Researchers at MIT have continued a study of climate risk and released a new report to show that even moderate carbon-reduction policies can substantially lower the risk of future climate change. It also shows that action is needed quickly if global emissions reductions are to provide a good chance of avoiding a temperature increase of more than 2°C above the pre-industrial level — a widely discussed target. But the researchers determined that failing to take prompt action could result in extreme changes that could become much more difficult, if not impossible, to control.

“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°C target," says 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. "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°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. He says the public discussion over climate change policies gets polarized as a debate between extreme views such 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."

That's why the research team looked at the probabilities of various outcomes, rather than focusing on the average outcome in a given climate model, he said. It was “a more useful way to think about it,” he added.

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

Not reinventing the wheel

Both studies used 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.

This new research involved hundreds of runs of the model using slight variations in input parameters, so that each run had about an equal chance of being correct based on what scientists know and understand currently. The MIT model is the only one that interactively includes detailed treatment of possible changes in human activities — such as the degree of economic growth, with its associated energy use, in different countries.

Quantifying the odds

"One of the common mistakes in the [scientific] literature," says Webster, "is to take several different climate models, each of which gives a 'best guess' of temperature outcomes, and take that as the uncertainty range. But that's not right. The range of uncertainty is actually much wider."

This study produced a direct estimate of probabilities by running 400 different probability-weighted simulations for each policy case, looking at the actual range of uncertainty for each of the many factors that go into the model, and how they interact. By doing so, the researchers say it produced more realistic estimates of the likelihood of various outcomes than other procedures — and the resulting odds are often significantly worse.

They say an earlier study, by Tom Wigley of the National Center for Atmospheric Research, estimated that the Level 1 emissions control policy — the least-restrictive of the standards studied - would reduce by 50 percent the odds of a temperature increase of more than 2°C, but the more detailed analysis in the new study finds only a 20 percent chance of avoiding such an increase.

The good news is that the team found that even relatively modest emissions-control policies can have a big impact on the odds of the most damaging climate outcomes. For example, under the strongest of the four policy options, the average projected outcome was a 1.7°C reduction of the expected temperature increase in 2100, but for the most extreme projected increase (with 5 percent probability of occurring) there was a 3.2°C reduction. And that's especially significant, the authors say, because the most damaging effects of climate change increase drastically with higher temperature, in a very non-linear way.

"These results illustrate that even relatively loose constraints on emissions reduce greatly the chance of an extreme temperature increase, which is associated with the greatest damage," the report concludes.

Webster says: "this is a problem of risk management," and adds that, while the technical aspects of the models are complex, the results provide information that's not much different from decisions that people face every day. He used the analogy of people using their seat belts and having a car with airbags to reducing the risks of injuries while driving, but that it didn’t mean they couldn’t still be injured or killed. "No, but the risk goes down. That's the return on your decision. It's not something that's so unfamiliar to people. We may make sure to buy a car with airbags, but we don't refuse to leave the house. That's the nature of the kind of tradeoffs we have to make as a society."