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Climate Change and Tropical Cyclones (Yet Again)

By Rasmus Benestad & Michael Mann
Hurricane Katerina
Just as Typhoon Nargis has reminded us of the destructive power of tropical cyclones (with its horrible death toll in Burma–around 100,000 according to the UN), a new paper by Knutson et al in the latest issue of the journal Nature Geosciences purports to project a reduction in Atlantic hurricane activity (principally the ‘frequency’ but also integrated measures of powerfulness).

The close timing of the Knutson et al and Typhoon Nargis is of course coincidental. But the study has been accorded the unprecedented privilege (that is, for a climate change article published during the past 7 years) of a NOAA press conference. What’s the difference this time? Well, for one thing, the title of the paper: “Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions” (emphasis added).

The idea that climate change might actually decrease the frequency of tropical cyclones (TCs) is not an entirely new idea. Indeed, similar conclusions have been reached in earlier work using climate model projections (e.g. Yoshimura et al., 2006, J. Meteorol. Soc. Japan; Bengtsson et al., 2006 J. Clim.; Chauvin et al., 2006 Clim. Dyn.). So what are the key developments in this latest work?

Some background

Before we tackle that question, it is helpful provide a bit more background on the problem. First, it needs to be recognized that GCMs are too coarse to provide a realistic description of ‘small-scale’ (mesoscale) features such as TCs. The atmospheric components of climate models were never really designed for the study of TCs, but the fact that they can produce features with TC-like character when run at sufficiently high resolutions, gives us increased confidence in the possibility that climate models can be used to analyze climate change impacts on TCs. In order to get a more realistic description of the TCs in coarsely resolved climate models, one needs howeveer to ‘downscale‘ the model results.

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).

Contrasting two recent studies

In certain respects, this new paper is closely related to a paper published last month by Emanuel et al in the Bulletin of the American Meteorology Society (‘BAMS’ to those in the know) which received some press of its own (some of it quite distorted). Emanuel et al . also use a downscaling approach applied to more-or-less the very same climate model simulations. And both studies project a decrease in the frequency of Atlantic tropical cyclones (though see caveats below). But here is where the similarities end.

Emanuel et al use a very different downscaling approach. The use a ‘seeding’ method to randomly generate small vortices analogous to ‘short wave’ tropical disturbances in the real world (the tracks they take are defined in terms of the background atmospheric circulation of the model combined with the so-called ‘self advection’ of the TC itself). They define the probability of development of these vorticies into TCs through a ‘genesis’ model that conditions the favorability of development on various characteristics of the background climate state, and they use a theoretical model to predict TC intensities.

The differences between the conclusions of the two studies are significant. Using essentially the same IPCC model projections, the two studies come to very different conclusions with regard to key projected quantities, such as the seasonally-integrated powerfulness of TCs or ‘power dissipation index’ (PDI). While the Emanuel et al study predicts a clear increase in PDI, the Knutson et al study does not. So which is right?

Well, Knutson et al fully acknowledge that their RCM still has too low resolution to produce realistic TCs (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. By contrast, Emanuel et al‘s approach does not suffer from such resolution limitations.

To their credit, Knutson et al openly acknowledge this weakness in their treatment of TC intensity and PDI. What about their conclusions about a projected decrease in Atlantic TC frequency, which are, after all, the central point of the paper? Here we have reservations as well (and if we were the betting kind might even put forward a wager with regard to future trends). In part, these reservations are a result of the very same issues. The limitations of the RCM for example, as the authors note, also lead to incorrect seasonal and geographic distributions of TC genesis.

Small-scale processes

In the real world, small-scale phenomena such as convection, clouds, gravity waves, and various sorts of eddies may influence mesoscale systems such as TCs, and these unresolved small-scale phenomena are represented through often somewhat simplistic statistical ‘parameterizations’. This limitation is of course common to essentially all atmospheric models, and in and of itself is no reason to dismiss the conclusions of the study.

But TCs do also play a role in terms of the larger scales, as they facilitate transport and a redistribution of heat, moisture and momentum (‘upscaling’). This action is simulated more explicitly in RCMs and more implicitly in GCMs by their parameterization schemes, but it is still not really known if these two levels of modelling provide a physically consistent picture of the scale interactions. The RCM solutions, however, are constrained by the results generated by the GCMs and thus depend on how well the parameterization schemes capture this upscaling effect. Yoshimura et al., 2006 have shown that the solutions may be sensitive to the choice parameterization schemes: they found an increase in TC number over the Indian Ocean if the model used the Kuo cumulus parameterization but a decrease if the Arkawa-Schubert cumulus parameterization scheme was used.


Also significant, perhaps, is the seasonality issue touched on above. The Knutson et al study involves an assumption of a fixed August-October TC season. Yet one important impact of large-scale climate factors which influence Atlantic TC frequency such as ENSO and the NAO (see e.g. one of our own papers on this topic) is their influence on activity during the latter part of the Atlantic hurricane season (one might argue for example that the primary reason for the low TC count during the 2006 season was the early ‘shut-down’ of the season due to increasingly strong El Nino-related wind shear in the autumn as the El Nino set in).

The role of ENSO

ENSO itself, and how it’s influences are represented in the analysis, is potentially an even more fundamental issue. It is well known (and openly acknowledged in both the Emanuel et al and Knutson et al studies) that tropical Atlantic TC frequency is heavily influenced by ENSO variability. This is primarily through its influence on vertical wind shear in the Caribbean and tropical Atlantic, which in turn determines how favorable of an environment incipient TCs encounter as they form and intensify. We have discussed this here in detail before.

