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May 20, 2009

MIT Finds Increased Warming Threat if Greenhouse Gases Stay Unchecked

The probable global temperature increases in degrees Celsius over the next 100 years are illustrated in two pie charts. The 'no policy' pie chart shows a less than 1% chance of a temperature increase less than 3 degrees Celsius, a 12% chance of 3 to 4 degrees, a 30% chance of 4 to 5 degrees, a 33% chance of 5 to 6 degrees, a 15% chance of 6 to 7 degrees, and a 9% chance of greater than 7 degrees. The

These two "roulette wheels" illustrate the probable global temperature rise if no action is taken to curb greenhouse gas emissions and if policies are enacted to limit greenhouse gas emissions.
Credit: MIT Joint Program on the Science and Policy of Global Change

In the absence of new policies to limit greenhouse gas emissions, calamitous global warming appears much more likely now than it did six years ago, according to comprehensive climate modeling by the Massachusetts Institute of Technology (MIT). The MIT Joint Program on the Science and Policy of Global Change uses a detailed computer simulation of global economic activity and climate processes and runs it 400 times, making slight changes to both the climate responses and the economic growth projections. The result is a probabilistic assessment of climate outcomes.

A similar study in 2003 found that a global temperature increase of 2.4°C by 2100 was the most likely outcome, but the newly updated study raised that to 5.2°C, with a 90% probability that the temperature increase would fall between 3.5°C and 7.4°C. In contrast, most climate scientists recommend that global temperature increases be maintained below 2°C. The scientists also examined the outcomes for greenhouse gas control measures that would stabilize the concentration of carbon dioxide in the atmosphere at 550 parts per million (an equivalent of 675 parts per million when all greenhouse gases are accounted for), and found a median warming level of 2.3°C, with a 20% chance of keeping global warming below 2°C.

According to the MIT researchers, the new study differs from the older study in several ways. First, it draws on improved economic modeling and newer data that shows less chance of lower greenhouse gas emissions. It also accounts for the effects of volcanoes, which masked some warming in the 20th century; for soot, which causes warming; and for a lower removal of carbon dioxide by the oceans. Yet the model does not include potential positive feedbacks, such as the emission of methane by melting permafrost, which would make the outcomes even more drastic. See the MIT press release, the study, and an in-depth description of the scenarios and outcomes examines in the study.

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