The big problem with much of the discussions about trends in hurricane activity is that the databases that everyone is working from are known to have significant inhomogeneities due to changes in observing practice and technology over the years. So it’s not surprising that a new re-analysis (Kossin et al, published yesterday) has been generating significant interest and controversy among the hurricane research community (see e.g. Prometheus or Chris Mooney). 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.
Kossin et al develop an algorithm based on North Atlantic data that can be theoretically used with the coarsest data available from the earlier parts of the record and in more remote regions. 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. The poorer performance in the other basins is surely a reason to anticipate that further work will be necessary to refine these estimates, and should serve as a caution to those wanting definitive conclusions.
How does this fit in with some of the previous work? Well, it confirms the large trend in the North Atlantic (seen in Emanuel, 2005), but doesn’t show significant trends in the other basins (from 1983). This isn’t directly comparable with Webster et al (2005) though, since their trends start in the 1970s, and the shortness of the new reanalysis (only 23 years) emphasizes interannual and decadal variability associated with e.g. El Nino. The Kossin et al study is therefore unlikely to shed much light on the potential global warming/hurricane intensity link.
In summary, read the papers and the comments but don’t believe the hype.
We’ll start off the discussion with a few comments we have already received on the provocative study:
Based upon the new results of Kossin et al. (GRL, 2007), it looks like the IPCC SPM just barely covered itself in its proclamations on observed hurricanes:
There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. 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. There is no clear trend in the annual numbers of tropical cyclones.From the results presented in Kossin et al. the “suggestions” of increases in intense tropical cyclone activity in regions other than the Atlantic basin are not really so well supported, at least for the last 23 years.
We’ve tried to make this point a bit more clearly here, despite the implications of the headline of Kossin’s press release.
-Chip
(disclosure: I have, to some degree, been funded by the fossil fuel industry since 1992)
1) The methodology is trained on the Atlantic. It has no parameters to allow for different structures or size of storms, and there is no good reason why it should work well on storms in other basins. Given the different land-sea configurations and the different role of ENSO in the different basins, and the fact that disturbances in other basins do not form from easterly waves from off of Africa, there is every reason to expect that storms in other basins have different characteristics. For instance, there is greater activity in the Pacific Northwest, and the tropopause is higher in the western Pacific, and this affects brightness temperatures at tops of clouds. If the size of storms differs then the fixed form of EOFs will not be able to capture that form. The analysis must be able to account for differences among basins in order to have confidence in variability or trends. It would be easy enough to test whether the storms in other basins had different characteristics by also performing an EOF analysis for each region. This basic test was not done. It should be.
2) The results are suggestive of these problems. In the SIO where the method gives 0, 1 or 2 storms vs up to 6 in the best track data, there is a serious bias. Similar large biases exist in the SPAC (up to 2 vs 5 in best track). Obviously the threshold is effectively different and it is a comparison of apples and oranges.
3) In addition, this version of the paper deals with PDI. The earlier version of the paper dealt with intensity of storms and that was abandoned because the results were not very good. In particular, the presumption is that the older results were the problem because operational methods have improved. But the Kossin et al results showed bigger and greatest discrepancies with those from best track in recent years: there is no convergence over time. This is harder to see with PDI, because the biggest storms are emphasized, but the question of why is there not good agreement in recent years is not answered.
3. Dr. Judith Curry:
The most vexing thing about the tropical cyclone data sets is the uncertainty that analyst subjectivity contributes to this. The Dvorak scheme for determining tropical cyclone intensity is notoriously subjective, see the recent BAMS article on this. The importance of what Jim Kossin has done is to take this subjectivity out of the analysis.
Kossin’s method matches well the historical data in the North Atlantic (NATL) and East Pacific (EPAC). The method was trained using North Atlantic data, and the East Pacific regime in terms of dynamical and thermodynamical conditions is very close to the North Atlantic conditions. However, Kossin’s method diverges substantially from the established data sets in the Western Pacific, South Pacific, and Indian Oceans. Does this mean that the established data sets or in error, or that Kossin’s method (trained in the Atlantic) does not translate well to the other ocean regions?
Owing to problems with dealing with historical satellite data, Kossin’s study was extended only back to 1983 (the period for which the satellite data are well calibrated), and it is almost certain that this data set cannot be extended back prior to 1977. By itself, this data set is too short to say anything about a trend in intensity. But it can in principle be used to assess uncertainties in the established data sets.
My own analysis of the discrepancies has focused on the Western Pacific (WPAC) data, where 40% of the global tropical cyclones form. During the period 1983-1987, Kossin’s data overlaps with aircraft reconnaissance data; I would expect the WPAC TC data during this period to be of comparable quality to that in the NATL and WPAC, but large discrepancies are seen. I would also expect the agreement to be better in more recent years with the advent of more sophisticated satellite systems, but the discrepancies are largest during the most recent period. Its would not be surprising for this method trained in the NATL not to work well in the other regions. The method does not allow for different structures or sizes of storms in different basins. The NATL cyclones form primarily from easterly waves, while those in other regions do not. The role of ENSO is different in the different basins. The tropopause height is higher in the WPAC. etc.
Many people in the tropical cyclone community have questioned the Emanuel and Webster et al. papers owing to uncertainties in the data sets. Other than anecdotal analyses, little has been done to quantitatively assess the uncertainties. Kossin’s paper is arguably the first important word on this subject, but it certainly won’t be the last word. To establish the credibility of Kossin’s data set outside the NATL, considerably more analysis is needed to understand discrepancies in individual basins and the nature of the discrepancies on a storm-by-storm basis.
#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.
Re #99:
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
Don
Don Keiller:
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.
Don
[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.
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.
An interesting article that covers that paper is available at http://news.mongabay.com/2006/0919-hurricanes.html , although their headline is in error, since this kind of study doesn’t address hurricane intensity. Furthermore, the article only quotes Chris Landsea on the issue – but there is no mention of the AMO hypothesis that Landsea et al put forward as the explanation for the increased activity:
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�� Planta 176, 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.
Hi Everyone:
Great discussion.
What really is ASTONISHING to me is that nobody seems to be talking about the biggest contributors to greenhouse cases, and thus global warming:
Cattle emissions contribute more to greenhouse gases and global warming than all the transportation in the world combined!
[Response: Even were that true, there are a lot more GHG emissions than just transport. But that page notable fails to cite its sources -W]
http://www.monkeytypesthebible.com/2007/02/cattle-emissions-contribute-more-to.html
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.
[edit]
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
Even if it were true, passing the buck to cattle for our contribution to GW is lame: the number of cattle not raised by humans approaches 0.
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.
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.
House of Representatives votes 269-150 to create a Select Committee exclusively to focus on climate change and energy independence:
http://www.speaker.gov/blog/?p=101
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.
The best I’ve found so far is:
http://earthobservatory.nasa.gov/Study/Iris/iris2.html
What else might provide a good clarification? Thank you.
Don’t know whether it’s “better”, but there’s Hartmann and Michelsen’s analysis here:
http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0477(2002)083%3C0249%3ANEFI%3E2.3.CO%3B2
and
http://www.atmos.washington.edu/~dennis/IRIS_BAMS.pdf
A new article in BAMS which you may be able to write a thread on:
“Mixing Politics and Science in Testing the Hypothesis
That Greenhouse Warming is Causing a Global Increase in Hurricane Intensity
A. T. J. DE LAAT
Royal Dutch Meteorological Institute (KNMI)
De Bilt, Utrecht, Netherlands”
http://ams.allenpress.com/archive/1520-0477/88/2/pdf/i1520-0477-88-2-251.pdf
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.
Thanks -