A new comprehensive MIT study of the probabilistic projections of climate change in this century found that absent aggressive intervention, warming will likely be about twice as severe as previously estimated by the MIT model six years ago.

The new projections, published this month in the American Meteorological Society’s Journal of Climate, indicate a median probability of surface warming of 5.2 °C by 2100, with a 90% probability range of 3.5 to 7.4 degrees. This can be compared to a median projected increase in the 2003 study of 2.4 degrees.

However, if recently published data, suggesting stronger 20th century ocean warming, are used to determine the input climate parameters, the median projected warning at the end of the 21st century is only 4.1 °C. Nevertheless all our simulations have a much smaller probability of warming less than 2.4°C, than implied by the lower bound of the IPCC AR4 projected likely range for the A1FI scenario, which has forcing very similar to our median projection. —Sokolov et al. (2009)

The difference is caused by several factors rather than any single big change. Among these are improved economic modeling and newer economic data showing less chance of low emissions than had been projected in the earlier scenarios. Other changes include accounting for the past masking of underlying warming by the cooling induced by 20th century volcanoes, and for emissions of soot, which can add to the warming effect. In addition, measurements of deep ocean temperature rises, which enable estimates of how fast heat and carbon dioxide are removed from the atmosphere and transferred to the ocean depths, imply lower transfer rates than previously estimated.

Study co-author Ronald Prinn, the co-director of the Joint Program and director of MIT’s Center for Global Change Science says these and a variety of other changes based on new measurements and new analyses changed the odds on what could be expected in this century in the “no policy” scenarios—that is, where there are no policies in place that specifically induce reductions in greenhouse gas emissions. Overall, the changes “unfortunately largely summed up all in the same direction,” he says. “Overall, they stacked up so they caused more projected global warming.”

The study uses the MIT Integrated Global Systems Model, a detailed computer simulation of global economic activity and climate processes that has been developed and refined by the Joint Program on the Science and Policy of Global Change since the early 1990s.

The new research involved 400 runs of the model with each run using slight variations in input parameters, selected so that each run has about an equal probability of being correct based on present observations and knowledge. Other research groups have estimated the probabilities of various outcomes, based on variations in the physical response of the climate system itself. But the MIT model is the only one that interactively includes detailed treatment of possible changes in human activities as well—such as the degree of economic growth, with its associated energy use, in different countries.

While the outcomes in the “no policy” projections now look much worse than before, there is less change from previous work in the projected outcomes if strong policies are put in place now to drastically curb greenhouse gas emissions. Without action, “there is significantly more risk than we previously estimated,” Prinn says. “This increases the urgency for significant policy action.”

The lead author of the paper describing the new projections is Andrei Sokolov, research scientist in the Joint Program. Other authors, besides Sokolov and Prinn, include Peter H. Stone, Chris E. Forest, Sergey Paltsev, Adam Schlosser, Stephanie Dutkiewicz, John Reilly, Marcus Sarofim, Chien Wang and Henry D. Jacoby, all of the MIT Joint Program on the Science and Policy of Global Change, as well as Mort Webster of MIT’s Engineering Systems Division and D. Kicklighter, B. Felzer and J. Melillo of the Marine Biological Laboratory at Woods Hole.

This work was supported in part by grants from the Office of Science of the US Dept. of Energy, and by the industrial and foundation sponsors of the MIT Joint Program on the Science and Policy of Global Change.

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