MIT Group Increases Global Warming Projections

By Andrew Freedman

Report: High odds of warming over 5°C (9°F) if no action



Warming possibilities for "no policy" and policy scenarios between 1990 and 2100. Size of pie slice indicates the likelihood of a given amount of warming under the different scenarios. Graphics courtesy MIT Joint Program on the Science and Policy of Global Change.

New research from MIT scientists shows that in the absence of stringent reductions in greenhouse gas emissions, 21st century climate change may be far more significant than some previous climate assessments had indicated.

The new findings, released this month by MIT's Joint Program on the Science and Policy of Global Change, showed significantly increased odds that by the end of the century warming would be on the high end of the scale for a so-called "no policy scenario" as compared with similar studies completed just six years ago. The main culprits: the cycling of heat and carbon dioxide in the climate system are now better understood and projections of future greenhouse gas emissions have increased.

The results also showed that even if nations were to act quickly to reduce emissions, it is more likely that warming would be greater than previous studies had shown. However, the increase in projected temperatures under the "policy scenario" was not as large as for the no policy scenario.

Keep reading for more information about MIT's revised projections...

The modeling experiments are not meant to provide precise forecasts of future temperature changes, but rather to serve as what one related MIT study calls "thought experiments" to help policymakers and the public understand how decisions regarding emissions reductions may affect the magnitude of climate change. They show how human activities are loading the dice in favor of a warming climate, and cast doubt on the feasibility of limiting temperature increases to the lower range of what the influential U.N. Intergovernmental Panel on Climate Change projected in its most recent assessment report in 2007.



To update their findings that were first unveiled in 2002, the MIT researchers used an in-house computer model known as the MIT Integrated Global System Model that incorporated new insight into how the climate system functions.

A main conclusion is that feedbacks within the climate system, which MIT says can be more accurately simulated in the updated version of its model, may act to increase the magnitude of climate change in response to increasing atmospheric concentrations of greenhouse gases. The more greenhouse gases in the atmosphere, MIT researchers reported, the more significant the feedbacks may be.

Results of the studies are depicted online in MIT's "Greenhouse Gamble" exercise that conveys the "range of probability of potential global warming" via roulette wheel graphics (shown above). The modeling output showed that under both a "no policy" scenario and one in which nations took action beginning in the next few years to reduce greenhouse gas emissions, the odds have shifted in favor of larger temperature increases.

For the no policy scenario, the researchers concluded that there is now a nine percent chance (about one in 11 odds) that the global average surface temperature would increase by more than 7°C (12.6°F) by the end of this century, compared with only a less than one percent chance (one in 100 odds) that warming would be limited to below 3°C (5.4°F) .

The median warming value, with even odds of the temperature increase being above or below that value, was 5.1°C (9.2°F). For comparison, the same research group's no policy scenario yielded a median value of just 2.4°C (4.3°F) in 2002.

"The take home message from the new greenhouse gamble wheels is that if we do little or nothing about lowering greenhouse gas emissions that the dangers are much greater than we thought three or four years ago," said Ronald G. Prinn, professor of atmospheric chemistry at MIT. "It is making the impetus for serious policy much more urgent than we previously thought."

According to the research group, there was no single factor that caused the new computer modeling to project a greater amount of warming compared with their 2002 simulations.

"In our more recent global model simulations, the ocean heat-uptake is slower than previously estimated, the ocean uptake of carbon is weaker, feedbacks from the land system as temperature rises are stronger, cumulative emissions of greenhouse gases over the century are higher, and offsetting cooling from aerosol emissions is lower," the group's web site states. "Rather than interacting additively, these different affects appear to interact multiplicatively, with feedbacks among the contributing factors, leading to the surprisingly large increase in the chance of much higher temperatures."

Under the policy scenario, in which global carbon dioxide concentrations would reach about 550 parts per million by 2100 (the current level is about 385 ppm), the projected magnitude of climate change is significantly less than under the no policy scenario, but it still would warm more significantly than the 2002 projections. Under the policy scenario, there is a 90 percent chance that climate change could be limited to below 3°C (5.4°F), compared to just a one percent chance of that occurring in the no policy case.



"If greenhouse gas emissions are controlled to relatively low levels then the Earth systems feedbacks are much lower and the slight difference in Earth system properties is not as important -- again a result of the way in which these different factors interact multiplicatively," the researchers stated.

As with any computer modeling work, there are uncertainties involved in the projections, and MIT researchers went to great lengths to quantify them in their estimates of temperature increases and future greenhouse gas emissions. Prinn said that rather than obscuring the uncertainties, the roulette wheels "give our best shot of showing what the uncertainties are and at the same time showing the value of a policy."

The MIT research is sponsored by a consortium of government agencies, including the Department of Energy and U.S. EPA, as well as companies that include several major oil companies such as ExxonMobil and Chevron.

Technical papers accompanying the results can be found here and here.

What do you think of the use of a roulette wheel to convey the odds of a warming climate? Might that approach be applicable to weather forecasts too? You're welcome to share your thoughts in the comments section below.