR imagery. Not exactly uber precise. Would hurricane specialists question the validity of our algorithm outside of the Atlantic? No. Would climate scientists question the validity of our algorithm outside of the Atlantic? Apparently yes (said with tongue planted firmly in cheek).
I thank Kevin Trenberth for his remarks and for his efforts in advocating for improved resources for data acquisition and analysis. The points raised about the bigger picture are good ones and are all too easy to forget when we get involved in the specifics of one study or one debate.
I just want to comment very briefly on the concern there is no convergence over time between the UW/NCDC and best track records. This might simply demonstrate the repercussions of making estimates over many years with virtually no validation. Ground truth measurements are exceedingly rare outside the financially privileged Atlantic basin, where we routinely have low- and high-level aircraft that drop sondes and measure surface wind with active microwave sensors. So although the satellite data has gotten better in the other basins, with no validation to calibrate the estimates against and uncover the biases, there may be no reason to expect better intensity estimates with better data. The biases and the fidelity of the variability might in fact become worse with time. The Dvorak technique is a subjective method, which is to say that two forecasters could form very different estimates from the same data. This happens routinely and is well-documented. Our algorithm is an objective form of the Dvorak technique. With these things in mind, perhaps it is not that surprising that the results don’t converge. This rather depressing situation truly underscores Dr. Trenberth’s call for improving current practices.
Re #51: Because of the large thermal mass, the ocean is expected to warm more slowly than the land. But the time period for your graph includes the 1940 to 1970 period of slight cooling between warming episodes before and after it.
I reproduced your graph here (basically the same except it has a coarser smoothing algorithm), then produced a graph covering only the recent warming period: (1975-1985) to (1995-2004), shown here. You can see there is relatively more warming in the ocean, but still less than on land.
Global warming is indeed expected to reduce the temperature differential between the poles and the equator. I understand the effect of this on hurricanes is minimal, but maybe some of the experts that we are lucky enough to have here can clarify this. However, the differential may act to reduce the intensity of temperate zone storms. Of course this is only one factor affecting such storms.
RC folks, I know you do not like to comment on political documents, but this document by the Senate Republican Policy Committee has to be about the worst case of cherry picking of published papers I have ever read. I agree with Lynn – it merits some comment.
Gavin, re 49: Of course many if not most of the serious scientists didn’t claim Katrina et al as proof of AGW. But there was a loud clammering for such from many others. I was just fighting ridiculous with silly since Ray lambasted all of us “denialists” for the reverse in 2006.
As someone else noted, the debate is fascinating. What I find somewhat odd is that there is no explicit consideration of simply variations in solar activity as a primary source of the energy for hurricanes and hence variations in their number and their intensity.
Second, I deal with numbers a lot and I get real nervous and skeptical when someone argues that a relative infrequent set of conditions that has to date an average of 7 associated events means that when 10 occurs that this is a “substantially” higher number. There must be something that I am missing or whoever thinks this should definitely stay away from Las Vegas.
RE#53, 51, Kerry Emanuel wrote a great short piece on hurricane physics, available at http://www.physicstoday.org/vol-59/iss-8/p74.html (“Hurricanes: Tempests in a Greenhouse”) which might help address that issue. The temperature gradient that matters for hurricanes is the difference between the sea surface and the top of the troposphere, and if the vertical structure breaks down due to wind shear the hurricane dissipates or won’t form. Thus, a decreasing pole-equator temperature gradient shouldn’t have much effect on hurricanes. Also, just because the average pole-to-equator temperature gradient is decreasing doesn’t mean that the seasonal variation won’t still be in place, and then there’s the whole issue of the hydrologic cycle intensification – a moister atmosphere carries more latent heat and thus may generate more intense mid-latitude storms as well. Such effects on mid-latitude storms, however, seem to be more uncertain than the effects on hurricane intensity and frequency. (see http://www.realclimate.org/index.php/archives/2006/12/on-mid-latitude-storms/ )
Regarding the issue of Katrina, note that in Sept 2004 Nature ran a brief news blurb entitled “Hurricane Ivan highlights future risk for New Orleans”. There is a relevant quote: “But scientists say that another disaster was only narrowly avoided – Hurricane Ivan missed the deeply vulnerable city of New Orleans by a tiny margin. In the face of future such storms, they are calling for action to restore the area’s wetlands, to act as a barrier against flooding.” Katrina could have been another Ivan – but throw enough darts at a coastline, and sooner or later you will hit a major city.
This news article ended with the claim that the hurricane season was at the peak of a 30-40 year cycle, however – a statement that seems poorly supported at best. For more on how the media has covered this issue, see http://www.cnn.com/2005/TECH/science/09/23/hurricane.cycle/index.html . Is the rather definitive statement that “The increased activity since 1995 is due to natural fluctuations (and) cycles of hurricane activity driven by the Atlantic Ocean itself along with the atmosphere above it and not enhanced substantially by global warming” at all supportable?
Re: 56. Rod, just for the record, although I am only a physicist and not a climate scientist, I was among those who counseled that one cannot infer trends from a single storm or a single season in 2005. When the initial studies came out suggesting an increase in hurricane intensity over time, I viewed it as intriguing, not compelling. Likewise, the current study. The thing is we in no way need an increase in storm activity to provide evidence for climate change. Such an increase is not expected to be evident at this stage. We have plenty of evidence from the places where the models tell us to look–at the poles, overnight low temperatures, dates of first and last frost, etc. I would continue to counsel that anyone who tries to make climate policy on the basis of the weather is unwise.
Re: 57, bjc. As someone who deals with Poisson fluctuations on a regular basis in my day job, I share your skepticism over drawing any rigourous conclusions. However, if one sees a trend in the Poisson mean over time, that may indicate that something is going on. Increases in insolation are easy to measure, so the trends can be compared.
Statistically, I would think that the Likelihood would be the proper tool, as it would be easy to compare various models and see if the increased model complexity for a particular trend in the mean actually yields increased information. To pursue your Vegas analogy, if you see a trend in payouts of slots over time, you might conclude to pursue your studies of applied probability (i.e. gambling) elsewhere.
Re Mike’s comments in #48, “In fact, 2006 was slightly above average w/ 10 total named storms, and substantially above average for an El Nino year (for which the typical number of named storms is closer to 7). Part of the explanation is that SSTs over the main development region were anomalously warm, favoring development even in the face of an unfavorable shear environment (associated with the incipient El Nino during Fall ’06).”
Are the studies controlling for these non-GW factors that would account for decreased and increased TCs? That, it seems, might yield better results. Also the other factors Kevin Trenberth (#47) mentioned.
