Hurricane Sandy and climate change

While the SSTs did not cause Sandy to curve into New Jersey, they quite likely caused Sandy to be stronger. Our new research shows that the limiting intensity of hurricanes (how strong hurricanes can get as a statistical limit) relates to SST at about 8 m/s/C.  With SSTs in the path of Sandy that were 2-3 C warmer than is typical, we would predict a strong hurricane to be twice as strong on average.  Read more.

What is Hurricane Climatology?

Hurricane Climatology on Prezi

Geographers poised to lead a new revolution in hurricane climate research

In my talk this week to the West Florida chapter of the American Meteorological Society I make a case that geographers are well positioned to lead a new revolution in hurricane climate research. Watch here.

Summit Summary

The 2nd International Summit on Hurricanes and Climate Change was held May 31-Jun 5th in Corfu, Greece. Judging by the level of science and by the participants enthusiasm for doing it again, the Summit was an overwhelming success. There were participants from 17 countries. The relative frequency of participants by country is plotted as a word cloud (using wordle). There were 25 participants from the USA, 8 from Germany and China, 7 from France, 6 from Australia and Bermuda, and 5 from the UK. A word cloud illustrating the relative frequency of keywords from the participants abstract titles is also shown.





Summary points arising from the talks and discussions (with help from Rick Murnane):

1. Although there were a few skeptics, most in attendance would agree that there appears to be an upward trend in the intensity of the strongest tropical cyclones worldwide. Although there does not appear to be a trend associated with the global frequency of tropical cyclones.
2. There is a growing appreciation among the participants for a significant feedback relationship between TC activity and weather and climate events on the inter-annual time scale and on spatial scales that extend across latitudes.
3. The availability of quality archival and proxy records of hurricane activity is increasing. These data hold the potential to provide new insights into the linkages between climate variation and tropical cyclone activity.
4. The idea that Atlantic tropical cyclone activity is controlled by changes in relative sea-surface temperature rather than local sea-surface temperature was discounted on physical and statistical grounds.

The Summit was sponsored by Aegean Conferences, the Risk Prediction Initiative, and Climatek.

Travel awards to help defray some of the costs for students were given to N. Jourdain, R. Hodges, H. Kim, S. Lavender, M. Lenard, J. Malmstadt, K. Sheitlin, A. Suzuki-Parker, E. Vincent, A. Werner. The talks are being made available here.

Plans are to have Springer publish an edited book of the proceedings.

Plans are underway to hold the 3rd International Summit on Hurricanes and Climate Change in 2011 on the spectacularly beautiful island of Santorini.

Follow the Summit

The 2nd International Summit on Hurricanes and Climate Change takes place in Corfu starting next Monday. We will archive podcasts of the talks here. The speaker list includes:

June 1
Session 1: M Chenoweth, K Walsh, JL McBride, F Chauvin, R Romero
Session 2: JB Elsner, JD Woodruff, AV Fedorov, K Oouchi

June 2
Session 3: RE Hart, B Soden, ME Mann, A Khain, J Done
Session 4: GJ Holland, YM Tourre, JP Kossin, S Gentile

June 3
Session 5: K Emanuel, SL Lavender, D Liu, B Owens, C Liu
Session 6: J Nott, C Welker, A Osso, AS Daloz

June 4
Session 7: AA Tsonis, K Scheitlin, DJ Vimont, C Wang, J Rumpf
Session 8: PS Chu, R Kumar, S Rahmstorf, MR Lowry

Prelude to the 2nd Summit on Hurricanes & Climate Change, Corfu, Greece

Comments & Replies

Our paper entitled: "The increasing intensity of the strongest tropical cyclones" published last September in Nature (henceforth EKJ08) garnered considerable attention in the scientific community. Here are replies to some of the comments we received about this work.

Comment: "If the total number of storms has not changed, and the number of strong storms has increased from 13 to 17, then surely the number of weak storms must have decreased proportionately."
Reply: We used statistics rather than arithmetic. The rate of strong tropical cyclones is much smaller than the rate of weak tropical cyclones. As the ocean warms the stronger tropical cyclones can "borrow" a few cyclones from below the threshold intensity that will significantly increase the rate of the stronger tropical cyclones while not significantly reducing the rate of weaker cyclones.

