Cyclones May Fan the Global Warming Flames

Hurricane-force winds whip the ocean so fiercely that the seawater belches dissolved carbon dioxide (CO2) into the atmosphere like a shaken bottle of soda, scientists have discovered. Three storms that raged near Bermuda in 1995 boosted the flow of CO2 into the air over part of the Atlantic Ocean by more than half, according to a report in tomorrow's issue of Nature. If global warming causes strong storms to grow even more fierce, as some climate models predict, that could trigger a self-feeding cycle that unleashes still more heat-trapping CO2 into the atmosphere.

Levels of CO2 in Earth's atmosphere have climbed steadily for decades, mainly due to industry and other human activities. Most climate experts believe this increase drives global warming, because CO2 is the dominant heat-trapping greenhouse gas. The planet's oceans absorb a large measure of that CO2, but oceanographers aren't sure exactly how much. Under some conditions--particularly when CO2-rich deep water wells up to the surface in the tropics--seawater also releases CO2 back into the air. Now, it appears hurricanes hasten that process.

A team led by biogeochemist Nicholas Bates of the Bermuda Biological Station for Research in Ferry Reach measured the effects of Hurricane Felix, which swept over two long-term ocean sampling stations near Bermuda in August 1995. By monitoring CO2 levels in the air and water before and after Felix struck, the team found that the hurricane caused local atmospheric CO2 levels to surge. Two later hurricanes, Luis and Marilyn, also spurred more CO2 release. All told, says Bates, the three cyclones churned 55% more CO2 into the air over the western North Atlantic that summer than usual. "Hurricanes agitate the sea surface, which greatly enhances the transfer of CO2," he says.

If extended to all hurricanes, this effect could force half a billion tons of carbon in the form of CO2 from the ocean into the atmosphere each year, Bates says. That amount--roughly 10% of the total released annually by human activity--could rise if global warming heats the sea and spawns storms with faster winds, he notes. Other experts say that the effect of hurricanes on global warming would probably be minimal as only the largest storms are expected to get stronger. "Overall, it appears global warming might not affect the strengths of most hurricanes," says meteorologist Chris Landsea of the National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami, Florida. But recent research, he adds, shows that the largest hurricanes--with the fastest winds--could become even more powerful CO2-spawning dynamos as the planet heats up.