Thank you for that! I had cut out all slides on hurricans from my presentation on climate change after reading the Nature Geoscience paper, because my sense of not trusting the conclusion of the paper was not good enough to stand ground in front of a skeptic. I will now immediately put those slides back in.
I love your blog – it is absolutely essential for non-climatologists like me to stay informed and well equipped to help educate the public.
Knutson et al project future changes in Atlantic TC behavior by using a regional climate model (RCM) which produces tropical cyclones (though ones that are too weak–see discussion below) to ‘downscale’ climate change impacts. This is accomplished by driving the RCM with boundary conditions provided from the various 21st century model projections described in the IPCC 4th Assessment report (IPCC AR4).
Section 10.3.4 ( Changes in Atlantic Circulation) of the 4th IPCC report states that:
“In spite of a slowdown of the MOC in most models, there is still warming of surface temperatures around the North Atlantic Ocean and Europe due to the much larger radiative effects of the increase in greenhouse gases.”
The problem here is that estimates of changes in sea surface temperature and the depth of the warm mixed layer might be very unreliable, since the general behavior of the Atlantic circulation is only now being directly observed – and the most recent findings are that flow rates vary over a whole order of magnitude:
John Church, Science 2007, “A Change in Ocean Circulation”
The RAPID/MOCHA (Rapid Climate Change/ Meridional Overturning Circulation and Heat Flux Array) array was deployed in March 2004 to continuously monitor the meridional overturning circulation at 26°N. These observations are a component of a larger set of activities of the World Climate Research Programme’s CLIVAR Project to monitor the North Atlantic meridional overturning circulation. . .
Cunningham et al. report a year-long average meridional overturning circulation of 18.7 ± 5.6 Sv (3), but with large variability ranging from 4.4 to 35.3 Sv over the course of the year. This range includes all five meridional overturning circulation values estimated from the snapshots analyzed by Bryden et al.; thus, the apparent long-term decrease inferred by these authors may merely be a result of large intra-annual variability. . .
The problem is the limited sampling – imagine if the only avaliable measurements of the global jet streams were from airplane-borne monitoring systems. Trying to infer changes in atmospheric circulation from that data would be a very tricky business – but until know, that’s what oceanographers have been stuck with – and the system could use a lot of expansion.
Some odd things are also going on with relation to large areas of spreading ocean anoxia – with the two most likely culprits being ocean warming and/or nutrient loading (global industrial fertilizer production).
That matters because the trickiest part of global climate models appears to be how they handle ocean-atmosphere interactions, and I really have no idea how well they link changes in local wind-driven upwelling to the net thermohaline circulation. In particular, estimates of any future changes in El Nino frequency and intensity seem pretty speculative – and the frequency component might be essentially unpredictable.
Isn’t it possible to run these regional climate models with a range of boundary conditions? Rather than saying “The IPCC predictions are ironclad”, simply include them as one of the boundary condition sets (toward the low end). That might be a better way to compare the output of different RCMs.
Soundbite version: “Global warming is expected to increase sea surface temperatures, create a thicker and warmer ocean surface layer, and increase the moisture in the atmosphere over the oceans – all conditions that should lead to a general increase in hurricane intensity and maybe frequency.”
Is it just folklore that hurricanes (I think these are TC’s) occur due to warm ocean water that causes air to rise over a region, drawing in air that then develops into circular winds?
[Response: Well, that’s part of it, but not the full story. All other things being equal, warmer SSTs mean greater potential latent heat release, which ultimately is the energy source from which the storms derive their power. However, as Knutson et al and others have pointed out, warmer SSTs won’t necessarily be more conducive to more TCs. In a very simple sense, thermally-driven circulations are associated with horizontal gradients in heating. The rising motion tends to take place over the relatively warm surface regions, and the subsiding over the relatively cool surface regions. relatively is the operative word. For example, if SSTs are increasing in the main development region for Atlantic tropical storms, but increasing even more over other regions, then the main development region may not become more favorable for TC development. On the other hand, larger-scale atmospheric circulation changes impacting vertical wind shear (which can place a constraint on TC development) are also important, and this is really where we get to the heart of the current uncertainties, because of issues like how the Walker circulation might change, and what impacts that would have on all of these considerations. I hope that provides a bit of clarification. -mike]
#1 Maiken: cutting out your hurricane slides may not be necessary, but you must be careful not to give an impression that we have a pretty good idea hurricane strength will increase–that will leave you open to just criticism, like Al Gore for one.
My own approach is to show pictures of hospitals during the 2003 European killer heat wave and pictures of hurricane Katrina submergence and refugees, and to say that it is highly likely that there will be many more such scenes in the future (that is, scenes of mortality, flooding and environmental refugees), due to the probable increase in heat waves and flooding. Confidence in the latter is not related to TC intensity or frequency; it comes from the probable increase in heavy rain events and the inevitable rise in sea level that will make it easier for storm surges to go inland.
That is why I had left out the hurricane slide in the first place. There is so much very clear evidence of the devastating effects anthropogenic climate change will have on the natural environment and on us humans, that subjects of debate could just distract from the clear catastrophe we are heading towards.
But from this sentence…
“So we know that (i) the warming is likely in large part anthropogenic, and (ii) that the recent increases in TC frequency are related to that warming. It hardly seems a leap of faith to put two-and-two together and conclude that there is likely a relationship between anthropogenic warming and increased Atlantic TC activity.”
..I was under the impression that TC activity is indeed, at least to a large part, related to anthropogenetic warming.
Actually, what alternative explanation is there for the increase of TC activity but anthropogenic climate change? And shouldn’t the relative T-difference even increase in the future as shallower coastal waters heat up more quickly than deeper ocean water (except probably in upwelling areas)?