[Response: Well, firstly tropical Atlantic SSTs are believed to be rising, in large part, as a consequence of anthropogenic climate impacts, so they are not necessarily unrelated to global warming. But there is a substantial body of research that has been done looking at how various factors (such as El Nino, the North Atlantic Oscillation, etc.) may influence Atlantic tropical cyclone numbers. James Elsner of Florida State University has, in particular, done quite a bit of interesting work in this area. - mike]
I guess if it weren’t for GW, the field wouldn’t be developing as rapidly as it is.
I’m even very impressed with the 3 dimensional doppler our weatherman used to show us Hurricane Emily. Perhaps that technology could get at the total size, though I imagine it would be quite expensive to use for each and every TC around the world.
Another thing about Emily, which I’ve raised here before, it brought a shower of hailstones to Brownsville, TX, which our weatherman said was unusual (he had never heard of that during a hurricane). I think all these idiographic factors need to be stored in data banks, so scientists can eventually explain them.
Comment by Lynn Vincentnathan — 4 Mar 2007 @ 8:49 AM
#57, RE small numbers, I’m no climate scientist, but I do know statisticians have methods, such as Chi-square and log-linear analysis (based on odds ratios), that are quite successful on data sets with small numbers of observations.
Comment by Lynn Vincentnathan — 4 Mar 2007 @ 9:09 AM
BJC, who told you that nobody considers solar variation? Who are you relying on for this? They’re not telling you the truth. You can look this up for yourself.
“… the Sun’s output fluctuates … by about 0.1 percent between maximums (1980 and 1990) and minimums (1987 and 1997) ….. Temporary dips of up to 0.3 percent and a few days’ duration are due to the presence of large sunspots …”
Read the numbers along the left side of the picture; http://ase.tufts.edu/cosmos/pictures/CambEncySun/Sun_ency_figs_3/Fig8_17_org_comp_7899f.jpg
The number of observations has to be a function of the complexity of the phenomena being studied. A rough analogy: How many rolls of a dice do you need before you can determine the nature of its bias. The more causes or sides of the dice you hypothesize the more data you need. You can do some things with small numbers but you need very strong theories or constraints. For how many El Nino’s years do we have reliable counts of hurricanes?
#57 Solar activity (if you mean TSI) has no clear trend from 1980s (if any, less pronounced than the difference bewteen a minimum and a maximum in a cycle, and I’m not informed of a statistical link between solar cycle variation and cyclone activity). On the other hand, I don’t know if there are correlation studies of the cyclone activity with the surface insolation trends in past decades and for each tropical bassin. Wong et al. 2006 find for example an upward trend (decrease of outgoing SW)for the mean tropical zone, but with no information on the regional signature of this trend.
Re #61. Lynn, as someone who often has to work with sparse datasets where things vary Poisson-wise about some mean, that also may be changing, I can attest that drawing rigorous and meaningful conclusions is not easy. First, if you look at the entire dataset of hurricanes, your mean is under 10 per year that hit the US. A Poisson distribution with mean less than 10 is nongaussian, so things like chi-square etc. do not scale properly. Now if you split your dataset–looking for example only at Cat. 4 and Cat 5 storms, or only years where there was no El Nino or La Nina, the situation gets much worse. Likelihood-based techniques are probably among the most powerful for assessing different models, but they get a little squirrelly with sparse datasets. This is a tough problem, and it would be difficult to draw meaningful conclusions at high confidence. However, if you establish what your model is telling you a priori, and look to see if you find those trends, you can increase confidence in your model.
No statistical technique can squeeze more info out of a dataset than is there to begin with, and since we’re dealing with a small signal in a complicated and noisy dataset, this is a really tough problem.
Mike, et al: a really late and simple question. I don’t comprehend how we believe the Atlantic SST is rising (over the long-haul) due to recent AGW. (Or is maybe “recent” not the appropriate word…) The IPCC thinks the global average temp has increased about 3/4 degree C. the past rough century, which is about 0.0075Â°C per year. Given the annual/periodic swings of SSTs, how is it we can conclude that SSTs are on a rising trend (O.K., maybe a decent tentative conclusion) caused by AGW (a great leap in logic??)?
[Response: See Santer et al (PNAS, 2006) for a fairly thorough investigation of precisely this question, using a standard detection and attribution approach. - mike]
So, on the one hand you have the claim that Atlantic hurricane intensity is controlled by the AMO, whose mechanism is poorly understood but which has something to do with the meridional overturning circulation, which is influenced by the sinking of water off of Greenland. The argument put forth by Gray and Klotzbach, among others, is that in 1995 the AMO entered a warm phase and that explains the increased Atlantic SSTs and the increased hurricane activity.
The argument is then that the reason trends outside the Atlantic are weak is that they aren’t being influenced by the AMO; the other explanation is that the other regions are already over the threshold, so that the Atlantic basin is more sensitive to changes in SSTs and atmospheric moisture than the other regions… or the data may be poor.
At the same time, the same groups (Pielke, Landsea, etc.) claim that the hurricane trend data is too questionable to support an increasing trend in intensity, and that no links can be made to global warming – Pielke, June 2005.
What they pointedly ignore is the question of what effect global warming will have on these natural oscillations. Attention has been focused on how El Nino will change in a warming world, and climate models do reproduce El Nino. If the AMO is tied to conditions off Greenland, then it is obvious that global warming will have some effect on it. Considering that the mechanism of the ‘natural AMO’ is so poorly understood, there’s no justification for immediately blaming increases in hurricane activity on it while entirely ignoring global warming effects on sea surface temperatures (and atmospheric moisture), for which very clear mechanisms do exist.
As in other climate phenomena, there may be a multitude of factors responsible for the observed trends – but are Gray, Pielke, Klotzbach and Landsea really claiming that global warming has no effect on SSTs? Do they really believe that there will be a decreasing trend in Atlantic SSTs as the AMO enters a ‘cool phase’ sometime in the future? This is a question that Roger Pielke Jr. doesn’t seem to want to answer, despite numerous requests.
What is fairly hypocritical is to publicly claim that the data over the past few centuries is good enough to support this AMO explanation for hurricane activity, while at the same time claiming that the data is too poor to produce a statistically relevant trend over the past few decades.
Incidentally, there are a few other people who are pointing to the high hurricane potential of the 2007 season (and this is also a good description of the factors involved in the 2006 season): Why has the 2006 Hurricane Season been so calm?
Hey folks, it looks like the skeptics have their own TV show. Apparently Drudge is linking to an upcoming documentary on British(?) Channel 4: The Great Global Warming Swindle. From the description, it appears to feature the usual suspects.
Causality is on tenuous ground here, unlike geologic temperature records, extreme weather event statistics are available for an infinitesmally small time period. It is impossible to know whether extreme weather events (drought,floods,hurricanes) are more or less prevalent in previous times of higher CO2 and temperatures. If ocean temperatures were deterministically generating them, they should be more easily forecast year by year using ocean thermometers than they are. It is entirely plausible that high enough global average temperatures may suppress more extreme events than are caused.