Comment: "Intuitively the number of tropical cyclones exceeding the mean rate plus 2 times the standard deviation as was shown in previous studies (e.g., Kossin et al. 2007) should be equivalent to number of tropical cyclones exceeding some upper quantile level as shown in EKJ08."
Reply: The number of cyclones exceeding plus 2 times the standard deviation is positively correlated to the rate of cyclones. A basin with a lower rate of tropical cyclones will have fewer cyclones exceeding plus 2 times the standard deviation compared with a basin with a higher rate, while the number of cyclones exceeding the 90th percentile is independent of the rate. Thus it is not appropriate to compare the differences between Kossin et al. (2007) and EKJ08 using this approach.

Comment: "I've regressed the most intense TC per season on year and my results do not match those presented in EKJ08."
Reply: A regression of the most intense TC per season is not the same as quantile regression on year as was done in EKJ08 for the following reasons. a) Quantile regression minimizes a linear absolute deviation statistic rather than a quadratic statistic, and b) quantile regression treats each intensity value equally; no wind speed contributes more to the model fit.

Comment: "Your results are only marginally significant and there are many factors contributing to hurricane intensification."
Reply: That is correct, but all else being equal, a warming of the tropical oceans where tropical cyclones form should increase their intensity. Since the strongest tropical cyclones are, on average, closest to there theoretical maximum potential intensity it stands to reason that if there is a warming signal it should be most apparent in the tendency of the strongest cyclones. Moreover, statistical inference is concerned with drawing conclusions based on data together with prior assumptions. Arguments that include the basic physics of the role ocean heat plays in tropical cyclone intensity have more weight before the data are examined.

Comment: "The authors claim that the increasing trend is consistent with theory, yet numerical modeling studies suggest a different sensitivity of tropical cyclone intensity to warming."
Reply: Numerical models are not theory. They are based on theory, but require many ad hoc empirical arguments that put them into the realm of "scenario generators." The theory we have in mind is the 2nd law of thermodynamics.

Comment: "I'm surprised that the relationship between intensity and sea-surface temperature is not stronger."
Reply: The physics of cyclone intensification works against the correlative relationship. An active year of tropical cyclones will effectively remove warmth from the ocean so that a seasonal average temperature will not correlate as strongly with tropical cyclone activity as one might expect even though the physical causality is strong.

Comment: "Yet when you look at scatter plots of these SST series versus number of intense TC’s there is no relationship in the warmer SST, more intense TC’s direction."
Reply: We did not look at the number of TCs; we looked at the intensity. There is no theory for TC formation. However, given a TC, there is a nice theory for the efficiency of intensification. So, we focused on intensity rather than on frequency. Given a TC in a nearly optimal dynamic environment, we should expect to see it reach a higher intensity with warmer SST. If on average 10% of the storms get within 5% of their MPI and the MPI increases then we would see the strongest storms getting stronger, assuming all else stays the same.

Comment: "Here's a hypothetical, what if the predictor had been another quantity that also shows a significant trend over the period 1981-2006, I don't know...my weight, perhaps...would one be discussing what the physical meaning of a non-significant correlation between the two was?"
Reply: This example has little to do with the relationship of TCs to warming seas since in the latter there is a theory linking the two, whereas with your weight and TCs there is none. In science this makes a big difference.

2nd International Summit on Hurricanes & Climate Change

Over the past several years the topic of hurricanes and climate change has received considerable attention by scientists, the insurance industry, and the media. Building on the successful inaugural summit on the topic of hurricanes and climate change held during the spring of 2007, we are organizing the second summit scheduled for May 31-Jun 5, 2009 in Corfu, Greece. The purpose is to bring together leading academics and researchers on various sides of the debate and from all over the world to discuss new research and express opinions about what is happening and what might happen in the future with regard to regional and global hurricane (tropical cyclone) activity. The goals are to address what research is needed to advance the science of hurricane climate and to provide a venue for encouraging a lively, spirited, and sustained exchange of ideas.