Mike, could you compare/contrast the results of the Wang et al paper (final draft version) that just appeared in G3? There appears to be a contradiction with Knutson et al:
“Atlantic Warm Pool Acting as a Link between Atlantic Multidecadal Oscillation and Atlantic Tropical Cyclone Activity”
“Multidecadal variability of Atlantic tropical cyclone activity is observed to relate to the Atlantic Multidecadal Oscillation (AMO) – a mode manifesting primarily in sea surface temperature (SST) in the high latitudes of the North Atlantic. In the low latitudes of the North Atlantic, a large body of warm water called the Atlantic Warm Pool (AWP) comprises the Gulf of Mexico, the Caribbean Sea, and the western tropical North Atlantic. AWP variability occurs on both interannual and multidecadal timescales as well as with a secular variation. The AWP multidecadal variability coincides with the signal of the AMO; that is, the warm (cool) phases of the AMO are characterized by repeated large (small) AWPs. Since the climate response to the North Atlantic SST anomalies is primarily forced at the low latitudes and the AWP is in the path of or a birthplace for Atlantic tropical cyclones, the influence of the AMO on Atlantic tropical cyclone activity may operate through the mechanism of the AWP-induced atmospheric changes. The AWP-induced changes related to tropical cyclones that we emphasize here include a dynamical parameter of tropospheric vertical wind shear and a thermodynamical parameter of convective instability. More specifically, an anomalously large (small) AWP reduces (enhances) the vertical wind shear in the hurricane main development region and increases (decreases) the moist static instability of the troposphere, both of which favor (disfavor) Atlantic tropical cyclone activity. This is the most plausible way in which the AMO relationship with Atlantic tropical cyclones can be understood.”
>(the model resolution is about 20 km, while theoretical estimates indicate that a resolution of about 1 km is likely required to simulate the inner core of intense TCs). TCs are very likely being artificially prevented from intensifying in a warmer climate in the Knutson et al study because of this.
Can you say what evidence justifies the second statement?
Comment by Steve Reynolds — 18 May 2008 @ 10:24 PM
…”The idea that climate change might actually decrease the frequency of tropical cyclones (TCs) is not an entirely new idea.’…
I can see where they are coming from if I keep in mind that there are several
schools of thought regarding quantum reality that physicists hold, and sub schools of thoughts.
Some believe there is no deep reality (Copenhagen interpretation), some do not, many worlds interpretation, quantum logic, neorealism, to name a few. Quantum theory schools of thought proponets range from Heisenberg to Schrodinger to Dirac to Von Neumann to Bohm to Bell to Feynman… Crowd waves (dilution), non cloud Frequency canceling, repetitious oscillations recycling in time and space, waves meeting in and out of phase, phase relations, periodicity, sum of amplititudes add, out of phase waves do not, superposition principle, and so on. Another words, the ocean can indeed be very calm if the next incoming wave arrives out of phase ( half a cycle or so), even if the preceding wave was two feet in amplititude due to two waves arriving in synch (on time). Waves adding or subtracting (subject to wave differences) is called interference (constructive, destructive). Wave’s intensity, (energy), amplitude squared.
Not that I could say whether I would agree or disagree with their findings, but I can understand where they are coming from.
The BBC, which quotes Held, emphasises the high resolution of the model and the high degree of confidence the authors have in the results:
Because there was a high degree of confidence that the sea surface temperature trend was going to continue to rise, Dr Held explained, people had “tried to conclude that hurricane activity will increase rather dramatically in the future”.
“We tried to simulate the fundamental fluid dynamics and thermodynamics that control hurricane genesis in the Atlantic in a numerical model to a very high resolution.”
Knutson acknowledges weaknesses in his computer model and said it primarily gives a coarse overview, not an accurate picture on individual storms and storm strength. He said the latest model doesn’t produce storms surpassing 112 mph.
… since the mainstream view, to the extent that such exists on the topic of hurricanes in the Atlantic, would seem to be that the number of hurricanes will not increase, but the intensity of the strongest storms will. It is this very point which the study is entirely unable to address.
Overall, there appears to have been a substantial 100-year trend leading to related increases of over 0.78C in SST and over 100% in tropical cyclone and hurricane numbers. It is concluded that the overall trend in SSTs, and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming.
They also make an important point: historical records of hurricane intensity are of no value prior to 1945 or so:
However, for the period prior to 1945, we consider that intensity estimates are too uncertain for definitive analysis, especially for differentiating between hurricane classes.
Holland and Webster’s discussion breaks the 20th century down into three regimes, as follows:
Between 1855 and 1900 cyclone numbers averaged 7–9 per year and the SST anomaly remained around -0.1 to -0.2C. This period was followed by a marked decrease of 30% to an average of six tropical cyclones per year from 1905 to 1930, which we designate climate regime TC1. Annual numbers then increased to a new stable regime averaging around 10 tropical cyclones per year from 1931 to 1994 (climate regime TC2). This increase was accompanied by a rise in SST of 0.2–0.3C and an upward shift of the annual variability to typically 6–14 named storms. Since 1995, the NATL has experienced a sustained upward trend in tropical cyclone numbers which may not as yet have settled into a stable regime. Accompanying this increase in tropical cyclone numbers is a marked SST warming to unprecedented anomaly levels exceeding +0.7C. We suggest that this trend may represent a transition to a third, and as yet undefined, climatic regime (TC3) in which cyclone frequency could stabilize at significantly higher numbers than characteristic of any previous period.
I don’t see what there is in Knutson’s work that challenges that in any meaningful way, so if there is an anthropogenic influence on sea surface temperatures, there also must be an anthropogenic influence on hurricanes.
