Kevin Trenberth
Distinguished Senior Scientist

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• 303.497.1318
• trenbert@ucar.edu
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Kevin Trenberth On Developments Related To Landsea

Written: July 2008

The vigorous hurricane season in the Atlantic in 2004, where 4 hurricanes hit Florida and flooding occurred along much of the eastern seaboard, attracted widespread media attention. Comments by a few scientists, including Landsea, suggested that there was no relation between these events and global warming. On October 14, 2004, a press conference called by the Harvard Medical School Center for Health and the Global Environment was held by teleconference with about 40 reporters to correct that impression and recognize that global warming is happening and hurricanes cannot be unaffected. Landsea took offense at Trenberth's participation in the news conference and what Trenberth said. Landsea issued a public letter on January 14, 2005 in which Landsea resigned from the IPCC but also made a number of charges that are not correct. There was considerable publicity about Landsea's letter, such as this Washington Post Article. Trenberth and UCAR (Trenberth's employer) disputed Landsea's comments and UCAR posted the news release and NCAR statement associated with Trenberth's comments online to show that several misquotes had been made.

When Landsea first expressed major dissatisfaction in a letter that was widely distributed, Trenberth responded with the following email response . Shortly after Trenberth's response Susan Solomon sent the following email response

"The global warming influence provides a new background level that increases the risk of future enhancements in hurricane activity"
Kevin Trenberth

Chris Landsea was not a Coordinating Lead Author (CLA) or a Lead author of the IPCC report. Landsea was asked by the CLAs to be a contributing author (there are 66 in Chapter 3) who write a half page or so on a particular topics that are assembled into the report by the Lead Authors. Landsea could easily refuse to do so, but to publicly resign the way Landsea did was a very political approach that had nothing to do with any scientific dispute. Actually one can't resign from something that one is not a member of!

Trenberth's response was to publish the scientific basis for the news conference criticized by Landsea in the June 2005 Science. Kerry Emanuel of the Massachusetts Institute of Technology independently published direct observational evidence in Nature [Article] only two months later. Emanuel showed that significant increases in cyclone intensity and duration around the world since 1970 have been strongly related to rising SSTs. Challenges from Landsea and other experts to Emanuel's work led to modest revisions in the specific correlations, Emanuel 2005b, but do not alter the overall conclusion. In September 2005 Peter Webster and his colleagues at the Georgia Institute of Technology published an article in Science that explicitly showed a substantial rise in the number of Category 4 and 5 hurricanes since 1970 and in the percent of total hurricanes that fit that description. Webster and colleagues concluded that the rise was to be expected, given the observed increase in SSTs. Trenberth and Shea subsequently published an analysis of the role of global warming in hurricanes [PDF]

Landsea's letter was written late in 2004, and subsequently, in addition to the many new published papers, the record breaking 2005 hurricane season occurred, including hurricane Katrina and 3 other category 5 hurricanes.

There is no doubt that there is large natural variability in hurricanes, and also disputes about how reliable the record is, points made by Landsea for example, Landsea 2005, Nature, [Article], and Chan 2006, Science, [Article]; but other articles have demonstrated that the changes in the hurricane environment have a human component such as Santer et al 2006, Proceedings of the National Academy of Sciences, [Article]

There is a wide range of scientific opinion on the issue, reflecting the genuine scientific uncertainties and developing nature of the science. Subsequently, the IPCC has also weighed in and the attribution of changes to a human role are clearly stated in chapters that were not authored by Trenberth.

As to Trenberth's credentials with regard to the issue, while it was true that Trenberth had not, in 2004, published articles on hurricanes as such, Trenberth had extensively published on changes in water vapor, precipitation, storms, and the hydrological cycle, and there is no doubt that hurricanes are a subset of storms. Subsequently Trenberth has published several articles on hurricanes and, along with other research, there is exceedingly strong support for the comments made in the news conference. On the other hand, there is no basis for Landsea's position.

Trenberth Views On Hurricanes As Of Late 2004

1) There is large natural variability of hurricanes. We can not say anything about increases in numbers or frequency from the record or how these may change in the future.

2) However, the environment in which the hurricanes are occurring is clearly changing, and those changes are part of global warming.

3) For the past 10 years the sea surface temperatures (SSTs) have been higher from 10-20°N in the Atlantic, where the hurricanes form and track, than at any other time in the record through the 20th C.

4) During this period 8 out of the 10 years had above normal numbers of hurricanes and the 2 exceptions were El Niño years when the main activity shifts to the Pacific.

5) Hence there is every reason to think that these changes should increase the intensity of hurricanes and rainfalls associated with hurricanes.

6) We can not say anything much about the 4 hurricanes that hit Florida, except that the rainfalls and flooding were likely enhanced by global warming.

7) The IPCC in 2001 also states that hurricanes are likely to become more intense with stronger winds and heavier rainfalls.

8) While the influence of climate change on hurricanes may not be detectable because of large natural variability, this does not mean that there is no influence.

Additional Comments

Landsea writes "It is beyond me why my colleagues would utilize the media to push an unsupported agenda that recent hurricane activity has been due to global warming." Yet no such claims were made. On the contrary, it was clearly stated that natural variability was dominant in observed the climate record. Many other mis-statements have been made by Landsea about what was said, and the fact that all statements were carefully caveated.