Recent research shows that tropical cyclones are more powerful than in the past with the most dramatic increases occurring over the North Atlantic and for the strongest hurricanes (Elsner et al. 2008). Although such increases are correlated with warming oceans and are consistent with the thermodynamic theory of hurricane intensity, there remains doubt about the interpretation, integrity, and meaning of these results. Thus we plan to invite leading scientists to present their latest research and participate in discussions on this topic. Invited speakers will be required to stay for the duration of the summit (4 days). All sessions will be plenary. An important part of the summit will be follow-up discussions during coffee breaks and meals to exchange opinions and ideas. We anticipate inviting 15-20 scientists and we expect between 80-100 participants (we had 75 participants for the 2007 Summit).

Keynote speakers: Greg Holland, NCAR; Jim Kossin, UW-Madison; Kerry Emanual, MIT, Robert Hart, Florida State University.

Topic keywords include: hurricane & climate theory; historical, paleo & modern data; empirical, statistical & dynamical models; intensity, frequency, size, paths, landfalls & rainfall, damage & losses; trends & cycles; hurricanes as a response to climate & hurricanes as forcing for climate. The intellectual merit of the summit is the likelihood of a significant advance in our understanding of climate processes associated with tropical cyclone activity. The broader impacts will be a better understanding of the future hurricane threat to the United States, Caribbean, Japan and elsewhere.

To register for the Summit click here.

• Elsner, J. B., J. P. Kossin, and T. H. Jagger, 2008: The increasing intensity of the strongest tropical cyclones, Nature, 455, 92-95.

In the eye of the storm

Storm World: Hurricanes, Politics, and the Battle Over Global Warming
by Chris Mooney
Harcourt: July 2007. 392 pp. $26

[An edited version published in Nature, v448, 648, August 9, 2007]

Chris Mooney’s follow-up to his The Republican War on Science (Basic Books, 2005) is a reconnaissance flight into the turbulent debate on a possible link between hurricane activity and global warming. The flight log is compelling enough for Hollywood. It records a clash between the empiricist climate scientist William Gray (think Ian McKellen) at Colorado State University (in a red state) and the theoretician Kerry Emanuel (think Tom Hanks) at the Massachusetts Institute of Technology (in a blue state). Journalist Mooney has a scriptwriter’s flair for pitting his protagonists against each other and dishing the historical and methodological back story in vivid prose: “If we’re really making the deadliest storms on Earth still deadlier, it will represent one of humanity’s all-time greatest foot-shooting episodes.”

The debate swirls about the cause of the recent upswing in severe hurricanes, especially over the Atlantic where evidence for a change is most compelling (here). There is no question that warmer tropical oceans will increase the potential intensity of tropical cyclones (all else being equal), but for Gray the causal chain ends with the ocean. “Nobody knows how the atmosphere works,” he says, feeling that it is far too complicated to be captured by a computer. Emanuel, on the other hand, adds a further link to the chain, placing the blame on human meddling with the composition of the atmosphere (aerosols and greenhouse gases). Having challenged Gray and his colleagues on their forecast methods, it's clear why it took someone with Emanuel's stature (Time Magazine) to spar with him on the climate-change issue.

Just a month before hurricane Katrina’s devastating strike on America’s south coast on 29 August 2005, Emanuel published a paper in this journal (Nature 436, 686–688; 2005) that ignited a scientific debate by linking storm strength to ocean temperatures. It also triggered a maelstrom of media coverage that resulted in the US National Oceanic and Atmospheric Administration (NOAA) closing ranks and claiming unequivocally that the increase in Atlantic hurricane activity since 1995 could be attributed solely to an ocean cycle unrelated to greenhouse warming. Mooney is at his best when describing the political tempest. By allowing what Emanuel calls the “party line” while discouraging dissenters, NOAA was, in Mooney’s words, “gaming the release of information and trying to shift the debate in their favored direction.”

Mooney revisits his call, propounded in his earlier book and in subsequent newspaper columns, for scientists to do a better job of communicating science to the public and media. He urges researchers to stop pretending that they are nothing but objective “fact machines” and to instead give more general interpretations of their results and put them into a broader context.