In fact, the evidence for a good link between human activities, sea surface temperatures and hurricanes is a good deal more robust than the evidence for a periodic or quasiperiodic or chaotic cycle in the Atlantic Ocean circulation which is supposed to bring more warm water north – “a positive phase of the AMO”.
Would someone care to propose or explain a mechanism for the AMO, maybe something along the lines of the explanation for El Nino / Southern Oscillation? I’ve never heard a single coherent explanation, other than that’s what you get when you squeeze the data through a time series analysis program… which can easily be nothing but applying Fourier analysis to noise and coming up with a quasi-periodic result. A “40-80 year time period” – based on one century of data? That’s pretty bad as an explanation for the observed increase in hurricanes. As it is, recent studies show that the annual yearly variation in thermohaline flow in the Atlantic can vary across an order of magnitude – how does that fit into the AMO hypothesis (which is still widely featured in news reports)?
The AP/Seth Borenstein article has spread all over the place, including an abbreviated local news version (here). Interesting title: “Study says global warming not worsening hurricanes“, when the study only seems to suggest that the frequency of Atlantic hurricanes isn’t being heightened by warming, while intensity remains a different story.
It seems there is no consensus on the effect of any warming on TCs. No one can agree on how they form, what forms them, whether there will be more or fewer of them or if they will be weaker or stronger. A TC is an important natural phenomenon, a product of climate manifested as weather. Clearly the science isn’t yet well understood and the significant gap in GCMs reflect this. Of course this begs the question of what else is not well understood by GCMs or is it just TCs?
Unprecedented privilege by NOAA in doing a Press Release for the Knutson article is clearly at odds with what the NOAA director Conrad Lautenbacher said in urging creation of a National Climate Service in NOAA,- quotation from May 13, 2008 story:
… Today everybody just cherry-picks the data that support their point of view, Lautenbacher said of the debate over climate change. “We need to deal with this in a scientific manner.” …
#12 Excellent point Ike. I will take actual data over model generated any day. It is hard to explain the observed increase in the number of major hurricanes without invoking global warming. The concluding sentence on Knutsen’s webpage on hurrioanes and global warming is “An implication of the GFDL studies is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse-gas induced warming may lead to an increasing risk in the occurrence of highly destructive category-5 storms.” http://www.gfdl.noaa.gov/~tk/glob_warm_hurr.html
Re. no 14: The slightest hint of a genuine scientific dispute, and someone always interprets it to mean that the whole of AGW theory is riddled with uncertainties. But it just isn’t like that: it’s clear that there must be broad agreement on how TCs form and what forms them, because if there wasn’t, the Knutson paper could not have been written; it uses that agreement as its starting point. The ‘significant gap in GCMs’ is not because ‘clearly the science isn’t yet well understood'; it just corresponds to the fact that Global Climate Models are not Regional Climate Models. And even if there was significant uncertainty about the probability of global warming, that would be no cause for complacency, since it could mean that things were going to turn out worse than predicted.
Australia’s most respected climate scientists and australian of the year last year has sounded this dire warning today..here are some of his key points. 1/ there is now considerably more greenhouse gasses in the atmosphere to cause catastrophic and permanent climate change. 2/ cutting GG’s now, alone will NOT save the planet, we have to extract the CO2 out of the air en-masse, convert it to charcoal and plough it back into farmer’s feilds….but 3/ in the meantime we have to pump sulphur via aircraft exhausts into the upper atmosphere to cause global dimming (this is not the solution long term but it may buy us valuable time in the short term). This procedure is untested and unprecedented but may save most life on earth. This will change the color of the sky however. 4/ all wealthy farmers or individuals or corporations should pay poor farmers in the equatorial regions to plant trees, still having enough food crops to live by but plant O2 producing trees for income. He says that most if not all our natural feedback mechanisms are now failing and working against us and it is absolutely vital that we act decisively NOW. 5/ On a more national level he said that the Rudd gov should lift the means test for the $8000 solar panel rebate. He has based his speech today on the latest and most accurate climate data to date.
I could only wish the CC skeptics out there could read this..and maybe just maybe the penny would finally drop.
Comment by Lawrence Coleman — 19 May 2008 @ 7:47 AM
I wonder how long it will take the Bush administration to start shoving Chris Landsea in front of every available camera like in the past.
On a completely unrelated matter, are going to deal with that paper by Ferenc Miskolczi at some point? It’s being cited by climate change skeptics quite often and I would like to know how much merit it has.
[Response: None. – gavin]
[Response: The paper does have some value as a teaching tool in undergraduate physics or climate classes. It’s like those puzzles in kids’ books our Swedish friends call “Finn Fem Fel.” I ran the paper past a guest class I was teaching at Bowdoin and with a little encouragement the students were able to find and understand the two elementary mistakes Miskolczi made in the first 9 pages, which invalidate the rest of the paper. The students in that class are writing up their work, and it’s my intention that we’ll put it up on RealClimate as a guest post, once it passes muster with the rest of the RC crowd. I’m still waiting for the first draft. Meanwhile, you can try your own hand at finding the mistakes. The only hint I’m giving at this point, so as not to spoil the fun, is that you needn’t look beyond the first 9 pages. –raypierre]
Does anyone want to address the issues North American tornadoes? Was the early start of the tornado season this year, in any way attributable to AGW? Can we expect greater numbers of more intense tornadoes over a larger geographical range in the course of longer tornado seasons?
We appreciate your interest in our recent paper on Atlantic hurricane frequency and global warming. Our goal in this ongoing work is to move this discussion to firmer physical grounds by bringing new kinds of modeling, based more closely on the underlying fluid dynamics and thermodynamics, to bear on the question. Relying on statistical studies can take us only so far. Our regional modeling effort is far from perfect, as you and others have noted, but we feel that it is a significant advance to begin addressing the hurricane/global warming connection with these kinds of simulations. To get a feeling for the kind of model on which our paper is based, we encourage readers to look at the animations in the paper’s supplementary material (or see here .)
