Contra Costa Times (California), September 11, 2006


[Rachel's introduction: "The human induced build-up of greenhouse gases in the atmosphere appears to be the primary force behind increased hurricane activity," said oceanographer Robert Correll of the American Meteorological Society.]

By Betsy Mason

Livermore, Calif. -- The results are in -- mankind is largely responsible for the rise in hurricane intensity in recent years.

A new study being released today, and led by Lawrence Livermore Laboratory, found man's unmistakable fingerprint on the pattern of increasingly powerful hurricanes in both the Atlantic and Pacific oceans.

"The human induced build-up of greenhouse gases in the atmosphere appears to be the primary force behind increased hurricane activity," said oceanographer Robert Correll of the American Meteorological Society.

A growing number of studies during the last year has convincingly shown that hurricanes have become more destructive during the last three decades, and that this is due to rising sea-surface temperatures.

And now, a team of 19 climate scientists from 10 different institutions led by Livermore Lab's Benjamin Santer, has closed the loop. The study used computer-generated climate models to reveal that natural climate fluctuations, also known as climate noise, cannot account for the upswing in ocean temperature.

"This clearly shows that the observed increases in ocean temperatures in these Atlantic and Pacific hurricane breeding grounds simply can't be explained without positing a large human affect," Santer said. "Climate noise alone just won't cut it."

Most climate scientists agree that man has contributed to global warming through emissions of carbon dioxide and other greenhouse gases. But the specific nature and causes of the warming varies in different parts of the globe. Some spots may have heated up naturally without help from carbon emissions while others may have gotten a bigger boost from man.

Previous studies have looked at the average warming over large parts of the globe, such as entire oceans. But Santer's team used virtually every climate model developed so far in the world, 22 in all, to analyze all of the possible causes behind the heating up of the sea surface in the specific parts of the oceans where hurricanes are born.

Livermore Lab keeps an archive of the results from the world's climate models, so the raw materials for the study were already at Santer's fingertips.

"We're sitting on top of a wealth of information," he said. "It's like a scientific gold mine."

The team compared results from models that tried to recreate the actual sea-surface temperatures using only natural forces such as the sun and volcanic eruptions, with results from the same models when human greenhouse gas emissions were included.

The results from the experiments that used only natural forces didn't come close to reality. But when human, or anthropogenic, influence was included, the results closely matched the actual temperatures recorded over the last century.

"We found that the dominant cause for the modeled sea-surface temperature changes in these (hurricane formation) regions was anthropogenic increases in the concentration of greenhouse gases," said co-author Tom Wigley of the National Center for Atmospheric Research in Boulder, Colo.

The team estimated there is a very good chance that as much as two- thirds of the temperature increase can be attributed to man. The research appears today in the Proceedings of the National Academy of Sciences.

While it is impossible to pin Hurricane Katrina, or any other single event, on carbon emissions and the warming oceans, it is clear that boosting the temperature makes powerful hurricanes more likely, Santer said.

The ocean temperature in the hurricane nurseries has gone up less than one degree over the last century, but the number of category 4 and 5 hurricanes has nearly doubled over the last three decades.

This doesn't bode well for the future, said team member Michael Wehner, an atmospheric scientist at Lawrence Berkeley Laboratory. He is using the same models to project ocean temperatures into the future.

Wehner looked at several possible scenarios including continued unabated carbon emissions as well as drastically curbed emissions.

Even if we had stopped emitting greenhouse gases yesterday, it will still get warmer in the future because it takes time for the carbon to affect temperatures, he said.

"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," he said. "You ain't seen nothing yet."

The worst-case scenario of undiminished future carbon emissions, which predict ocean temperatures will rise as much as nine degrees by the end of the century, could be catastrophic, Wehner said.

"The results give me cause for concern," Santer said. " We can't stick our heads in the sand here."