Drawing on scientific conferences and on interviews with hurricane and climate scientists during 2006, Mooney covers plenty of ground, from heat engines and synoptic meteorology to computer modeling, and all without equations. At times it feels hurried, US-centric and somewhat uneven, jumping between history, science and politics. But Mooney presents an accurate account of the clash between two of the most prominent climate scientists today. He is a good writer — “Scientists, like hurricanes, do extraordinary things at high wind speeds” — and his stories are consistently about people, giving the book a wide appeal.

In the end he gives us a clear picture of what the hurricane–climate change debate is about and where it might go next (sensitivity). As there are no answers, Mooney provides none. Not surprisingly, Chris takes a liking to Bill, but cannot recommend his view that global warming has nothing to do with hurricane activity. I also detected a commentary about storm climatologists for which science sometimes plays second fiddle to entertaining sound bites.

While the inner-core dynamics are well resolved in Storm World, a hurricane needs its outer rain bands and there are many scientists contributing to the whirlwind surrounding what is arguably the most important climate debate in history. Neither side is completely wrong and both would do well to study the full breadth of literature.

Storm World is a great summer read. The story continues however with more answers likely in the sequel.

Summary: Summit on Hurricanes and Climate Change

An International Summit on Hurricanes and Climate Change was held May 27-30, 2007 at the Aldemar Knossos Royal Village in Hersonissos, Crete. It was hosted by Aegean Conferences and supported by the Bermuda Institute for Ocean Sciences (BIOS) Risk Prediction Initiative and by the U.S. National Science Foundation. It was organized by myself and Richard Murnane (BIOS). There were 77 participants from at least 18 countries with a mix of academics and stakeholders (insurance and risk modelers). The purpose was to gather leading researchers in the field of tropical cyclone climate for sustained discussions on the state of the science and to elevate the discourse above the fray. In this spirit, it was appropriate to convene at the birth place of the Socratic method.

Traditionally tropical cyclones are analyzed as a passive response to climate forcing: the hurricane as a product of its environment. A warm ocean provides sustenance, a calm atmosphere nurturing, and a subtropical high pressure cell forward direction. An increase in oceanic heat will raise a hurricane's potential intensity, yet an increase in shearing winds could counter by dispersing the heat in a fledgling storm. This perspective is useful for identifying the mechanisms responsible for making some seasons active while others inactive. In this regard it was argued that data modeling is superior to data analysis (trend lines, etc) as it avoids cherry-picking the evidence and provides a framework for making use of older, less reliable data.

For example, a Poisson distribution is useful for modeling tropical storm counts over time. The benefit of this approach is that it provides a context that is consistent with the nature of underlying physical processes, analogous to the way the laws of physics provide a context for studying meteorology. It was shown that smoothing (filtering) the count data introduces low frequency patterns that may not be significant and that a data model of Atlantic hurricanes indicates a recent upswing in the number of strongest hurricanes with little multidecadal variation. The figure above shows annual hurricane rates using a data model of basin-wide Atlantic hurricane counts. Results are shown for different categories of hurricane intensity. Note that for each year the model provides a distribution on the estimated annual rate as indicated by a box plot. The model also provides a distribution for the number of rate changes over the 63-year period (1943-2005). Note the absence of a multidecadal pattern.

Although the question of whether we can ascribe a change in tropical cyclone intensity to anthropogenic climate change (attribution) is still open, it was argued based on data models for extreme winds that the difference in U.S. hurricane intensity between globally warm and cool years is consistent in sign and magnitude with theory and simulations. In this regard it was noted that the discrepancy between numerical model results and observations is likely due to a reliance on data analysis rather than data models.