One quantifiable effect, which is spreading of severe weather events to places where they have not previously been experienced regularly, may not necessarily translate into continuing increase in probability of that occuring. It may be just the current temperature signature, which may well change with continuing increases in average temperature.
I am not saying that this research is invalid in general, but no weight should be given to this aspect with regards to policy. If climate change’s quantifiable effects other than hurricanes are not enough for a policy shift in themselves, they shouldn’t be with the consideration of hurricanes.
Here, the author draws causality relationships between global mean near-surface air temperatures and Atlantic sea surface temperatures and hurricane power dissipation indexes using statistical causality tests. This is one of the very few papers that directly addresses the issue of whether the observed trends in hurricane intensity can be blamed on an Atlantic Multidecadal Oscillation, or whether they are due to global warming. The causality path is different for each hypothesis, to quote:
“Figure 1 illustrates the two competing hypotheses concerning Atlantic hurricanes. The climate change hypothesis asserts that changes in radiative forcing resulting from increased greenhouse gas build up in the atmosphere increases GT and causes Atlantic SST to rise at least during the hurricane season months of August through October. On the other hand, the AMO hypothesis asserts that natural changes in the deep water circulation of the Atlantic Ocean drive hurricane season SST resulting in changes to both hurricane activity and GT. Under both hypotheses local SST plays a direct role in helping to power hurricanes by providing moist enthalpy and instability. Thus the point of departure for the two competing hypotheses is the causal connection between GT and Atlantic SST. The climate change hypothesis suggests the causality goes from GT to Atlantic SST whereas the AMO hypothesis implies it is the other way around.”
Based on the results of the causality tests, the author concludes that it is global near-surface air temperature that influences sea surface temperature, and not the other way around – which supports the global warming-induced increase in hurricane intensity.
Being a parochial West Australian I had to read the original article by Kossin et al to find out what it had to say about the S. Indian Ocean – basically that the dataset is suspect and no trend is particularly observable. Fair enough.
But a couple of anecdotal observations seem in order. This summer cyclone activity has been low and the season has been late. But this seems not to be due to El Nino but more localised conditions, specifically the SST have been unusually cold on the west coast of Oz and high off the east coast of Africa.
About two weeks ago, we had simultaneous formation of two cyclones(Gamede and Humba) in the Indian Ocean and currently we have George tracking across the Kimberley and a yet-to-be-named cyclone forming near Christmas Island. These double cyclone systems interact with one usually ending up dominating(Gamede). But they presumably also work against each other in terms of kinetic energy. So this raises the question of how double systems like these affect conclusions about cyclone intensity and cyclone numbers. Kossin et al limit their observations to cyclones above a certain category. So, are two simultaneous Category 2 cyclones( one of which goes to Cat 3 and the other dies) more or less important than one category 4(say) in terms of describing the removal of heat from the ocean or in terms of reaching conclusions about the relationship between cyclones and GW?
[[Second, I deal with numbers a lot and I get real nervous and skeptical when someone argues that a relative infrequent set of conditions that has to date an average of 7 associated events means that when 10 occurs that this is a "substantially" higher number. ]]
Depends on the standard deviation, doesn’t it? You seem to be assuming it’s large compared to the mean.
Re #60, thanks, Mike. I guess the non-global warming factors I was thinking of are the things that reduce hurricanes even when SST are rising, such as wind factors (I don’t actually know what I’m talking about here). But then it occurred to me that these, too, may be impacted by GW.
I don’t have time to read those articles now, but I’ve saved the links. I’m thinking some might have to do with how GW is impacting el ninos, which involves warming waters. I know RC has addressed this before and it’s complicated.
All I know is many decades ago I learned el ninos occurred roughly every 7 years. Then in the early or mid 90s there were more frequent and more intense el ninos, and also the weatherman blamed all the weird weather on el ninos.
It seemed to me that GW was likely impacting the el ninos (warm waters were also found responsible for die out of plankton & the fish that thrive on it). It seemed sort of like blaming a hired gun for killing a person and letting the big culprit off the hook. Prior to those weather assurances by the weathermen, people were beginning to think GW may be responsible for the weird weather, but the U.S. weathermen IDed el nino (like Perry Mason IDing the actual killer, only it was probably a hired killer and not the guy (GW) behind the crime).
So once the (supposedly natural) el nino cause was established, Americans forgot about GW and went back to business as usual….and now our GHGs are expected to rise 20% by 2020, over our 2000 emissions. And the rest will be history.
Comment by Lynn Vincentnathan — 5 Mar 2007 @ 9:28 AM
Re 73: Barton, higher moments–especially the skew can also be important when you are dealing with Poisson fluctuations, as this can introduce systematic errors into the analysis. Basically in statistics, we can handle things vary normally and maybe lognormally, but anything more complicated than that gives us trouble. I’ve recently played around with re-deriving the Chi-square distribution assuming Poisson distributed fluctuations vs. normally distributed. How important these fluctuations are depends on the Poisson mean, the size of the effect you are looking for and the level of confidence you require. For a Poisson distribution with small mean, the standard deviation is always large (square root of the mean). Note that if you want to play around with this stuff, Excel gives you most of what you need.
My basic point is that with a small number of datapoints SDs tend to be driven by the next piece of data. Since we don’t “know” what that might be, making judgements as if we did are, as I said originally, troubling. Ray Ladbury’s more thorough explanation make sense to me.
What is fairly hypocritical is to publicly claim that the data over the past few centuries is good enough to support this AMO explanation for hurricane activity, while at the same time claiming that the data is too poor to produce a statistically relevant trend over the past few decades.
Note there is an out. The data in the Atlantic are so much better, it would not be a stretch to claim the data in the Atlantic is good enough to detect a trend, but the global data is not. I don’t know that any of Gray, Pielke, Klotzbach, or Landsea are taking it, however. (Related – much of the media has been claiming ‘Global warming is honing in on the Atlantic – but this almost the opposite of the truth, which is that hurricane researchers, due to what might be called historical accidents, are honed in on the Atlantic. ) Although I am sure you have already read it, I think your set of AMO-related links is not complete without Atlantic Hurricane Trends Linked to Climate Change (Emanuel, Mann, 2006) (supplemental material here ) which may be the best available explanation for the AMO. In that paper, Kerry and Mike show the combination of global SSTs and regional aerosol forcing are a good predictor of Atlantic tropical SSTs and thereby Atlantic TC counts, and that there is very little Atlantic tropical SST variation left for a multi-decadal cycle to explain. This implies (though the paper does not directly state) that the AMO is just a case of mistaken pattern recognition (an idea Urs Neu has been promoting in comments here for a very long time).
