How Hurricanes May Add to Global Warming

Scientists tracking the global warming gas carbon dioxide (CO2) should look at what hurricanes do to the surface of the sea.

The ocean soaks up about a third of the CO2 coming from fossil fuel burning and forest clearance. But new research shows that hurricanes pump some of that CO2 back into the air - and could hold important implications for global warming.

Nicholas Bates at the Bermuda Biological Station For Research at Berry Beach calls this feedback effect "significant." He explains that "hurricanes, essentially, are making oceans lose CO2." This enhances the accumulation of the climate warming gas in the atmosphere. That, in turn, has impacts - that are still unclear - for forecasts of global warming. "It's another complication" among "many uncertainties" that designers of computer programs that model climate change have to take into account, Dr. Bates says.

The Bermuda Station is strategically placed to study this unexpected hurricane effect. It maintains research sites nearby in the Sargasso Sea. Records of such sea surface conditions as air and water temperatures and salinity at one site go back some 40 years.

Bates calls this "a great window on climate change in the Sargasso Sea." It provides a basis for assessing the influence of passing hurricanes. In 1995, hurricane Felix passed over the research site giving scientists an ideal opportunity to measure hurricane effects. Bates, his Bermuda colleague Anthony Knap, and Anthony Michaels with the University of Southern California in Los Angeles report their findings in the Sept. 3 issue of Nature.

They found that Felix plus two other hurricanes - Luis and Marilyn - increased summertime feedback of CO2 to the atmosphere by 55 percent. Luis and Marilyn did not pass directly over the research site.

Scientists have known that hurricanes cool the sea as they roil the surface and bring cooler water up from below. But Dr. Knap notes that the effect on how the air and sea surface exchange gases had been unknown. Cooler water has less ability to hold CO2. Splashing waves and breaking bubbles also release CO2.

The three scientists made a preliminary estimate that indicates hurricanes provide a significant secondary feedback to the global climate system. Seasonally, the ocean soaks up CO2 from the atmosphere in winter. It gives back more than it absorbs in summer. Hurricanes enhance that feedback.

What this means for long term global warming is unclear. Some scientists speculate that warming would bring more hurricanes and increase hurricane intensity. That, in turn, would increase hurricane CO2 feedback. Chris Landsea with the National Oceanic and Atmospheric Administration's hurricane research division in Denver calls such speculation "overplayed." He says it now seems clear that any warming-induced change in hurricane frequency or intensity would be "lost in the noise" of normal year-to-year hurricane variability.

Bates says that, whether or not global warming increases hurricane activity, he thinks hurricanes are contributing to the year-to-year variability of uptake of CO2. He notes there are other unknown factors, such as El Nio, that influence the variability and should be studied. The research off Bermuda, he adds, "is clearing up a little bit of the puzzle."