Our work is a “downscaling” study, in which we first simulate past hurricane seasons, using as input observed sea surface temperatures (SSTs), the observed state of the atmosphere at the boundaries of our Atlantic domain, as well as the largest scales in the atmospheric flow over the Atlantic. We then perturb this input with the change in the seasonal mean SSTs and the seasonal mean state of the atmosphere as projected by an ensemble mean of global models for the end of the 21st century. Several of your reservations about our work are related to possible inadequacies in this input from the global model projections. Our paper does not address this issue. We are interested, as you are, in improving these large-scale projections, but we feel that the ensemble mean of the CMIP3 models is a logical place to start in this kind of downscaling study.
We are in the process of downscaling individual global model projections, and preliminary results (as mentioned briefly in the paper) show substantial departures from the result using the ensemble mean. This is a slow process, as these simulations are at the limit of what we can do with the computer power available to us. But we see no indications, as yet, that any of these calculations will show an increase in Atlantic hurricane frequency remotely comparable to that obtained by extrapolating the recent trend. (As mentioned in the paper, when downscaling the GFDL CM2.1 model, rather than the ensemble mean, the number of hurricanes stays roughly unchanged by the end of the 21st century, and we see a substantial increase in the strongest model storms, those that exceed the surface pressure criterion for category 3. Of the models that we have examined to date, this is the one that shows the greatest tendency to enhance activity).
You seem to criticize us for putting observed statistical relations aside and trusting in our dynamical model. But the question is whether or not these statistical relationships have any predictive value. If one examines our model’s control simulations for the 1982-2006 period, which show a trend towards increasing hurricane activity over this period, and correlates this activity with SST in the Main Development Region, and then tries to use this correlation to predict the 21st century behavior of the model, it clearly doesn’t work. Our results are supportive of a very reasonable physical picture in which relationships between hurricane activity and spatial structure in SST are more relevant than those connecting activity to local SST only. We believe that storm frequency in our model is controlled by shears and by the gravitational stability of the vertical temperature profile (we are currently trying to determine the relative importance of these factors). Both of these factors, in turn, are primarily controlled by the spatial structure in the SST field.
Since our results represent, to our knowledge, the first calculation of this type, we do not know, and are anxious to learn, if different assumptions made in the downscaling result in substantially different results concerning changes in storm frequency, or if most of the uncertainty arises from the global model input.
Turning to very important question of the frequency of the strongest storms, it is entirely possible that a large increase in category 4-5 storms will result from increasing greenhouse gases, despite an overall reduction in hurricane numbers. Our model adds little new information on this question because of its failure to simulate these very strong storms.
[Response: Thanks for your comments. The way I look at it is whether the CMIP3 runs – which I too consider to be the best data set for future projections – are capable of providing regional details with sufficient accuracy and fidelity so that they can be used to make projections for TCs. There is a fundamental limitation to RCMs, as they are constrained by the coarse resolution boundary conditions provided by the GCMs (which tend to favour a more El Nino like state on average). These GCMs were – as far as I know – never designed to study TCs, but are suitable for studying the larger picture of our climate. RCMs provide more details, but don’t the ‘question’ the environment on which TCs depend. In Scandinavia, new efforts with RCMs have started, where RCMs are now coupled to regional ocean and sea-ice models, as there are air-sea coupled processes that become important for these small-scale processes and uncoupled models are not sufficiently realistic (otherwise, people wouldn’t bother). Likewise, cyclones play a role in the global heat, moisture and momentum budgets, and my concern is that the GCMs still have some problems capturing all these aspects in a realistic manner through simple parameterization schemes (they tend to have regional biases) even though they provide a realistic over-all description. Such details may or may not be important depending on your scientific question, but when the outcome depends on a fine balance of competing ‘forces’, then we really have to be careful. I would furthermore like to argue that one always should back up RCM studies with statistical downscaling which at least provides an independent assessment (although it may suffer from different caveats). Additionally, one should look at the 20C3M runs to see if they capture the trends of the past. -rasmus]
[Response: Guys, thanks for your comment. The study is an important one and points us in the right direction in terms of approach. I just think that the confidence in the conclusions has in places been overstated. There will no doubt be other similar analyses in the months ahead using different approaches, and it will be interesting to compare the results and conclusions. -mike]
Aaron, Google: tornado shear helicity
will help; lots of recent info, not enough data collected over enough years to tell if wind shear/helicity numbers are changing overall. My speculation — wind shear defusing hurricanes but pumping up tornados — didn’t seem apt, the size/scale is so different between the two kinds of storms that wind shear means something different for each type.
Do any GCMs have variable grid sizes? I wonder because in some types of modelling one puts more grid points in areas where more interesting (or changable) things are happening. I could imagine (at a push) that say the modelling of the climate over a vast flat area of ocean might be achievable in a few less grid points than over say the Himalayas or Rockies. If one divides the earth into 50x50km blocks then I suppose that’s about 200k grid points (times however many layers you want in the atmosphere) and as for the oceans I have no idea. Did anyone try using different distributions of those 200k grid points, with more points in interesting places (I don’t know, erm, say places where hurricanes generally form)? Or do any of the models out there dynamically change the grid size, keeping the total number of points the same, but putting more grid points in at places where gradients in variables are largest? If you would expect better results from 1km grid, which is just not possible computationally at the moment, why not try other ways on tipping the balance in favour of even better results. I suspect however, that this has been tried, and the answer I’ll get will be something like: tried it, but the gains are marginal…
Back to #4’s response…. just me a non-science layperson trying to understand this; water vapour rising over warm water… condensing over cooler water…. but as the oceans are warming, there is less relative difference between the warm and cool, so less frequent release, meaning fewer storms, but of greater intensity. ?
