TUNDRA THAWING

Climate scientists also worry that permafrost in the Arctic tundra will thaw, releasing enormous amounts of CO₂ and methane gas into the atmosphere. According to Field, the most critical, short-term concern is the release of CO₂ from decaying organic matter that has been frozen for millennia. "The new estimate of the total amount of carbon that's frozen in permafrost soils is on the order of 1,000 billion tons," he said. "By comparison, the total amount of CO₂ that's been released in fossil fuel combustion since the beginning of the Industrial Revolution is around 350 billion tons. So the amount of carbon that's stored in these frozen soils is truly vast."

Much of the carbon is locked up in frozen plants that were buried under very cold conditions and have remained in deep freeze for 25,000 to 50,000 years, he added. "We know that the Arctic is warming faster than anyplace else," he said. "And there is clear evidence that these frozen plants are very susceptible to decomposition when the tundra thaws. So melting of permafrost is poised to be an even stronger foot on the accelerator pedal of atmospheric CO_2, with every increment of warming causing an increment of permafrost-melting that shoots an increment of CO₂ into the atmosphere, which in turn increases warming.

"There's a vicious-cycle component to both the tundra-thawing and the tropical forest feedbacks, but the IPCC fourth assessment didn't consider either of them in detail. That's basically because they weren't well understood at the time."

For the fifth assessment report, Field said that he and his IPCC colleagues will have access to new research that will allow them to do a better job of assessing the full range of possible climate outcomes. "What have we learned since the fourth assessment? We now know that, without effective action, climate change is going to be larger and more difficult to deal with than we thought. If you look at the set of things that we can do as a society, taking aggressive action on climate seems like one that has the best possibility of a win-win. It can stimulate the economy, allow us to address critical environmental problems, and insure that we leave a sustainable world for our children and grandchildren. Somehow we have to find a way to kick the process into high gear. We really have very little time."