The AMO seems related to the same phenomena that result in claims like ‘it was awfully warm back in the 1930s and 1940s’ . It seems the aerosol induced cooling between the 1950s to the 1970s has served the ‘its not happening’ camp so well, despite having been explained some time ago.
[Response: Thanks for mentioning this. Actually, we don't go as far as arguing that the AMO itself is an artifact. I for one have published a number of papers very much arguing for its existence (e.g. Delworth and Mann, 2000 and Knight et all, 2005, you can find reprints of both here), and must confess that I actually coined the term "Atlantic Multidecadal Oscillation" in an interview with Richard Kerr of Science in 2000. I'm happy to let Kerr (2000) take the credit nonetheless :). Kerry Emanuel and I do point out in our piece that past work indicates that the AMO is unlikely to project substantially onto tropical Atlantic SST, and that the attribution of recent trends in tropical Atlantic SST to the AMO is indeed probably an artifact of the impact on the tropical Atlantic of competing anthropogenic forcings. -mike]
“The physical expectation is that TCs are apt to become
more intense and bigger with warming. But because one big storm takes much more heat out of the ocean than two smaller storms, the expectation is that numbers may decline.”
While I understand the logic behind this idea, I think it is worth noting that, at least anecdotally, it does not seem to agree with what the history is telling us. For example, 2005 Atlantic had exceptionally high SSTs, the largest concentration ever of very strong storms, and also, by far, the largest number of named storms.
Further, looking back at hurricane records, there does not appear to be an inverse correlation between strong storms and number of storms, as one might expect if the assumption you refer to is correct. There may be a variety of explanations to account for this, and my observation is hardly rigorous, but it still strikes me as interesting. I wonder if the fact the higher SSTs will lead to somewhat longer seasons (and 2005 certainly had that), and larger sea-surface area where TCs might form, would serve to counteract the effect you describe, and still lead, net-net, to an increase in the number of tropical cyclones with higher SSTs.
RE: #35 #45 #54
This paper is smoothly seductive and well written in the sense that it has the look and feel of an authoritative document. Strip it to the bare bones and it’s the usual controversy-debate-uncertainty biaised tripe.
Also spotted is Lindzen, Ross McKitrick and the “Cooler Heads Coalition” being referenced. Well there’s a red rag waving right in front of you.
Maybe a word from the gurus? ‘Course it’s a full time job debunking deniers but silence implies consent.
[[My basic point is that with a small number of datapoints SDs tend to be driven by the next piece of data. Since we don't "know" what that might be, making judgements as if we did are, as I said originally, troubling. Ray Ladbury's more thorough explanation make sense to me. ]]
I’m sure it does. Is there a known time series for this number, so we can calculate the mean and standard deviation? I mean, Poisson corrections and so on aside, if we know what the standard deviation is, then the fact that this kind of problem tends to lead to large standard deviations is rather beside the point, isn’t it?
Well, if AGW does cause an increase in North Atlantic SST then this might be another positive feedback. This is a link to a Science Now story about research into the winds around Greenland. http://www.sciencedaily.com/releases/2007/03/070305085344.htm It mentions offhand about a 2003 melting event on the icecap caused by the passage of a tropical storm. A warmer ocean will allow storms to retain their tropical characteristics as they venture into the north Atlantic and dump copious amounts of warm rain onto Greenland more frequently. This effect of warmer SST will occur whether or not tropical cyclones become more frequent or more intense. Not to mention the increased chance that a tropical storm will dump 20 or 30 inches of rain onto northern Europe. I recall that Portugal saw a tropical storm last year and that a couple of other storms just wouldn’t call it quits as they wandered northward due to very warm SST.
Re 80: Barton–by all means if you know the standard deviation, but how do we know the standard deviation unless we estimate it from data. The thing about the Poisson is that there is only one parameter to the distribution–the mean, so if you determint that, you know all the moments. In this case though, the mean is assumed to be a function of time, and there are other variables that affect it as well (e.g. El Nino), and we don’t even know if we have a reliable dataset (e.g. has our ability to determine hurricane strength improved with time). So this is not an easy problem. Finally, there’s the question of the confidence level for the results–you don’t want to be fooled by statistical fluctuations. I am by no means an expert here. I’m just going by how I’d look at the problem based on how I’ve solved others.
My reaction was the same. The absence of data is the problem. Making statements about the relative distinctiveness of the number of events with no knowledge of its mean and distribution is very odd. I am sure the person just mispoke. EOD
Re: #71 On the other hand, the AMO hypothesis asserts that natural changes in the deep water circulation of the Atlantic Ocean drive hurricane season SST resulting in changes to both hurricane activity and GT. Under both hypotheses local SST plays a direct role in helping to power hurricanes by providing moist enthalpy and instability. Thus the point of departure for the two competing hypotheses is the causal connection between GT and Atlantic SST. The climate change hypothesis suggests the causality goes from GT to Atlantic SST whereas the AMO hypothesis implies it is the other way around.”
This analysis is useful in comparing two theories of “the past”. Whether this has anything to do with future higher GT’s, we are assuming that the relationships(between GT and SST’s and between SST’s and Hurricanes/cyclones) continues and that it is linear when we “extrapolate” outside the range of historical data. I am not claiming to have an alternate hypothesis that debunks these two, but extrapolating for one set of extreme events that we suspect may increase, and ignoring other kinds of extreme events that may reduce in risk, is cherry-picking, and presumptious. This is different to the case for predicting GT’s in the first place. We have geologic records of temp and CO2 – we don’t have geologic records of temp and hurricanes full stop.
The very late moderate El-Nino of 2006-07, is turning to La-Nina extremely fast! Implications for hurricanes are obvious. But the changeover speed is a bit fast? Or is this normal?
[Response: Not that unusual (see e.g. here). There is a tendancy for the tropical Pacific to 'rebound' out of an El Nino into a La Nina, as the underlying dynamics are fundamentallly oscillatory. -mike]
[[My reaction was the same. The absence of data is the problem. Making statements about the relative distinctiveness of the number of events with no knowledge of its mean and distribution is very odd. I am sure the person just mispoke.]]
I fear you may be making a mistake when you refer to the small number. The number referred to is the number of data points; I interpret your original post as implying that you believe the value of the number is the small number in question — i.e., because 7 or 10 is a small number, the distinction is obscure. But that 7 or 10 is only one data point. By changing the units we can make the value as small or as large as we like; the value of the point means nothing as to the ability to interpret a number of such points. How long has the number of named storms, or the number of large storms, been kept track of? I would be surprised if it were less than 30 years, and that is enough to give 95% confidence, other things being equal. If the typical numbers reported are as, let us say, 2,7,9,4,3,8 and so on, then a value of 10 is not very surprising. But if they are more like 7,6,7,8,8,7, then a value of 10 is surprising indeed. The person who mentioned the large value is someone who works professionally with the analysis of such storms, is he not? If so, I would be inclined to take his word for it when he says 10 storms rather than 7 indicates a meaningful deviation.