Re: 26. It’s unlikely the early start to the tornado season was in any way attributable to AGW. It’s likely that the January-February tornado count will be the highest on record (adjusting for changes in reporting practices), but it’s not clear that it’s qualitatively different from the 1971 and 1999 starts. It’s also important to note that 2002-2004 are three of the six slowest starts to the season and 2001 and 2005 were also well below normal.
There’s really no evidence to support an expectation of greater numbers of more intense tornadoes over a larger geographical range in the course of longer tornado seasons. If we restrict attention to F1 and stronger tornadoes, which are the strongest 1/3 of current reports and seem to have been relatively consistently reported, the biggest era in the US was the early 1970s, when the US average temperature was at its lowest since ~1920. The correlation of F1 tornadoes and annual temperature in the US since 1954 is -0.1, but that’s essentially zero. There’s also no particularly evidence to suggest that the temporal pattern of the tornado season has changed in any appreciable way. There is a lot of interannual variability. If we break the first two months of the year down into the most frequent and least frequent tornado years since 1950 (the official database goes back then, although it didn’t get really started until 1954), and count how many years in each decade are in the “big” and “little” starts to the year, we get
(There are more in the big years because of a tie for 19th fastest start.) Other than the fact that the late 1970s through 1980s were slow starters (9 of the 12 years from 1977-1988 make the little list), there’s not much to say about the list.
The climate model studies (three papers in the last year) have focused primarily on conditions associated with large hail, suggesting that environments supportive of the largest hail will become more frequent. There’s also some support for historical increases in large hail frequency in the US
“Back to #4’s response… water vapour rising over warm water… condensing over cooler water…. but as the oceans are warming, there is less relative difference between the warm and cool, so less frequent release, meaning fewer storms, but of greater intensity?”
Water vapour does not condense over cooler water. It cools when it has been raised to a colder altitude. This is obvious in the case of a hurricane where the clouds form above the warm water which is generating the hurricane, (not over the trail of cold water left behind the hurricane.
But warm air can be lifted in two ways; natural convection and forced convection. Natural convection is due to the low density of the air caused by the lighter water vapour. One type of forced convection is when the cold air from the poles flows under the warmer subtropical air, which results in wind shear.
This would imply, that in a warmer world where the Arctic ice had vanished, there would be less wind shear so allowing more hurricanes to form in the tropical North Atlantic.
I can’t imagine that Knutson included the disappearing Artic ice in his model :-(
No time to read all this, but here are a few of my ideas:
1. That would be wonderful if at least Atlantic TC reduce or do not increase with GW, since GW is and will be doing so much greater harm thru droughts, floods, disappearing glaciers, disease spread, ocean anoxia (with HS outgassing likely to follow), species loss, heat deaths,…am I leaving anything out? TCs are only a small portion of the tremendous harm GW is and will be causing, perhaps for many millennia (brought to you future peoples by our generation). So that’d be great if Atlantic TCs decrease. We can put that along side less deaths due to a decrease in snow shoveling. The net harm from GW, however, is and will be vast.
2. As pointed out in the post, increasing sea surface temps due to GW is a necessary, if not a sufficient cause for increased TC intensity and frequency. And I’d think predicting all those nitty-gritty sufficient causal variables would be harder to do than predicting global warming. Why they can’t even predict rain accurately a few days in advance. So if skeptics are attacking GW science, finding all sorts of (mostly bogus) problems with it, they ought to be able to rip TC causal variable prediction (minus the SST variable) to shreds.
3. I’m still buying insurance for my house near the Gulf of Mexico; and I’m still reducing my GHGs.
4. RE #31 & increasing hail. During Hurricane Emiliy in 2005 there was a serious hail storm in Brownsville, TX, breaking windows, etc. Our weatherman said he had never heard of hail during a hurricane. When I asked here at RC about it, someone who seemed to know what he was talking about said that indeed hail was not expected during hurricanes. So this might be some avenue to research — possible hail storms during hurricanes (even if perchance they do reduce with GW).
Okay Alistair, So what Knutson seems to be saying is that it’s kind of a zero-sum game. He is saying his modelling predicts fewer hurricanes, but of greater intensity. So, as to my understanding then, it’s not the cold water, but the cooler atmosphere, and as the atmosphere is warmer, then… there will be less relative difference and fewer occurances…. ?
To those of you interested in seeing the actual 43-page, peer-reviewed article and its citations, over and above the curt dismissals of gavin, “raypierre”, and Nick Stokes, here is the link to the work itself:
“Meanwhile, you can try your own hand at finding the mistakes. The only hint I’m giving at this point, so as not to spoil the fun, is that you needn’t look beyond the first 9 pages.”
Okay, I tried reading the paper. Are the mistakes related to the use of virial theorem and Kirchhoff’s law?
[Response: Got it in one! There are probably other mistakes in there as well, but these will do. As a further exercise, you can ask whether the virial theorem, correctly applied, gives you any further information about an atmosphere in hydrostatic balance. –raypierre]
As you climb up a mountain the temperature drops by 1F for every 300 feet of altitude you ascend. Eventually, if the mountain is high enough you will reach the snow line.
It is the same with rising air. It cools, and if it get high enough any water vapour it contains that has not already turned into cloud, condenses into ice clouds. Global warming will not stop the water vapour condensing. It may make it condense at a higher altitude, but since the air at the surface will be warmer and hold more water vapour the condensation level may remain at the same height.
