FOREST-CARBON FEEDBACK

Of particular concern is the impact of global warming on the tropics. "Tropical forests are essentially inflammable," Field said. "You couldn't get a fire to burn there if you tried. But if they dry out just a little bit, the result can be very large and destructive wildfires."

According to several recent climate models, loss of tropical forests to wildfires, deforestation and other causes could increase atmospheric CO₂ concentrations from 10 to 100 parts per million by the end of the century. This would be a significant increase, given that the total concentration of CO₂ in the atmosphere is currently about 380 parts per million, the highest in 650,000 years.

"It is increasingly clear that as you produce a warmer world, lots of forested areas that had been acting as carbon sinks could be converted to carbon sources," Field said. "Essentially we could see a forest-carbon feedback that acts like a foot on the accelerator pedal for atmospheric CO₂. We don't exactly know how strong the feedback could be, but it's pretty clear that the warmer it gets, the more likely it is that degradation of tropical forests will increase the atmospheric CO₂."

The ocean is another vital reservoir for carbon storage. Recent studies show that global warming has altered wind patterns in the Southern Ocean, which in turn has reduced the ocean's capacity to soak up excess atmospheric CO₂. "As the Earth warms, it generates faster winds over the oceans surrounding Antarctica," Field explained. "These winds essentially blow the surface water out of the way, allowing water with higher concentrations of CO₂ to rise to the surface. This higher-CO₂water is closer to CO₂-saturated, so it takes up less carbon dioxide from the atmosphere."