Given that ENSO is the dominant source of variability on interannual timescales, it is likely that future changes in ENSO (more specifically, the mean state of the climate and whether it is more “El Nino” or “La Nina” like, i.e. is there a strengthened or weakened ‘Walker Circulation’) could have a profound influence on Atlantic TC frequency. Although the IPCC models project overall a more El Nino like mean state with a weakened Walker circulation, there is far from a consensus among the models. Several credible state-of-the-art coupled models project precisely the opposite. And all of the models used in the IPCC assessment suffer to a varying extent from certain fundamental biases (the inability to produce a realistic ‘ITCZ’ over a large part of the equatorial Pacific ocean–the so called ‘split ITCZ problem’).

The CMIP3 model projections are essentially evenly split as to whether they project an increase or decrease in the magnitude of individual El Nino and La Nina events. Yet the frequency of large El Ninos and large La Ninas means everything in terms of the likelihood of very active Atlantic tropical storm seasons. If all of this sounds familiar to you, its because we made essentially the same point about a year ago in response to a paper that was more or less making the same argument as Knutson et al, though not quite as fleshed out.


The fact that the RCM-based downscaling approach can reproduce the observed changes when fed modern reanalysis data is used by Knutson et al as a ‘validation’ of the modeling approach (in a very rough sense of the word–there is in fact a non-trivial 40% discrepancy in the modeled and observed trends in TC frequency). But this does not indicate that the downscaled GCM projections will provide a realistic description of future TCs in combination with a multi-model GCM ensemble mean. It only tells us that the RCM can potentially provide a realistic description of TC behavior provided the correct input.

Indeed, other purely statistical approaches using large-scale climate predictors of Atlantic TC activity, and which seem to imply different relationships between projected climate change and future Atlantic TC activity (more on this in the future!), also pass similar validation tests with flying colors. So validation against the modern record alone (be it with a dynamical or statistical model) cannot demonstrate the reliability of the future projections. It can only indicate the self-consistency of the analysis.

Future research

Scientifically, where do we go from here? How do we achieve greater clarity on these issues? Obviously, there is need for significant increases in resolution of the RCMs, as past studies indicate a significant sensitivity of results to model resolution. The arguably required, aforementioned 1 km resolution may not be practically achievable in the near term, but the community must strive to move in that direction, particularly if projections of future changes in TC strength, intensity, and power dissipation are to be useful.

Better yet would be to run the coupled ocean-atmosphere models themselves at very high resolution (e.g. 10 km or even finer). This could in principle eliminate many of the thorny issues discussed above, including the potential artifacts of using embedded models with one-way only coupling. But this may be wishful thinking, at least for the foreseeable future.

Final Thoughts

Of most fundamental significance to assessing the reliability of these current projections, in our view, is the “junk in/junk out” factor. The detailed projections made using either the RCM approach of Knutson et al or the ‘random seeding’ approach of Emanuel et al, can only be as good as the large-scale scenarios used to drive them. And since key aspects of those large-scale scenarios as far as Atlantic TC activity is concerned (i.e. what really happens to the ENSO mean state and amplitude of variability) are currently not confidently known, neither can we be confident using the model projections to say what will happen to Atlantic TC activity in the future.

In this respect, we have to consider the entirety of currently available evidence that can inform our assessment of climate change impacts on Atlantic TCs. We know, for example, from the work of Santer et al. that the warming trend in the tropical Atlantic cannot be explained without anthropogenic impacts on the climate. Knutson et al. do not contest this. Furthermore, they do not dispute that the late 20th century increase in Atlantic TC frequency is tied to large-scale SST trends (though they argue that the influence may be non-local rather than local). 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.

What Knutson et al are asking us to do in essence is to put all that aside (because, they argue–in short–that its not the warming but the pattern of warming that matters here) and instead take on faith the perhaps not-much-more-than 50/50 proposition that the mean changes in ENSO state and variability projected by the IPCC multimodel ensemble (which are a key determinant in the projected future Atlantic TC activity) should be trusted.

Given these considerations, we would argue that coastal homeowners, insurers, the re-insurance industry, and every other potential stakeholder in this debate would be wise not to take false comfort from the notion (which the headlines resulting from this paper will inevitably feed) that climate change poses no future Atlantic hurricane threat. In fairness to Knutson et al, they do explicitly point out that their projected decrease in frequency is mostly coming from the weak end of the TC intensity spectrum. In principle, therefore, we imagine that they might perhaps even agree with this message themselves. Indeed, we invite them to comment here!

Update: The authors of the paper have put out an FAQ about their research.

Correction (5/21/08): It has been brought to our attention by NOAA representatives that the NOAA press conference for the Knutson et al paper was not an unprecedented event in recent years. In fact, similar press conferences were held for two other papers (both also questioning the premise that climate change is likely to lead to an increase in tropical cyclone activity) by Wang and by Vecchi and Soden

62 Responses to “Climate Change and Tropical Cyclones (Yet Again)”

  1. 51
    Andrew says:

    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.

  2. 52
    Jim Dukelow says:

    In #51, Andrew wrote:

    “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 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.

    Best regards.

  3. 53
    Bocco says:

    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.

  4. 54
    Jim Eaton says:

    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.

  5. 55

    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.

  6. 56
    Aaron Lewis says:

    re 44 & 45

  7. 57
    Jim Galasyn says:

    Speaking of ocean-consuming anoxia:

    Oxygen Depletion: A New Form of Ocean Habitat Loss

    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.”

  8. 58

    Officially speaking, TC Nargis is not a typhoon, because it was formed in the Indian Ocean sector. See, for example, or .

  9. 59
    Nick Bone says:

    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.

  10. 60
    pete best says:

    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, 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.

  11. 61
    Joe Andersen says:

    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.


  12. 62
    Isaac Held says:

    We have created a FAQ sheet in response to questions that have arisen with regard to our paper, Perhaps this will of interest to readers here.