[Response: TC numbers are essentially Poisson distributed, so the standard deviations are approximately the square root of the mean rates. For El Nino years, that gives 7+/2.6 , i.e. 10 storms is a bit more than one standard deviation above the mean rate for El Nino years. Very roughly (keeping in mind that the Poisson distribution is heavy tailed), 10 or more counts should happen about 1 in 6 times randomly in an El Nino year. Clearly not outside the range of possibility, but relatively rare. The main point is simply that any claims that the the 2006 Atlantic TC season was in any way 'inactive' by historical standards, is simply silly. -mike]
[Response: I just now performed a quick calculation using the total number of named Atlantic TCs from 1870-2006 (the 1870-2004 data are available here, I updated w/ the latest two years numbers, 28 and 10). The mean rate for all years is 9 named storms, the mean rate for El Nino years (that is, years where an El Nino event was building over the course of the storm season, based on DJF Nino3.4 index > +1) is 6.3. Fitting a Poisson distribution to the series of El Nino year TC totals gives an 11% probability of equally or exceeding 10 storms. So my estimate above was too conservative. 10 or more storms in an El Nino year really is fairly unusual. -mike]
Mike, thanks for the link. The data look pretty Poisson, even though there’s a hint of bimodality. A maximum likelihood fit yields a mean annual number of TC of 9, with 90% confidence that the mean is actually between 8.5 and 9.5. The data for El Nino and La Nina years are much sparser, but I basically get a Max. Likelihood mean of about 7.5 with a 90% confidence interval of 5.75 to 9.75. So we can’t really say with high confidence that 2006 was way high, but it would certainly be flat wrong to say that it was low by historical standards for an El Nino year. Has anyone been looking at the possibility of reconstructing which years prior to the ’80s were El Nino, La Nina and “normal”? That might help us out in reconstructing any trends.
RE #3: There are several folks working on paleotempestology as it’s called. The ones I’m most familiar with is Kam-Biu Liu at LSU and Andy Reese at USM. The focus is using pollen records from lake sediment cores. I had a few courses with Professor Liu at LSU…intresting stuff.
Seems to me the debate about AGHG global warming and increasing TC frequency/intensity/duration boils down to the fact that as sea surface temperatures, as well as deeper water temperatures rise, the wallop of any TC over warmer seas without mitigating circumstances like wind sheer and dry air off land masses entrained in the cyclone will likely be much more devastating.
Do total numbers matter to most people if a large percentage of storms veer away from coastlines? Do any of the parameters one way or the other count for much if just one storm exacerbated by warmer oceans results in catastrophic effects?
Of course having the science nailed down is essential. But focusing the debate within societies on overall frequency or intensity or degrees of this or that in this or that geographical basin as a way to evaluate the relationship between climate change and tropical storms is confusing the issue. How many Hurricane Katrinas does it take to make this point?
No one can say global warming caused this hurricane. But the intensity of Katrina a day before landfall was category five. This was due to warm water in the GOM loop current. As warmer waters are entrained in currents throughout the gulf isn’t it just common sense to expect that as warm deep waters inevitably occur closer to land the intensity of violent storms way well be sustained as they make landfall?
The debate in the US is about whether or not capping emissions is necessary. Warmer water means more evaporation and a bigger heat engine offshore. I’d say without attempts to reduce emissions to mitigate impacts we’re playing Russian Roulette with violent cyclones whether there are other weather/climate patterns like El NiÃ±o or the NAO influencing occurrence or not.
Just monitoring the science will be insufficient to avoid economic consequences over-riding the consequences of doing nothing as our elected representative seek to avoid economic consequences to big oil, big coal, transportation and utility campaign contributors. Perhaps the Congress should meet in New Orleans or Galveston or Biloxi, Miss, from now on?
If it hadn’t been for a large amount of dust and dry air from Africa during the middle of last summer, there would have likely been a few additional tropical storms. Some of the tropical waves that left the African coast were quite “promising”, but the dry dusty air did them in. With the increasing SSTs and forecast of more drought in Africa, I would imagine that we would likely see a decrease in the number of tropical storms (particularly in the eastern Atlantic because of dry dusty air), but the ones that develop will more likely be in the cat 4/5 range since there will not be as many storms to mix the waters. Even with dust reflecting some radiation back into space, the dust will also act as a blanket still allowing SST to increase. With warmer SSTs, I would imagine there will be more storms like Wilma that will increase their intensity from TS to cat 4/5 level in a very short period of time. This has to be causing some nightmares for the forecasters at the NHC and emergency preparedness officials. For example, the 5 pm advisory says the storm has 65 mph winds and a hurricane watch is issued for a heavily populated portion of the coast. Residents go to sleep thinking the next day they will have a cat 1 storm. The next morning, the residents find they have a cat 3 heading to cat 4 that will be making landfall. Andrew did a increase just before landfall as well as Hugo. Hugo picked up energy from the gulf stream just before landfall that increased its winds by about 20 mph. I lived on the east side of Columbia, SC (100 miles inland) and we took a nice hit from Hugo (lost power for 3 days). I now live 150 miles from the coast and I am beginning to think it is too close. I hope someone can shoot some holes in the above line of thought.
Re #92, the Russian Roulette analogy is good. That’s a one in six chance something bad will happen. I think that’s about the right standard for false-negative avoiders (people trying to avoid harms — which should be nearly everyone). We don’t play Russian Roulette; we mitigate at an 18% or greater certainty GW is happening, or GW is enhancing hurricanes (given the good theories and logic, which have been around 100 years, that we could expect GW with increased GHGs). Which means we should have started seriously mitigating at least by 1990.
Another point is that the extra amount of hurricane intensity or precipitation caused by GW, even if small, could be the “umph” that could blow the house down or breach the levee (the straw that breaks the camel’s back). The last “umph” will likely be much more devastating than the first “umph” of equal amount (can someone tell me what the unit of TC intensity is called?), and the last few inches of precip, than the first few inches. So the impact of GW on TCs or precip does not have to be very much to cause a tremendously greater amount of damage and harm.
Comment by Lynn Vincentnathan — 6 Mar 2007 @ 9:52 PM
Re #96, I know your feeling, Jim. We live in about 50 miles from the Gulf, in the Rio Grande Valley. As we moved down here the signs were pointing in the opposite direction, “Hurricane Evacuation Route.” They still talk about Hurricane Beulah (1967), and getting flooded 3 feet in their homes, and how it spawned a lot of tornados, as well. I’d only been thinking about getting on GreenMountain wind energy and growing tropical plants, & didn’t even consider the hurricanes. That’s the problem, mostly they don’t come, so we forget them rather quickly.