Knutson et al. are saying that “Using projected boundary conditions for the end of the twenty-first century, the frequency of Atlantic tropical cyclones and hurricanes in a regional climate model of the Atlantic basin is reduced compared with observed boundary conditions at the end of the twentieth century. This is inconsistent with the idea that higher levels of atmospheric greenhouse gases will result in increased Atlantic hurricane activity.”
In other words, global warming will lead to less North Atlantic hurricanes, not more as had been generally expected because of the rise in sea surface temperatures.
What I am saying is that they used a regional climate model which did not include the changes in the Arctic region, and the boundary conditions they used were those predicted using GCMs that we know got the Arctic ice wrong! Their results are useless, but no scientist would dare to point that out. It is like saying the king is wearing no clothes :-)
where Ossi wrote “Okay, I tried reading the paper. Are the mistakes related to the use of virial theorem and Kirchhoff’s law?”
You seem to have found that problem pretty easy, although I have spotted another error. Perhaps you would like another challenge. Can you find where Miskolczi has got it right? No need to restrict yourself to the first 9 pages. But perhaps I should admit that I can’t answer that problem :-)
I am trying to understand in lay terms, why Knutson et al feel there would be fewer low intensity hurricanes. Alastair points out that the model they use doesn’t deal with known variables, has initial presumptions that have been shown to be incorrect and that their sample is essentially too small.
Okay, these criticisms I can understand. But it doesn’t explain plainly why they got the results they got.
I appreciate that you guys are all quite the scientists and that kind of hard criticism needs to be leveled at these studies, no matter what. Part of the problem is with news media summing up stuff glibly, and uneducated critics trumpeting those ‘conclusions’ as some kind of proof.
So how are they modelling these TCs? Same as everybody else? Why would they choose to use faulty sources?
I feel that there is a plausible expectation that more latent heat as a result of AGW would lead to more tornados of increasing intensity. Moreover, HB points out a shift in seasonality. In an example that he does not use, there were tornadoes in Wisconsin this last January, and January tornadoes had not been seen in Wisconsin since 1844. It is worth remembering that in the “old days” farms were smaller, so there were more farmsteads per section, and every farmstead had to keep an eye on the weather. My point is that we are seeing weather that has not been seen for a long time.
We have an observed trend in the number of tornadoes supported by an expectation of more tornadoes as the result of more available latent heat to drive them. Why is everyone so reluctant to say, “Global Warming” when we have an exceptionally fine swarm of tornadoes?
I don’t deny the possibility of an increase in tornado frequency or severity, but I haven’t yet seen hard evidence of it. Improvements in tornado detection are undeniable; the increase in population alone has an impact. The record of “wide” tornadoes (the measure I used to distinguish more powerful ones) shows a distinct change in 1995 but no other sign of any change. And since an active researcher in that field commented that the definition of, and measuring procedures for, width data were changed in 1994 (first affecting 1995 data), it’s also undeniable that the change in that year is at least in part (and possibly, entirely) due to the changed data procedures.
I repeat, it’s by no means impossible that tornado frequency or severity has changed — but the data I’ve seen don’t establish it reliably.
We also have an expectation of fewer tornadoes with decreasing vertical wind shear. Since shear is more important for tornadoes, given that you’ve got some CAPE, it’s not at all clear what impact global warming will be on tornado occurrence. It’s just as reasonable from physical principles to expect a decrease as to expect an increase.
Actually, the last January Wisconsin tornado was an F2 or F3 in 1967 in Green and Rock counties.
There’s no trend in the number of F1+ tornadoes, going back to 1954. The increase is entirely in F0 tornadoes, which we have good reason to believe are better observed now than in the past. Personally, my years of researching the climate and severe thunderstorm connection give me no evidence to predict a change in frequency or intensity of tornadoes. (I think the support for an increase in very large hail in the US is stronger, from theoretical and modelling perspectives, as well as a hint in the observational database. There’s also some support for the notion of a decrease in small hail associated with a raising of the freezing level height, based on observations from China.)
I think one of the reasons why many people don’t jump on tornado outbreaks as associated with global warming is that it’s hard to say why this year is particularly different than 1999 or 1998 or 1971, which had big starts to the year and continued big through most of the year. You also have to explain the very slow starts to the tornado season from 2001-2005.
I thought the key issue with TC impacts is increasing storm power (power dissipation index), not increasing storm frequency. It seems to me that the popular press frequently does not make this distinction. At the end of their comment #27 the authors state “Turning to very important question of the frequency of the strongest storms, it is entirely possible that a large increase in category 4-5 storms will result from increasing greenhouse gases, despite an overall reduction in hurricane numbers. Our model adds little new information on this question because of its failure to simulate these very strong storms.”
So isn’t not fair to state that we see increase in TC power in recent decades, and that this is expected giving rising SST, which are driven by greenhouse gas forcing. The new paper does not suggest a decline in storm power, and even if power does not continue to grow at the current rate we can still expect a future with more powerful and damaging TCs.
Re: #32 “Natural[free]convection is due to the low density of the air caused by the lighter water vapour.”
Nonsense! The “low density” of which you speak is a result of the release of latent heat that occurs when water vapor condenses to form clouds. (See any introductory meteorology textbook!)
You are right that when the water vapour condenses the released latent heat causes further convection, especially in hurricanes.
But in order for the air to reach the cloud base where condensation begins, the air has to be lifted from the sea surface (or an evapotranspiring land surface.) That happens because the density of the air is “diluted” by the addition of water vapour with a molecular weight of only 18 compared to air with a molecular weight of 29.
It seems a little odd to validate the models only at a seasonal macro
scale after adjusting small scale parameters since that would
aggregate the effect of parameterizations across large scale pattern
changes within each season and make it impossible to determine cause
and effect. It would seem better to validate the models using
individual historical storms by replicating the actual historical
conditions in the model.
