February 16, 2009
Noting that industrial emissions are well out-pacing even the most aggressive forecasts, Dr. Christopher Field said the IPCC's 2007 climate assessment failed to account for potentially devastating feedback cycles that could be triggered by rising temperatures, including large-scale ignition of tropical forests and catastrophic melting of Arctic tundra, which would greatly accelerate the release of greenhouse gases.
The loss of these giant carbon sinks would have substantial climate consequences.
"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."
Many climate models suggest that forests in the Amazon and southeast Asia will become more susceptible to drought and fire should CO2 levels continue to rise. Burning of these forests will in turn feed the cycle, boosting atmospheric CO2 concentrations from 10 to 100 parts per million by the end of the century.
"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."
In the Arctic, warming is thawing the permafrost, raising the prospect that enormous amounts of frozen methane could be released into the atmosphere.
Change in duration of snow-covered ground north of 50°N. The number of days in a year in which the ground is snow covered has decreased by an estimated average of 7.5 days from 1970 to 2000. Source: Euskirchen and others 2007, image courtesy of UNEP
"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 CO2, 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."
Field said that the next IPPC assessment will look include these, and other factors, in 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," he said. "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."