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Study Links Human Activities to Warmer Oceans, Stronger Hurricanes

September 15, 2006

New research shows that rising sea surface temperatures (SSTs) in hurricane breeding grounds of the Atlantic and Pacific Oceans are unlikely to be purely natural in origin. These findings, according to a team of researchers including Michael Wehner of CRD’s Scientific Computing Group, complement earlier work that uncovered compelling scientific evidence of a link between warming SSTs and increases in hurricane intensity.

Previous studies to understand the causes of SST changes 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.

Using 22 different computer models of the climate system, atmospheric scientists from 11 research centers have shown that the warming of the tropical Atlantic and Pacific oceans over the last century is directly linked to human activities.

“We’ve used virtually all the world’s climate models to study the causes of SST changes in hurricane formation regions,” said Benjamin Santer of Livermore’s Program for Climate Model Diagnosis and Intercomparison, lead author of a paper describing the research that appears online recently in the Proceedings of the National Academy of Sciences. The bottom line is that natural processes alone simply cannot explain the observed SST increases in these hurricane breeding grounds. The best explanation for these changes has to include a large human influence.”

For the period 1906-2005, the researchers found an 84 percent chance that external forcing (such as human-caused increases in greenhouse gases, ozone and various aerosol particles) accounts for at least 67 percent of the observed rise in SSTs in the Atlantic and Pacific hurricane formation regions. In both regions, human-caused increases in greenhouse gases were found to be the main driver of the 20th century warming of SSTs.

"It's safe to say that even the conservative estimates of the 21st century will see significantly larger increases in the temperature in this region that we examined than we've already seen," Wehner told the Contra Costa Times. "You ain't seen nothing yet."

Hurricanes are complex phenomena and are influenced by a variety of physical factors such as SST, wind shear, moisture availability and atmospheric stability. The increasing SSTs in the Atlantic and Pacific hurricane formation regions isn’t the sole cause of hurricane intensity, but is likely to be one of the most important influences on hurricane strength.

Using the computer models, Wehner created scenarios involving different levels of carbon emissions and generated ocean temperature forecast. The result: Curbing the rise of SSTs will take a while, even if humans stop emitting greenhouse gases right away. If no actions are taken to curb emissions, then the ocean temperature could rise as much as nine degrees by the end of the century and help create more catastrophic natural disasters.

The team also included researchers from the National Center for Atmospheric Research, the University of California, Merced, the Scripps Institution of Oceanography, the University of Hamburg in Germany, the Climatic Research Unit and Manchester University in the United Kingdom, the NASA/Goddard Institute for Space Studies and the National Oceanic and Atmospheric Administration’s National Climatic Data Center.

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