Comment by Lynn Vincentnathan — 6 Mar 2007 @ 10:08 PM
Re #3 and #89:
Thanks for the pointer Royce. Without a doubt Paleotempestology is the most fascinating new word I’ve learnt in a long time!
These pages provide good descriptions of the discipline.
1) Proxies with promise that are mentioned include;
– physical analysis of sedimentary deposits in coastal lagoons and marshes.
– isotope and physical analysis of rings in corals, trees and cave .
– isotope and physical analysis of layers in lakes especially those with varves (annual layers) and in arid areas.
2) It seems to be a relatively young discipline with lots to be done and breakthroughs to be made.
Sedimentary deposits are probably limited by the fact that the sea level has only been at roughly it’s current level for 6 or so thousand years, before which it was rising and therefore erasing deposits as it went. But some of the others offer amazing possibilities. For example check out the paper of Frapier (2005) describing the testing of a method for identifying tropical cyclone events using isotope ratios in the cores of stalagmites. They used a modern 23 year period with a known cyclone history and were able to discern 8 out of 10 cyclones but found only one false positive out of 1200 sample points – impressive.
A very interesting (and a wonderful piece of detective work) paper by Miller et al (2006).
It shows that late 20th century Atlantic hurricane frequency is not at all unusual.
I suggest that this paper finally ends the speculation that recent hurricane activity is driven by/linked to AGW.
What a pity that this seminal paper will not be included in the latest IPCC assessment.
Miller, D.L., C.I. Mora, H.D. Grissino-Mayer, C.J. Mock, M.E. Uhle, and Z. Sharp, 2006. Tree-ring isotope records of tropical cyclone activity. Proceedings of the National Academy of Sciences, 103, 14,294-14,297.
I was wondering if you guys could comment at all on the study that just came out in EOS by Lau and Kim regarding a possible explanation for the non-extreme 2006 hurricane season. Their main hypothesis was that an increase in Saharan dust blown over the Atlantic decreased solar radiation, lowering SSTs. Interesting.
[Response: Interesting, but unconvincing. To anyone who was observing the TCs as they were organizing (or attempting to organize), it was fairly obvious that the development was being hindered by strong westerly winds aloft -- this is reasonably attributable to the substantial incipient El Nino conditions that were in place last fall. A good test will be what happens this next season. We still have historically high SSTs, and it looks like a substantial La Nina will have taken hold, which should lead to decreased wind shear. This should provide a similarly favorable environment for TCs to that which was in place during the 2005 season, and a forecast of more than 15 total named storms would seem reasonable at this point. The authors of the Eos article should their own forecast. Let the best hypothesis prevail! -mike]
[Response: One thing that bothers me about the study is that no mention is made of what the covariability is between Nino3 SST and the Saharan dust index used. It it possible that ENSO impacts are masquerading in their study as dust impacts simply because of the covariability between the two. I would want to see that addressed before taking the results more seriously. The article is provocative and interesting, but at this point should be considered somewhat speculative. Hopefully, it will spur others to look at this connection more carefully. -mike]
#99, I would assume, though, that higher SST correlates with increased hurricanes, since while it may not be a SUFFICIENT CAUSE (other factors are also required to create a hurricane), it does seem to be a NECESSARY CAUSE (hurricanes cannot happen without higher SST).
And if increasing SST correlate with increasing hurricane intensity (and we know SST is lagging behind air temp warming — “a watched pot never boils,” esp if it’s the Atlantic), then we only have worse to expect in the future….worse than Katrina.
Katrina is now water over the levee. The reason we should mitigate GW & reduce our GHGs ASAP is hopefully to reduce the chance for “Super-Katrinas” in the future, as well as a host of other potential harms.
It IS a shame the article didn’t make it into the current AR4, since by the next AR we’ll have more hurricane data to work with, and the question will perhaps be more settled, and Miller et al will either be disproved, or one among many still better studies saying the same thing.
I sincerely hope their study is just one among many saying the same thing (hurricanes don’t increase with increasing SST – if that’s what they’re saying), but I’ll keep on engaging in measures to reduce my GHG, bec there are just so many other reasons to do so.
Comment by Lynn Vincentnathan — 7 Mar 2007 @ 9:37 AM
That paper can be viewed here. They were able to use oxygen isotope ratios in tree rings to reconstruct a remarkably accurate picture of the hurricanes that have passed over or within 400km of the site in southern Georgia since 1770. However, it don’t agree that it is a definitive statement on the lack of a link between recent hurricane activity and anthropogenic global warming. In particular, the oxygen isotope ratios can reveal that a hurricane has occurred, but as far as I can see give little indication of strength. It certainly doesn’t say anything about the possibility of correlations emerging as we get warmer. It’s one reconstruction at one site and there is no statistical analysis of the significance of the observed sequence. It seems to me that we will to need to see more such studies before a clear picture emerges. The key message I get from the paper is that oxygen isotope ratios in tree rings can provide a very effective hurricane incidence proxy.
Lynn (#comment 101). The article does not try and link SST with hurricane activity. What it shows is a 220 year record of Atlantic hurricane activity using a novel proxy method.
It also demonstrates that hurricane activity has always been variable and what we are now experiencing, in terms of hurricane activity, is not unusual. You can access the paper at; http://www.pnas.org/cgi/reprint/0606549103v1
A very interesting (and a wonderful piece of detective work) paper by Miller et al (2006).
It shows that late 20th century Atlantic hurricane frequency is not at all unusual.
I suggest that this paper finally ends the speculation that recent hurricane activity is driven by/linked to AGW.
PNAS has made that paper available on open access, see here .
It draws no conclusions about the effect of AGW on hurricanes (neither ‘climate change’ nor ‘global warming’ occurs in the paper). Nor does it claim to have shown that ‘late 20th century Atlantic hurricane frequency is not at all unusual’ . Indeed, the most recent year they examine is 1990, whereas 1995 is usually viewed as the start of the recent high activity.
The method they use is important, but it can only detect whether hurricanes which cause significant O18 depleted rain to fall on the trees sampled. (Note they fail to detect the above average seasons of 1893 and 1898.) Further, they make no claim to be able to detect the number of hurricanes in a season, or the intensity of a hurricane. Although they do not attempt to calculate the proportion of total Atlantic activity their method detects, it does not seem amiss to suggest their method detects only a small fraction of total Atlantic activity. Had it been extended to include recent hurricane years (not necessarily feasible with tree-ring studies), it might well be insufficient to detect a trend.
I was looking at the data on TCs and the El Nino. Is the data equally reliable throughout the period covered? What actually is the data source for the earlier periods say pre-1939? Is it compiled from weather stations or shipping records such as those maintained by Lloyds? Are TCs that are true mid-Atlantic storms as likely to be counted. I am curious since there is a clear upward trend and the first thing is to make sure that the data sources are standard.
