June 5, 2001 CHAMPAIGN, Ill. — In the wake of mounting evidence of global warming, decision-makers are wrestling with related policy issues. Now, researchers at the University of Illinois have shown that the probability of severe climate change is much greater than many scientists or policy-makers had thought. "The size and impacts of anthropogenically induced climate change strongly depend on the climate sensitivity – the change in equilibrium surface warming due to a doubling of the concentration of carbon dioxide in the atmosphere," said Michael Schlesinger, a UI atmospheric scientist. "According to the Intergovernmental Panel on Climate Change, the climate sensitivity lies between 1.5 and 4.5 degrees Centigrade."
If the climate sensitivity is less than 1.5 degrees Centigrade, then climate change may not be a serious problem, Schlesinger said. "If, however, the climate sensitivity is greater than the IPCC’s upper bound, then climate change may be one of humanity’s most severe problems of the 21st century. By judging the likelihood of the climate sensitivity having any particular value – that is, by its probability density function – the crafting of robust adaptive climate-change policy could be greatly facilitated."
Schlesinger and UI atmospheric scientist Natalia Andronova used a simple climate/ocean model and the near-surface temperature record to estimate the probability density function for climate sensitivity. They considered 16 radiative-forcing models, which included such factors as greenhouse gases, anthropogenic sulfate aerosol, solar irradiance and volcanoes. For each model, the changes in global-mean near-surface temperature were calculated for the years 1765 through 1997.
The researchers found that, as a result of natural variability and uncertainty in the radiative forcing, the climate sensitivity could lie between 1 and 10 degrees Centigrade. "Consequently, there is a 54 percent likelihood that the climate sensitivity lies outside the IPCC range," Schlesinger said. "Our results show that the probability density function very strongly depends on which radiative forcing factors have actually been at work during the period of the temperature measurements," he said. "At present, the most likely scenario is one that includes anthropogenic sulfate aerosol forcing but not solar variation. Although the value of the climate sensitivity in that case is most uncertain, there is a 70 percent chance that it exceeds the maximum IPCC value. This is not good news."
One way to reduce the uncertainty of which probability distribution is the appropriate one to use in impact and policy studies is "to determine whether the sun’s irradiance has actually changed during the past 150 years," Andronova said. "Another way would be to consider the net radiative forcing of all the anthropogenic aerosols, not just the sulfate aerosol."
A paper discussing the researchers’ findings has been accepted for publication in the Journal of Geophysical Research. The National Science Foundation and the U.S. Department of Energy supported the work.
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