Ice age analysis suggests global warming may be less severe than predicted
After crunching ice-age climate numbers, Oregon researchers and colleagues from Harvard, Princeton, Cornell and Barcelona came up with two encouraging conclusions about future global warming.
The planet appears less sensitive to carbon dioxide changes than expected, their study says, so extreme temperature increases in the near future appear highly unlikely. And future warming may also be less than predicted by the Intergovernmental Panel on Climate Change in 2007, particularly at the upper end of the "likely" range.
The study, funded by the National Science Foundation's paleoclimate program, drew on the known extent of ice sheets in the past and levels of carbon dioxide, methane and nitrous oxide taken from air trapped in Antarctic ice cores. Researchers also mapped ice-age temperatures based on pollen levels on land and concentrations of temperature-sensitive microorganisms in the ocean. Schmittner and colleagues then ran a climate computer model at different "climate sensitivities" -- the climate's reaction to greenhouse gas levels -- to see which sensitivities best pinpointed actual ice-age temperatures.
Their conclusion: The climate appears less sensitive to greenhouse gases than prior estimates. Based on the computer runs, doubling carbon concentrations would likely increase the world's average temperature from 3.1 to 4.7 degrees Fahrenheit over preindustrial levels, the study predicts. That's lower than the IPCC's likely temperature range for a doubling of carbon dioxide: 3.6 to 8.1 degrees. And it's a far cry from increases of up to 18 degrees held out as low probability possibilities. The study also concludes that increases greater than 11 degrees from a doubling of C02 "should be assigned near-zero probability." Computer runs using such severe climate sensitivity modeled the globe as entirely covered in ice during the ice age, the study said. Schmittner said the actual number was closer to 10 percent.