Can the small scale parameterizations also be validated (for example
in smaller scale models)? For example, the predicted change in PDI
would seem to be dependent on the rapid intensification process, which
you imply is not modeled.
One parameter for intensification is obviously SST which could dictate
a percentage of storms of each intensity. Or more realistically, the
time to rapid intensification could be based on SST. But SST is not
the cause of intensification, only supportive of it. The other
factors, storm dynamics, wind shear, vertical temperature profile,
humidity, etc, can be added to the parameterization, but should be
subject to small scale validation (i.e. weather forecasting).
Once intensified, the SST would help sustain storm intensity, again
with paramterizations that include the other factors subject to
Seasonality questions: does negative NAO always go away in the summer
in the models? What makes it go away and come back in the fall?
[Response:The NAO is really just a measure of the dominant mode of variability of the Northern Hemisphere jet stream over the North Atlantic and neighboring regions. The jet stream is strongest (and most equatorward) during the boreal winter, and weakest (and most poleward) during the boreal summer. For these reasons, the NAO has the greatest opportunity to impact tropical cyclone behavior in the late fall, when its reach is beginning to extend into the northern tropics, its impact is beginning to become substantial, and the TC season is still in full gear. A similar argument holds for El Nino influences. – mike]
Hurricanes and tropical storms will become less frequent by the end of the century as a result of climate change, US researchers have suggested.
But the scientists added their data also showed that there would be a “modest increase” in the intensity of these extreme weather events
and what the authors say above
Turning to very important question of the frequency of the strongest storms, it is entirely possible that a large increase in category 4-5 storms will result from increasing greenhouse gases, despite an overall reduction in hurricane numbers. Our model adds little new information on this question because of its failure to simulate these very strong storms.
TC deaths are mostly due to storm surge and inland flooding. These are positively correlated to storm size and intensity. Wind shear is important, but so are sst and the depth to which warm water extends. The warm loop current greatly intensified both Katrina and Rita (NWS statements).
As SST’s are expected to increase by most GCM’s under AGW then: increasing SST will lengthen the TC season, increase the area northward (and southward if southern Atlantic TC become something more than freaks) in which TC’s can occur, and increase the area northward in which intense and large TC’s can occur.
I also believe that increasing SST’s will increase inland flooding from heavy rainfalls whether or not they are associated with TC’s and more importantly, increase the area northward in which intense rainfalls occur including into areas where the infrastructure can not handle such rains.
Super intense rainfalls seem to occur where tropical air masses moving eastward collide with the westerlies and stagnate. I don’t know that these occur much north of where I now live which is in Houston. We (Alvin, TX) hold (arguably) the world record for intense rainfall events (38″ in 24 hours). This was almost matched by TS Allison in 2001 (32″ in 24 hours) and by a non TC rain in October of 1994 (30+”). Likewise, similar events have occurred in other parts of Texas (San Antonio). Massive property damage and many deaths result. The common driver is tropical air masses colliding with the westerlies often in late spring or early fall as the easterlies are either coming or going.
I think that the Katrina argument is leading TC researchers to go off on a somewhat unproductive tangent and may well be arguing about how many angels can fit on the head of a pin.
If Houston’s climate is matched by Washington D.C. in 2080 (NASA), then will D.C. begin recieving regular 30-40″ 24 hour rainfall events in the future? What about mountainous West Virginia?
Perhaps projections about the movement of the extent of the areas subject to the seasonal easterlies and tropical air masses would provide more answers than whether we will lose more property in 2.5 cat 2’s or 1.8 cat 2’s plus an additional 0.3 cat 4’s? My guess is that this question would be more easily and robustly answered by GCM’s and coupled RCM’s.
“We (Alvin, TX) hold (arguably) the world record for intense rainfall events (38″ in 24 hours). This was almost matched by TS Allison in 2001 (32″ in 24 hours) and by a non TC rain in October of 1994 (30+”). Likewise, similar events have occurred in other parts of Texas (San Antonio). Massive property damage and many deaths result. The common driver is tropical air masses colliding with the westerlies often in late spring or early fall as the easterlies are either coming or going.”
Well, not quite the record. The Banqiao Dam wiki at en.wikipedia.org/wiki/Banqiao_Dam reports that the failure of Banqiao Dam in Henan province in China in 1975 was the result of several days of rain averaging 1060 mm per day, that rain the result of the collision of a tropical air mass, Super Typhoon Nina, and a (westerly) cold front over Henan province. In all, there were cascading failures of Banqiao and 61 other dams in Henan. The flooding caused 26,000 deaths; subsequent epidemic and famine caused another 145,000. The disaster was a state secret for twenty years until Human Rights Watch Asia blew the whistle. The Peoples Republic finally declassified their records in 2005.
This was probably history’s worst technologically- exacerbated disaster, although that is a pretty vaguely defined category.
RE:29. Ok, I read up a bit on this myself. According to Ar4 most models use a rigid grid system, with some using more grid points in equatorial regions for the ocean parts of the GCMs. It wasn’t particularly clear to me what spectral advection meant in terms of the resolution of the atmosphere, and what hybrid vertcial coordinate meant in terms of the ocean. But, if gives me the feeling that there is more to gain in tuning of the vertical resolution than the horizontal.
On the subject of tropical cyclones and rainfall, the 72-hour record was set last year on the South Indian island of Reunion when Cyclone Gamede slowly moved offshore of the island. In three days, 12.9 feet (3.93 meters) of rain fell. Over 5 days, 16.3 feet (4.98 meters) was recorded.