Re #104, I never stated that the paper makes any conclusions about the effect of AGW on hurricanes. I did state that it shows that late 20th century Atlantic hurricane frequency is not at all unusual. That is my reading of this paper.
Although they do not attempt to calculate the proportion of total Atlantic activity, they were able reconstruct a remarkably accurate picture of the hurricanes that have passed over or within 400km of the site in southern Georgia since 1770.
Now “within 400km” is a pretty large area and I suspect would capture a fair proportion of total activity.
What I am saying is that I do not believe recent measurements of hurricane activity capture anything like the natural variation in activity. Hence it is not possible for us to make any conclusions about effects of AGW on hurricane activity.
[Response: Objectively speaking, it does appear as if your comments misrepresented the conclusions of that paper. Several readers have pointed that out above. Your statement here that "it is not possible for us to make any conclusions about effects of AGW on hurricane activity" is simply silly. The consensus of the worlds scientists studying the problem is quite the opposite, albeit w/ the nuance and caveats that are appropriate in legitimate scientific discourse. As this particular comment thread is increasingly becoming argumentative rather than substantive in nature, we're going to leave it at that. -mike]
I could see how it might be hard to get a correlations or association between hurricanes and SST, if the non-SST factors are dominant in causing hurricanes. That is, you might not get hurricanes in cooler waters, but in warmer waters you may or may not get them, depending on these other dominant factors….which would make it hard to get good & significant stats for incrementally warming waters in known hurricane areas.
But it’s fairly well established that warmer SST means more hurricanes in general, but not always.
Comment by Lynn Vincentnathan — 7 Mar 2007 @ 12:21 PM
Thanks llewelly for that link (#77); I hadn’t actually seen that one. Obviously there is natural variability, and some of it is cyclic, but the anthropogenic warming signal is very clear. Unfortunately, various commentators continue to try and blame the warming signal on natural and cyclic variability, whether it’s solar cycles, the AMO, or El Nino. The real question for issues like El Nino, however, is how a warming planet will affect these cycles – will frequency and intensity of El Ninos increase? In the case of El Nino, predicting the response in a warming world still is uncertain, but is an important question – see this Jan 2007 news report on the issue.
Don Keiller – as others have pointed out, that paper doesn’t attempt to address the issue of the recent increase in Atlantic hurricane intensity, or the ongoing increasing trend of increasing sea surface temperatures and atmospheric moisture content. The study area is fairly small, and it’d be interesting to see if a broader regional study would still produce a discernable signal. Furthermore, the paper clearly states that it doesn’t address hurricane intensity, but only the landfalling hurricane record – and as others have noted the landfalling hurricane record doesn’t really relate to the total hurricane frequency. The paper seems to be a description of a new proxy method that may be suited to detecting the effects of hurricanes on localized regions, but that says little about Atlantic basin hurricane variability. The method should also be tested against the hurricane-related oxygen isotope data collected in 2004 and 2005, if that is possible for recent tree growth. As other comments here note, there are very few proxies for storm activity, so this method is notable – but your interpretation doesn’t make much sense.
Our main conclusion was that a), we had, in fact, gone back to a busy period in the Atlantic, Gulf of Mexico, and the Caribbean, and b), it was caused by a natural fluctuation in the Atlantic Ocean and the atmosphere, called the Atlantic multi-decadal oscillation. And we could say this because we had historical records for hurricanes and ocean temperatures, as well as other studies of proxy records of temperatures, suggesting that these busy and quiet periods tend to last 25 to 40 years each. Thus we concluded that it was likely to stay busy for another decade or two to come.
Even if we ignore the poor hurricane data on a global basis, it seems that Landsea, Pielke, Gray, et. al are still pushing the AMO hypothesis while claiming that there is no reliable baseline for comparison of the past 20 years of increasing intensity data in the Atlantic basin. In particular, their claim that hurricane activity (which seems to be a poorly defined phrase – does it mean frequency + intensity?) should decrease in a ‘decade or two to come’ based on the AMO claim has very little support – they don’t even seem to have a hypothetical mechanistic explanation for how the AMO will reduce Atlantic basin SSTs in the future.
Dr Don Keiller has posted once here before, under the nom de guerre of Archskeptic (#58). It was, I think, an inauspicious start by him. (Gavin obviously thought so.)
He was a nearly single-issue correspondent on the old BBC S&N boards on all things sceptical about GHGs and AGW (IR bands are saturated (Beer-Lambert), sea level, glaciers, etc., most of the usual stuff you’d expect, including the hurricanes issue and Dr Landsea’s distancing of himself from the IPCC). Sadly, I think all those posts on the old BBC boards are now gone.
However, a flavour of his contributions remains on a BBC R4 forum: #12, #17, #26, #35, #37, #40, #41, #43, #44.
I have the feeling that in raising some of those arguments here he might well become unstuck when faced with correspondents whose day job is in climatology and atmospheric physics … pretty much like an atmospheric physicist would feel, I would imagine, in commenting on the merits of something like, say, â��Polyclonal antibodies raised to phycocyanins contain components specific for the red-absorbing form of phytochromeâ�� Planta176, 391â��398 (not that there appears to be much to argue about there, not that I’m qualified to express an opinion on it anyway).
Perhaps Dr Keiller’s added input will cause the temperature here to rise sufficiently to set off a few local topical [sic] storms. And perhaps the added heat of argument may add to the intensity of the odd seasonal hurricane hereabouts, until the storm hits a patch of cool green comment that is.
I look forward to more of his contributions on here … or, rather, I look forward to the ensuing comments and correspondence. Life won’t be dull. Possibly tedious, but not dull.
I used to work with Ralph Nader and he always said the most important stories are under reported. He is still right!
[Response: These things are not difficult to check. Transportation is about 17% of global CO2 emissions, ruminants are 24% of anthropogrenic CH4 emissions. If you assume these are reasonably good estimates of historical emissions, you can apply those percentages to the appropriate radiative forcings. So transportation is 17%*1.6W/m2 = 0.27 W/m2 and ruminants 24%*0.8W/m2 = 0.19 W/m2 (less if you use the SPM numbers which deal with the indirect impacts of CH4 on O3 etc.). Therefore, while the same order of magnitude, transportation is in fact significantly larger. Not sure what we are supposed to conclude from this though... - gavin]
reading the comments here really depresses me. looks like i chose the wrong field of science to pursue.
i couldn’t believe it when i read the claim that 2006 was an above-average year (for an El Nino year).
ok, granted no one claims they can predict individual hurricanes or transitions in the southern oscillation, but the fact of the matter is that you have a significant statistical description of El Nino and no one
bothered to include it in the 2006 forecast? Bayes is roling over in his grave. are there any
serious Bayesians in this field?
