Limiting the Risk of Climate Catastrophe

Photo: Ronald Prinn, director of MIT's Center for Global Change Science, and his group have revised their model that shows how much hotter the Earth's climate will get in this century without substantial policy change. Standing with the group's "roulette wheel" are, from left to right, Mort Webster, professor in the Engineering Systems Division; Adam Schlosser, principal research scientist at the Center for Global Change Science; Prinn, the TEPCO Professor of Atmospheric Chemistry; and Sergey Paltsev, principal research scientist, MIT Energy Initiative. (Photo: Donna Coveney)

A new analysis of climate risk, published by researchers at MIT and elsewhere, shows that even moderate carbon-reduction policies now can substantially lower the risk of future climate change. It also shows that quick, global emissions reductions would be required in order to provide a good chance of avoiding a temperature increase of more than 2 degrees Celsius above the pre-industrial level - a widely discussed target. But without prompt action, they found, extreme changes could soon become much more difficult, if not impossible, to control.

Ron Prinn, co-director of MIT's Joint Program on the Science and Policy of Global Change and a co-author of the new study, says that "our results show we still have around a 50-50 chance of stabilizing the climate" at a level of no more than a few tenths above the 2 degree target. However, that will require global emissions, which are now growing, to start downward almost immediately. That result could be achieved if the aggressive emissions targets in current U.S. climate bills were met, and matched by other wealthy countries, and if China and other large developing countries followed suit with only a decade or two delay. That 2 degree C increase is a level that is considered likely to prevent some of the most catastrophic potential effects of climate change, such as major increases in global sea level and disruption of agriculture and natural ecosystems.

"The nature of the problem is one of minimizing risk," explains Mort Webster, assistant professor of engineering systems, who was the lead author of the new report. That's why looking at the probabilities of various outcomes, rather than focusing on the average outcome in a given climate model, "is both more scientifically correct, and a more useful way to think about it."

Illustration: To illustrate the findings of their model, MIT researchers created a pair of 'roulette wheels.' This wheel depicts their estimate of the range of probability of potential global temperature change over the next 100 years if no policy change is enacted on curbing greenhouse gas emissions. (Image courtesy: MIT Joint Program on the Science and Policy of Global Change)

Too often, he says, the public discussion over climate change policies gets framed as a debate between the most extreme views on each side, as "the world is ending tomorrow, versus it's all a myth," he says. "Neither of those is scientifically correct or socially useful."

"It's a tradeoff between risks," he says. "There's the risk of extreme climate change but there's also a risk of higher costs. As scientists, we don't choose what's the right level of risk for society, but we show what the risks are either way."

The new study, published online by the Joint Program in September, builds on one released earlier this year that looked at the probabilities of various climate outcomes in the event that no emissions-control policies at all were implemented - and found high odds of extreme temperature increases that could devastate human societies. This one examined the difference that would be made to those odds, under four different versions of possible emissions-reduction policies.