It’s getting hotter faster: a sobering new MIT study on the odds of global warming shows that without drastic action, the planet is likely to heat up twice as much this century as previously projected.

Researchers in the Joint Program on the Science and Policy of Global Change reached these conclusions using the MIT Integrated Global Systems Model, a detailed computer simulation of global economic activity and climate change that they’ve been refining since the 1990s. They ran the model 400 times, varying the values of the input parameters each time in such a way that each run had about an equal probability of being correct. This process reflects the fact that each of dozens of parameters–the rate at which the ocean’s surface waters mix with deeper waters, the rate at which new coal plants will be built–has its own range of uncertainty. The multiple runs show how these parameters, whatever their specific values, interact to affect overall climate.

Study coauthor Ronald Prinn, ScD ‘71, the program’s codirector, says the MIT model is the only one that looks at the effects of economic activity in concert with the effects of changes in atmospheric, oceanic, and biological systems. The new study, published in the American Meteorological Society’s Journal of Climate, takes into account recent changes in projections about the growth of economies such as China’s, as well as new data on some physical processes, such as the rate at which oceans take up both heat and carbon dioxide. When results of the different scenarios are averaged together, the median projection shows land and ocean surfaces warming 5.2 °C by 2100. The researchers’ 2003 study, which was based on results from an earlier version of the model, projected an increase of just 2.4 °C.

Prinn and the team used the new results to update their “roulette wheel,” a pie chart representing the relative odds of various levels of temperature rise (see “Wheel of Global Fortune,” January/February 2008). The new version of the wheel reflects a much higher probability of greater temperature increases if no actions are taken to reduce greenhouse-gas emissions.

To help raise general awareness of how serious the problem is, the MIT team also developed a method for estimating the current output of greenhouse gases around the world, which is used to continuously update a “carbon counter” on a giant billboard outside New York’s Madison Square Garden. Sponsored by Deutsche Bank, the 70-foot-tall display is similar to the famed national-debt clock in Times Square.

But there’s good news from the model as well: if substantial measures to curb greenhouse-gas emissions are put in place soon, the new projections show, the expected warming will be no worse than the earlier studies suggested. “This increases the urgency for significant policy action,” Prinn says