Scientists say they have found what could be the key to ending a yearlong debate about what is making hurricanes more violent and common -- evidence that human-caused global warming is heating the ocean and providing more fuel for the world's deadliest storms.
For the past 13 months, researchers have debated whether humanity is to blame for a surge in hurricanes since the mid-1990s or whether the increased activity is merely a natural cycle that occurs every several decades.
Employing 80 computer simulations, scientists from Lawrence Livermore National Laboratory and other institutions concluded that there is only one answer: that the burning of fossil fuels, which warms the climate, is also heating the oceans.
Humans, Ben Santer, the report's lead author, told The Chronicle, are making hurricanes globally more violent "and violent hurricanes more common" -- at least, in the latter case, in the northern Atlantic Ocean. The findings were published Monday in the latest issue of the Proceedings of the National Academy of Sciences.
Hurricanes are born from tropical storms fueled by rising warm, moist air in the tropics. The Earth's rotation puts a spin on the storms, causing them to suck in more and more warm, moist air -- thus making them bigger and more ferocious.
In that regard, the report says, since 1906, sea-surface temperatures have warmed by between one-third and two-thirds of a degree Celsius -- or between 0.6 and 1.2 degrees Fahrenheit -- in the tropical parts of the Atlantic and Pacific oceans, which are hurricane breeding grounds.
Critics of the theory that greenhouse gases are making hurricanes worse remained unconvinced by the latest research.
Chris Landsea, a top hurricane expert, praised the Proceedings paper as a worthwhile contribution to science, but said the authors failed to persuasively counter earlier objections -- that warmer seas would have negligible impact on hurricane activity.
Landsea, science and operations officer at the U.S. National Hurricane Center in Miami, noted that modern satellite observations have made hurricanes easier to detect and analyze, and that could foster the impression of long-term trends in hurricane frequency or violence that are, in fact, illusory. The surge in hurricane activity since the mid-1990s is just the latest wave in repeating cycles of hurricane activity, he said.
Philip Klotzbach, a hurricane forecaster at Colorado State University, said that "sea-surface temperatures have certainly warmed over the past century, and ... there is probably a human-induced (global warming) component." But his own research indicates "there has been very little change in global hurricane activity over the past 20 years, where the data is most reliable."
Researchers report in the Proceedings paper an 84 percent chance that at least two-thirds of the rise in ocean temperatures in these so-called hurricane breeding grounds is caused by human activities -- and primarily by the production of greenhouse gases.
Tom Wigley, one of the world's top climate modelers and a co-author of the paper, said in a teleconference last week that the scientists tried to figure out what caused the oceans to warm by running many different computer models based on possible single causes. Those causes ranged from human production of greenhouse gases to natural variations in solar intensity.
Wigley said that when the researchers reviewed the results, they found that only one model was best able to explain changing ocean temperatures, and it pointed to greenhouse gases in the atmosphere. The most infamous greenhouse gas is carbon dioxide, a product of human burning of fossil fuels in cars and factories.
Wigley estimated the odds as smaller than 1 percent that ocean warming could be blamed on random fluctuations in hurricane activity, as some scientists suggest.
The debate among scientists was triggered in August 2005, a few weeks before Hurricane Katrina struck New Orleans, when hurricane expert Kerry Emanuel of MIT wrote an article for the journal Nature proposing that since the 1970s, ocean warming had made hurricanes about 50 percent more intense in the Atlantic and Pacific oceans.
Later, two scientific teams, both at Georgia Tech, estimated that warmer sea-surface temperatures were boosting both hurricane intensity and the number of the two worst types of hurricanes, known as Category 4 and Category 5 storms.
Nineteen scientists from 10 institutions were involved in the Proceedings paper. In addition to Lawrence Livermore, other U.S. institutions included Lawrence Berkeley National Laboratory, the National Center for Atmospheric Research, NASA, UC Merced, Scripps Institution of Oceanography in La Jolla (San Diego County), and the National Oceanic and Atmospheric Administration.
Santer's co-authors included six Livermore colleagues -- Peter J. Gleckler, Krishna AchutaRao, Jim Boyle, Mike Fiorino, Steve Klein and Karl Taylor -- and 12 other researchers from elsewhere in the United States and from Germany and England.
Assuming that warmer water equals more bad hurricanes, scary times could be ahead for inhabitants of hurricane-prone regions.
That's because "the models that we've used to understand the causes of (ocean warming) in these hurricane formation regions predict that the oceans are going to get a lot warmer over the 21st century," Santer said in a statement. "That causes some concern."
How Hurricanes form in the Atlantic Ocean Hurricanes are born in far western Africa, where modest windstorms known as tropical disturbances pick up moisture from the warm sea and begin to whirl. As atmospheric pressures drop, tropical depressions form with wind speeds up to 38 mph. As they speed westward they become tropical storms, lashing the ocean with sheets of rain and winds blowing up to 70 mph or more, finally building into hurricanes with winds exceeding 100 mph.
Hurricane Source: NOAA, The New York Times Joe Shoulak / The Chronicle
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