[Response: Its easy to make such pronouncements without doing a little bit of homework first. The El Nino was indeed taken into account in most forecasts once it was apparent there was one in place. The problem is that El Ninos typically only emerge during the boreal autumn season, and primarily impact the latter half of the Hurricane season. When the initial predictions were made last season by NOAA they were based on the assumption of neutral El Nino conditions. As it became increasingly clear that an El Nino was building, the forecasts were systematically downgraded over the course of the season. We've criticized NOAA Hurricane Center folks before on certain issues (e.g. their attribution of recent Tropical Cyclone trends to the "AMO") but on this issue they are quite sound. By the way, here's a google scholar search result on "Bayesian" and "Hurricane". Wouldn't have been so hard to do this first, would it? Elsner's work, by the way, was cited in one of our earlier comments in this thread. -mike]
RE #110, here’s a solution to the beef v. transport debate on which is worse for GW….just stop driving around eating at burgers drive-thrus. Or, turn off the engine at drive-thrus, and order veggie burgers!
RE the argument (I know it’s largely unsupported) that current hurricanes are not impacted by GW, but within natural variation, does not give me one iota of solice. If Katrina & other current hurricanes are just run-of-the-mill, what will the hurricanes be like once GW really starts kicking in?
So if skeptics are trying to assure people all is okay, nothing bad is happening, that’s not a good argument. Sort of like their “GHGs trail, don’t precede, GW, in past warming events” which just reinforces my concern about positive feedbacks, and the whole thing spiralling into a super-duper warming, well beyond what our human emissions are causing.
Comment by Lynn Vincentnathan — 8 Mar 2007 @ 10:02 AM
RE#111 and El Nino / La Nina, this looks like a difficult forecasting problem. Back in Jan and Feb of 2006, the cautious predictions were for an onset of La Nina conditions (see NOAA Feb 2 2006 and NOAA Jan 12 2006) However, by spring 2006 all signs of La Nina had disappeared, and the NOAA forecast for August 2006 was for a potential El Nino (Aug 10 2006). So, the hurricane forecasters took all this into account, apparently.
It does make one wonder about the current La Nina forecasts, however, and their potential effects on the oncoming hurricane season.(Pacific Shows Signs of Morphing From Warm El Nino to Cool La Nina). The statistical argument that La Ninas are likey to follow El Ninos may support this – but as climate continues to change, the historical statistical predictions will become less and less useful.
It seems that the effects of global warming on hurricane intensity are better understood than effects on the El Nino oscillation. Figuring this out will be important for understanding regional climate change – is persistent drought in the American West going to be the result, or will regional precipitation become highly variable in space and time?
Hello. I was hoping that someone from ClimateScience would give their take on this article. Thanks.
A Climate-change Amplifying Mechanism
Science Daily – During the past ninety thousand years there were alternating hot and cold periods lasting several thousand years each which resulted in a modification of global oceanic circulation. With the help of paleoclimatic and paleooceanographic indicators, scientists at CEREGE1 have highlighted a feedback mechanism of ocean circulation on the climate which reinforces this heating or cooling. This mechanism relies on a close link between the circulation of the North Atlantic and the tropical hydrology of Central America. This study, published in the February 22, 2007 edition of the review Nature, should allow us to better understand and therefore better predict the effects of climate change on oceanic circulation.
Average rainfall variations, simulated by models, after a collapse of deep ocean circulation (increases in blue, decreases in red; from Stouffer et al. 2006). The dotted arrow shows the actual flow of water vapour. The solid arrows indicate the multiple flows (trade winds -> rivers -> marine currents) during a climatic anomaly. Crosses show the paleoclimatic study sites.
In the past, major and rapid climatic variations which took place notably during the last glacial period (Heinrich period) disturbed ocean circulation. Climatic archives (marine and lake sediment, polar ice, stalagmites) show the close relationship existing between climatic variations and oceanic circulation. Changes in oceanic circulation in the North Atlantic have influence on a planetary level by affecting, in particular, the water cycle. These changes are accompanied by a shift in the climatic equator which separates the trade wind systems of the two hemispheres: southwards during cold events and northwards during hot ones.
Central America, a narrow continental strip which separates the Atlantic and Pacific oceans, plays a key role in this system. On the Atlantic side surface waters evaporate, which increases salinity. The water vapour is transferred by the trade winds to the Pacific where it is deposited as rain, thus lowering salinity there. This enormous transfer of water (several hundred thousand cubic meters per second) maintains a contrast in salinity between the two oceans. The surface waters of the tropical Atlantic are then transported, via the Gulf Stream, towards the high latitudes where they warm the atmosphere before plunging into the abysses in the convection zones situated in the seas of Norway, Greenland and Labrador. The deep waters formed by this process then flow into the world ocean, purging the North Atlantic of part of its excess salt.
The scientists at CEREGE1 reconstituted the variations in surface water salinity in the area where the water vapour coming from the Atlantic is deposited. To do this they worked on the measurements taken in marine sediments collected in 2002 west of the Isthmus of Panama by the French oceanographic ship the Marion Dufresne. This study shows that the cold Heinrich periods correspond to increases in salinity in the east Pacific. This is synonymous to a decrease in the transfer of water vapour. By comparing their results to other studies done in the Atlantic sector and in South America, the scientists were able to describe a feedback mechanism which amplified the climatic disturbance. During cold periods the trade winds, loaded with humidity, migrated southwards. Unable to cross the Andes part of the rain, which would normally have lowered the salinity of the East Pacific, fell in the Amazon basin. This feedback had the effect of re-injecting rainwater into the Atlantic, thereby decreasing the ocean’s salinity. This water was then transported to the higher latitudes, contributing to the weakening of deep oceanic circulation, thereby reinforcing the cooling above and around the North Atlantic.
Today, the fact that global warming could disturb the water cycle and lead to a slowing down of the North Atlantic circulation is a real subject of concern. Oceanographic data from the last 50 years suggest that hydrographic changes (temperature and salinity) as well as a lessening of the flow of water transported by certain surface and deep-sea marine currents have already occurred in the North Atlantic. The risk of an even greater variation of oceanic circulation by the end of this century or the beginning of the next needs to be taken seriously and actively studied.
Note: This story has been adapted from a news release issued by CNRS.
Hello. I am searching for the best information available on responses to Lindzen’s “Iris” effect. Perhaps I am looking in the wrong place, but Realclimate does not seem to have a substantial post on the topic.
With AGW increasing SST’s generally world wide, a cooling troposphere why would it not be plausible to conclude an increase in intensity of TC’s.
Data omission and even trends aside, other than near permanent El Nino ie. sub tropcial sheer, what are the other detractors, perhaps increased subsidence, Sahel dust, Blocky/cut off lows at least in the Atlantic basin? And a question to anyone, Has anybody closely examined the S. Atlantic TC or realtionship of recent strong Indian Ocean TC’s.