There are always differences of opinion among scientists on issues of science. NCAR encourages responsible dialog and discussion. But we do not condone name-calling and deliberate attempts to mislead. The IPCC process is robust, open, and subject to extensive reviews, checks and balances. We have confidence that it will be balanced and represent the consensus. However, the process is helped if people contribute rather than withdraw from it.

Updates

There was a tremendous amount of publicity, often ill-informed, following all of this. Trenberth remained quiet and did not respond in person to the often personal attacks on his integrity. Instead Trenberth documented the state of the science and the views Trenberth espoused at the press conference in this article:
Trenberth, K., 2005: Uncertainty in hurricanes and global warming. Science, 308, 1753-1754. [Summary] [Paper]

Shortly thereafter, two other publications came out which also provided strong support:
Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Chang, 2005: Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309, 1844-1846. [Paper]
Emanuel, K. , 2005a: Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688. --, 2005b: Emanuel replies. Nature, 438, E13, doi:10.1038/nature04427. [Paper] [Reply]

And these in turn produced the following article in November 2005:
Pielke, R.A., C. Landsea, M. Mayfield, J. Laver, and R. Pasch, 2005: Hurricanes and Global Warming. Bull. Amer. Meteor. Soc., 86, 1571-1575. [PDF]

The following was a rebuttal of Pielke et al (2005) by the news conference participants:

Pielke et al (2005) provide their assessment of the situation with regard to hurricanes and global warming. They state there is "lack of a theory for future changes in storm frequencies" and "the state of understanding of tropical cyclogenesis provides too poor a foundation to base any projections about the future". Given the lack of any physical understanding of how hurricanes work it follows then that " ...until a relationship between actual storm intensity and tropical climate change is clearly demonstrated, it would be premature to conclude that such a link exists or is significant ..." . It is unfortunate that they overlook the considerable understanding and modeling capabilities that do exist in tropical storms, even while recognizing that the theory is incomplete. Indeed, there is plenty of research indicating that hurricanes do respond to the environment in which they are embedded, as summarized by Trenberth (2005).

Observed changes in hurricanes are masked by poor data, especially prior to the 1960s when the satellite era began. Moreover changes in atmospheric temperature throughout the atmosphere are distorted by changes in radiosondes and instruments. Rising sea surface temperatures and increased atmospheric water vapor provide the primary fuel for tropical convection through latent heat release, and both are increasing. Atmospheric dynamics play a key role in determining where storms occur and their tracks, and changes are less certain in association with global warming. But the evidence suggests increases in intensity of storms, once they are formed, and increases in heavy rains and risk of flooding. The Pielke et al. essay never does discuss the most widespread impact of such storms which is already flooding.

However, the essay also attacks the "misguided" participants in a telecon news conference held to discuss the changes and impacts of the record breaking 2004 hurricane season in Florida. The transcript of the news conference has been made available by UCAR to correct the many misquotes of the statements made at that news conference. In fact the comments by Trenberth are fully consistent with those in Trenberth (2005). Pielke et al. further misquote the IPCC (2001). There is no doubt that social changes of people placing themselves in harm's way contributes substantially to hurricane damage, but they seriously underestimate the changes in nature that also contribute. But then what should one expect from a bunch of guys who have no expertise in climate change? The article is exceedingly political, and can not even get the strong link between burning fossil fuels and energy and changes in carbon dioxide and atmospheric composition changes right.

Misguided news conference participants (Jim McCarthy, Paul Epstein, Kevin Trenberth).

It was also followed up by this article and response:
Anthes, R. A., R. W. Corell, G. Holland, J. W. Hurrell, M. C. McCracken, and K. E. Trenberth, 2006: Hurricanes and global warming--Potential linkages and consequences. Bull. Amer. Meteor. Soc.,87, 623-628. [PDF]
Pielke, R., C. Landsea, M. Mayfield, J. Laver, and R. Pasch, 2006: Reply to "Hurricanes and Global Warming Potential Linkages and Consequences". Bull. Amer. Meteor. Soc., 87, 628-631.[PDF]

Of course the 2005 season broke so many records that it further demonstrated that global warming was playing a role.

Related Articles

• Trenberth, K. E., 2005: Uncertainty in Hurricanes and Global Warming. Science, 308, 1753-1754. [Summary] [Paper]
• Trenberth, K. E., and D. J. Shea (2006), Atlantic hurricanes and natural variability in 2005, Geophys. Res. Lttrs., 33, L12704, doi: 10.1029/2006GL026894. [PDF]
• Trenberth, K. E., C. A. Davis and J. Fasullo, 2007: Water and energy budgets of hurricanes: Case studies of Ivan and Katrina . J. Geophys. Res., 112, D23106, doi: 10.1029/2006JD008303. [PDF]
• Trenberth, K. E., and J. Fasullo, 2007: Water and energy budgets of hurricanes and implications for climate change. J. Geophys. Res., 112, D23107, doi:1 0.1029/2006JD008304. [PDF]
• Trenberth, K. E., and J. Fasullo, 2008: The energy budgets of Atlantic hurricanes and changes from 1970. Geochemistry, Geophysics, Geosystems, 9, doi: 10.1029/2007GC001847. [PDF]

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