Re: Mike Mann’s reply to Comment #50:
Statement (i), that "the warming [of the tropical Atlantic Ocean] is likely in large part anthropogenic." is reasonable, taking “anthropogenic” to mean “greenhouse gas”, given the work of Santer et al (2006, PNAS), Knutson et al (2006, J. Clim.), and Gillett et al (2008, G.R.L.). To quote from Gillett et al:
…our results indicate that greenhouse gas increases are indeed likely the dominant cause of [tropical Atlantic] warming…
However, statement (ii), that "the recent increases in [Atlantic] TC (tropical cyclone) frequency are related to that warming" is vague – with "related to" allowing an interpretation that includes anything from a negative relationship, to a minor contribution, to local SST warming being the dominant dynamical control on TC frequency increase. Some might interpret "related to" to mean "are dominantly controlled by", and we think the evidence does not justify such a strong statement. In particular, the results of Knutson et al (2008) do not support such an attribution statement,if one focuses on the greenhouse gas part of the anthropogenic signal. Quoting from page 5 of the paper:
Our results using the ensemble-mean global model projections (Fig. 4) are inconsistent [emphasis added] with the notion of large, upward trends in tropical storm and hurricane frequency over the twentieth century, driven by greenhouse warming
We agree that TC activity and local Atlantic SSTs are correlated but do not view this correlation as implying causation. The alternative, consistent with our results, is that there is a causal nonlocal relationship between Atlantic TC activity and the tropical SST field. The simplest version uses the difference between Atlantic and Tropical-mean SST changes as the predictor (Swanson 2008, Non-locality of Atlantic tropical cyclone intensities, G-cubed, 9, Q04V01). This picture is also consistent with non-local control on wind shear (e.g. Latif et al 2007, G.RL.), atmospheric stability (e.g., Shen et al 2000, J. Clim.) and maximum potential intensity (e.g., Vecchi and Soden, 2007, Nature).
We view the SST change in the tropical Atlantic relative to the rest of the tropics as the key to these questions. Warming in recent decades has been particularly prominent in the northern tropical Atlantic, but such a pattern is not evident in the consensus of simulations of the response to increasing greenhouse gases. So, whether changes in Atlantic SST relative to the rest of the tropics – that according to our hypothesis have resulted in the changes in hurricane activity – were primarily caused by changes in radiative forcing, or whether they were primarily caused by internal climate variability, or (most likely) whether both were involved, is obviously an important issue, but this is not addressed by our paper.
Scientists confirm computer model predictions that oxygen-depleted zones in tropical oceans are expanding, possibly because of climate change
Scripps Institution of Oceanography/UC San Diego
MAY 1, 2008, 11 A.M. U.S. PDT
An international team of physical oceanographers including a researcher from Scripps Institution of Oceanography at UC San Diego has discovered that oxygen-poor regions of tropical oceans are expanding as the oceans warm, limiting the areas in which predatory fishes and other marine organisms can live or enter in search of food.
The new study is led by Lothar Stramma from the Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel, Germany, and is co-authored by Janet Sprintall, a physical oceanographer at Scripps Oceanography and others. The researchers found through analysis of a database of ocean oxygen measurements that levels in tropical oceans at a depth of 300 to 700 meters (985 to 2,300 feet) have declined during the past 50 years. The ecological impacts of this increase could have substantial biological and economical consequences.
“We found the largest reduction in a depth of 300 to 700 meters (985 to 2,300 feet) in the tropical northeast Atlantic, whereas the changes in the eastern Indian Ocean were much less pronounced,” said Stramma. “Whether or not these observed changes in oxygen can be attributed to global warming alone is still unresolved. The reduction in oxygen may also be caused by natural processes on shorter time scales.”
“I think it’s uncharted territory,” said Levin, who was not affiliated with the study. “Thicker oxygen minimum zones could affect nutrient cycling, predator-prey relationships and plankton migrations. Where the expanding oxygen-minimum zones impinge on continental margins, we could see huge ecosystem changes.”
RE: 22 and 24. I get a very strong impression of “fiddling while Rome burns”.
We should all be amazed that Tim Flannery’s shock statement has received so little media attention, or at best ridicule on his point 3 about global dimming and sky colour (whereas points 1 and 2 are much, much more important).
However, this roughly matches the media response to Friends of the Earth’s Climate Code Red report in February (zilch) or to James Hansen’s recent papers on 350ppm being the new 450ppm (very nearly zilch, and with no real understandimg of the implications).
The public faces of AGW are finally saying we have a really massive problem on our hands, and we need to do something *now*. Meaningless targets to reduce CO2 emissions by 60% by 2050 or whatever aren’t the solution anymore.
But it seems the message is too radical for the mainstream to take seriously yet. So instead we engage in displacement behaviour: suggesting bets on whether one decade will be colder or warmer than another, or quibbling whether there will be more or fewer hurricanes by 2050 or whatever. Maybe the first pictures of an ice-free Arctic will finally jolt people’s attention – the “ozone hole” moment. Don’t hold your breath though.
Re #59, its even worse than that to be fair. Governments are in the pcokets of the very powerful lobbyists who have millions of pounds to spend. A very good story has arisen recently regarding so called “clean coal” or CCS to give it its proper name. Apparantly the IEA and the coal industry founded the CSLF as detailed in this article, http://www.greenleft.org.au/2008/751/38813 which says it all. In order to get the power plants built now they have decided to get the G8 to agree to fund CCS without it being ready or even proven on the scale required.
If this is all true (always a question mark) but further rooting around seems to deem that it is true makes me worry for if the USA builds these 150 new power plants without CCS fitted presently then I doubt it will be fitted even if the power plants are “Carbon capture” ready. The G8 meeting takes place this July but nothing much will be said of it I guess.
I think the assumption of present day variability is the most telling flaw in this paper. The lack of a good estimate of the future variability is a good excuse – but I didn’t see Knutson mentioning it at all.
I saw Knutson talking about the present day simulations last year – those results are pretty impressive.