Human activities are boosting ocean temperatures in areas where hurricanes form, new study finds

BOULDER -- Rising ocean temperatures in key hurricane breeding grounds of the Atlantic and Pacific oceans are due primarily to human-caused increases in greenhouse gas concentrations, according to a study published online in the September 11 issue of the Proceedings of the National Academy of Sciences (PNAS).

Using 22 different computer models of the climate system, Benjamin Santer and six other atmospheric scientists from the Lawrence Livermore National Laboratory, together with Tom Wigley, Gerald Meehl, and Warren Washington from the Boulder-based National Center for Atmospheric Research (NCAR) and scientists from eight other research centers, have shown that the warming sea surface temperatures (SSTs) of the tropical Atlantic and Pacific oceans over the last century is linked to human activities.

NCAR's primary sponsor is the National Science Foundation.

"We've used virtually all the world's climate models to study the causes of SST changes in hurricane formation regions," Santer says.

Research published during the past year has uncovered evidence of a link between rising ocean temperatures and increases in hurricane intensity. This has raised concerns about the causes of the rising temperatures, particularly in parts of the Atlantic and Pacific where hurricanes and other tropical cyclones form.

Previous efforts to understand the causes of changes in SSTs have focused on temperature changes averaged over very large ocean areas, such as the entire Atlantic or Pacific basins. The new research specifically targets SST changes in much smaller hurricane formation regions.

For the period 1906-2005, the researchers found an 84 percent probability that human-induced factors--primarily an increase in greenhouse gas emissions--account for most of the observed rise in SSTs in the Atlantic and Pacific hurricane formation regions.

"The important conclusion is that the observed SST increases in these hurricane breeding grounds cannot be explained by natural processes alone," says Wigley. "The best explanation for these changes has to include a large human influence."

Hurricanes are complex phenomena that are influenced by a variety of physical factors, such as SSTs, wind shear, water vapor, and atmospheric stability. The increasing SSTs in the Atlantic and Pacific hurricane formation regions are not the sole determinant of hurricane intensity, but they are likely to be one of the most significant influences.

"It is important to note that we expect global temperatures and SSTs to increase even more rapidly over the next century," Wigley says.

According to Santer, "In a post-Katrina world, we need to do the best job we possibly can to understand the complex influences on hurricane intensity, and how our actions are changing those influences."

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Other institutions contributing to the study include the University of California, Merced; Lawrence Berkeley National Laboratory; Scripps Institution of Oceanography; the University of Hamburg; the University of East Anglia; Manchester Metropolitan University; NASA's Goddard Institute for Space Studies; and NOAA's National Climatic Data Center.

The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under primary sponsorship by the National Science Foundation. Opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Notes for Journalists

Journalists who agree to abide by the embargo can obtain a pdf of the paper by e-mailing PNAS at pnasnews@nas.edu

Title: "Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions"

Authors: B.D. Santer, T.M.L. Wigley, P.J. Gleckler, C. Bonfils, M.F. Wehner, K. AchutaRao, T.P. Barnett, J.S. Boyle, W. Brueggemann, M. Fiorino, N. Gillett, J.E. Hansen, P.D. Jones, S.A. Klein, G.A. Meehl, S.C.B. Raper, R.W. Reynolds, K.E. Taylor, and W.M. Washington.

Additional background

See Q&A with Benjamin Santer and Tom Wigley about global warming and ocean temperatures http://www.ucar.edu/news/releases/2006/oceantempsfacts.shtml

To receive NCAR and UCAR press releases via e-mail or to unsubscribe, send name, title, affiliation, postal address, fax and phone number to yvonnem@ucar.edu.

Additional contacts:
Anne Stark, Lawrence Livermore Public Affairs
925-422-9799
stark8@llnl.gov

Tom Wigley, NCAR Scientist
303-497-2690
wigley@ucar.edu


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