Hurricane-human link seen

Jim Erickson, Rocky Mountain News

Published September 12, 2006 at midnight

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The human-caused buildup of carbon dioxide and other heat-trapping gases is mainly to blame for rising ocean temperatures that fuel intense hurricanes such as Katrina, according to Boulder researchers and their colleagues.

During the past year, more than a dozen studies have looked at possible links between global warming and recent increases in hurricane intensity.

Opposing camps of scientists have debated the relative contributions of natural climate variability and human activities - mainly the burning of fossil fuels.

Authors of the latest report, published online Monday in the Proceedings of the National Academy of Sciences, say their work substantially strengthens the chain of evidence pointing to humans.

"The work that we've done kind of closes the loop," said Tom Wigley, of the National Center for Atmospheric Research in Boulder.

Three NCAR researchers and colleagues from eight other institutions looked at long-term changes in sea-surface temperatures, or SSTs.

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

Several prominent hurricane researchers said the new study supplies the missing link that ties human activities to a big increase, during the past 30 years, in the number and proportion of intense hurricanes.

But longtime Colorado State University hurricane forecaster William Gray called the paper a desperate attempt "to push the steady drumbeat of human-induced climate warming."

And National Hurricane Center scientist Chris Landsea said that the paper "doesn't address the real key issues that are involved in this hurricane and climate-change controversy."

Those issues include the sensitivity of hurricanes to small changes in ocean temperature and the reliability of decades-old hurricane records, Landsea said.

"There's nothing wrong with the paper. But I don't think it's really a big advance in the field," he said. "I think it reconfirms what just about everybody already thought."

In the past century, ocean temperatures in hurricane-forming regions of the tropical Atlantic and Pacific have increased 0.6 degrees to 1.2 degrees Fahrenheit.

Last year, MIT researcher Kerry Emanuel reported a strong correlation between the warming oceans and increasing hurricane intensity.

NCAR scientists and their colleagues took the next step.

They used 22 state-of-the-art computerized global climate models to determine what's causing the ocean warming.

They looked at the output of more than 80 simulations using models developed at 15 research institutions around the world. The simulations allowed the researchers to examine the roles of various factors that affect climate, including variations in the sun's output, volcanic eruptions, changes in ozone levels and greenhouse gases.

Previous efforts to understand the observed ocean warming have focused on temperature changes averaged over large ocean areas, such as the entire Atlantic or Pacific basins. The new study targeted much smaller hurricane-formation regions in those oceans.

For the period 1906 to 2005, the researchers found that human activities - primarily an increase in greenhouse gas emissions - account for at least 67 percent of the observed temperature rises.

The new work shows that "those regions in which hurricanes are born, the hurricane genesis regions, are clearly being influenced by global warming," said Robert Corell, of the American Meteorological Society.

But University of Colorado climatologist Roger Pielke Sr. said the claim that humans are responsible for at least 67 percent of the warming "overstates the capabilities of the (climate) models."

"No one has demonstrated any ability for these models to skillfully predict on the regional scale," he said.

Gray said that changes in deep- ocean circulation, not the buildup of greenhouse gases, likely explain the observed rise in sea-surface temperatures. "The big question they don't answer is this: Even if SSTs go up, does that make hurricane activity more frequent and more intense? They imply that it does, but we have no observations to show that it's true," said Gray, a global warming skeptic.

According to Gray, the Atlantic Ocean is in a period of increased hurricane activity that began around 1995 and could last another 15 or 20 years before tapering off. The recent surge of activity is part of a natural cycle and has little to do with global warming, he contends. "This paper is going to carry a lot of weight because it has such prominent authors," he said. "But I'm sure they're going to be proven wrong."

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