The collective role that hurricanes play in changing the climate was a point of emphasis at the Summit. In the Atlantic, heat and moisture transport out of the tropics by an ensemble of hurricanes moving poleward in a given season was shown to have a detectable influence on the baroclinic activity at high latitudes the following winter which in turn influences the preferred hurricane track type (recurving or straight-moving) during the subsequent hurricane season. Thus a communication between the tropics and the middle latitudes on the biennial time scale is accomplished through tropical cyclone track changes and middle latitude baroclinicity. Also, the relationship between global warming and ENSO was explained in terms of warming rather than warmth. A warming planet is associated with more El Nino events which on the biennial time scale leads to cooling. These are intriguing hypotheses about climate change and tropical cyclones that merit further investigation. It was also shown that super typhoons in the western North Pacific need a deep ocean mixed layer for rapid intensification only in regions where the sub surface water temperatures are marginally supportive of tropical cyclone intensification. It was demonstrated that high aerosol concentrations lead to an enhancement of the ice/water cloud microphysics leading to the invigoration of the convection inside a tropical cyclone.

Another important theme of the Summit was paleotempestology. Paleotempestology is the study of prehistoric storms from geological and biological evidence. For instance, coastal wetlands and lakes are subject to overwash processes during hurricane strikes when barrier sand dunes are overtopped by storm surge. The assumption is that during landfall the waves and wind-driven storm surge reach high enough over the barrier to deposit sand in the lake. In a sediment core taken from the lake bottom, a sand layer will appear distinct from the fine organic mud that accumulates slowly under normal conditions. Sediment cores taken from the northeastern Caribbean show more sand layers during the second half of the Little Ice Age when sea temperatures near Puerto Rico were a few degrees C cooler than today provides some evidence that today's warmth is not needed for increased storminess. Not surprisingly intervals of more hurricanes correspond with periods of fewer El Nino events. Sedimentary ridges in Australia left behind by ancient tropical cyclones indicate that activity from the last century under represents the continent's stormy past. Proxy techniques based on oxygen isotopes from tree rings and cave deposits also show promise for studying prehistoric tropical cyclone events.

It was mentioned that a spatially limited set of proxies or historical records is not able to resolve changes in overall activity from changes in local activity due to shifts in tracks. While the northeastern Caribbean region is in the direct path of today's hurricanes, was it always? The answer is important as more hurricanes locally could mean changes in steering rather than changes in abundance. Proxy data from the U.S. Gulf coast show a pattern of frequent hurricanes between 3800 and 1000 years ago followed by relatively few hurricanes during the most recent millennium which is explained in terms of the position of the subtropical North Atlantic High. Moreover it was shown that recent increases in typhoon intensities affecting Korea can be explained by an eastward shift in the subtropical North Pacific High allowing the storms to recurve over the warmer waters of the Kuroshio Current rather than the colder subsurface waters of the Yellow Sea. In order to understand how climate influences local changes in tropical cyclone activity, more research is needed to identify factors influencing tropical cyclone tracks.

Results from various high resolution numerical models, including a 20 km-mesh model, were consistent in showing stronger tropical cyclones in a warmer future. Most models indicate an overall decrease in the number of storms, attributable in one study to greater atmospheric stability and a decrease in the vertical mass flux. Not all models agree on the change in individual basin numbers with some models showing an increase in the Atlantic and others a decrease. It was shown that models without tropical cyclones need to remove the oceanic heat in the tropics through stronger trade winds. It was noted that models may be better at identifying changes to the large-scale genesis fields and that models still do not have the resolution to be useful to society. Climate model projections can be downscaled to construct tropical cyclone climatologies using a method that combines rejection sampling of genesis points with simple physical models for storm motion and winds. A few presentations focused on the perception and politics of tropical cyclone risk in a changing climate.

The summit was a success. All sessions were plenary and attendance averaged greater than 90%. There were 4 days of talks with each day broken into an early and late morning session of 4 to 5 speakers each. Coffee, tea, soft drinks, and snacks were served during the poster session between the oral sessions. The posters were available for viewing during the entire summit. Lunch was served on the beach after the late morning session followed by free time. In the evening participants and their company were treated to a full course meal at a local restaurant. Wednesday afternoon featured a tour of the Minoan Palace of Knossos near the city of Heraklion. The Summit concluded with a gala dinner featuring traditional Greek and Cretan music and dancing. Invited speakers were provided full accommodations. Travel awards were given to six students and two additional participants. The conference proceedings will be published by Springer as an edited volume in 2008.