According to new research at the University of Illinois at Urbana-Champaign (UI), the probability of climate change exceeding even the United Nations’ Intergovernmental Panel on Climate Change highest estimates of up to 5.8°C by 2100 (see related story) by examining the climate sensitivity – the change in equilibrium surface warming due to a doubling of the concentration of carbon dioxide in the atmosphere.

UI researchers Michael Schlesinger and Natalia Andronova used a simple climate/ocean model and the near-surface temperature record to estimate the probability density function – the likelihood of the climate sensitivity having any particular value. 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°C. “Consequently, there is a 54% 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 sulphate aerosol forcing but not solar variation. Although the value of the climate sensitivity in that case is most uncertain, there is a 70% 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 sulphate aerosol.

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… the crafting of robust adaptive climate-change policy could be greatly facilitated.”

University of Illinois at Urbana-Champaign

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