Climate Action May Shift Warming Odds

With aggressive curtailment of global carbon emissions, we have a 50-50 chance of stabilizing the world's average temperature increase at around 4 degrees Fahrenheit (2 degrees Celsius), a commonly cited target for avoiding the worst effects of climate change.

So says a new report led by Massachusetts Institute of Technology Assistant Professor Mort Webster. The team combined economic data and climate models to calculate the probabilities of various climate outcomes and how much each would cost to achieve.

Aggressive action on emissions could help to avert some of the most catastrophic consequences of global warming, such as rising sea levels that swallow up coastlines and disruptions of agriculture and natural ecosystems.

"Some people are saying the world ends tomorrow if you don't do everything. Other people are saying, 'It's all a lie, don't do anything.' It has become so polarized," Webster said.

"We wanted to present a more balanced view, to say the purpose of climate policies and targets is to manage risks," he added. "You can never drive them to zero. How much do you want to manage?"

A problem with climate discussions, Webster said, is that they tend to focus on specific outcomes, like limiting temperature increases. They assume that if we achieve a particular atmospheric carbon dioxide concentration, the temperature increase will follow.

In reality, he said, the connections are much more uncertain, so he prefers to talk about the probability that a given temperature increase will result from a set of actions.

"People are walking around with this false sense of security that if we do 'this' then we get 'that'. If you do 'this', you still have some chance that you'll go over 2 degrees (Celsius), but it's less," Webster said. "The way to think about choosing among these targets is, 'How much is it worth it to pay to reduce the risk?'"

For instance, the study finds that with no action to curb emissions, there is a one in four chance that the global average temperature increase would be more than 10 degrees Fahrenheit (6 degrees Celsius). But even the least stringent emission reductions the group considered drop the chance to one in 400.

Such an increase would bring pretty catastrophic changes, however. The least stringent emission reductions give a one in four chance that the temperature change will be limited to 7 degrees Fahrenheit (4 degrees Celsius).

The most stringent set of emission reductions are required for a one in four chance of keeping the temperature change under 3.6 degrees Fahrenheit (2 degrees Celsius), the team found.

The work shows that at a certain point, the marginal gain from further risk reductions gets smaller.

Even with uncertain outcomes, he notes, it makes sense to manage risk: "You wear a seatbelt, but on the average day, you don't get into an auto accident. You're trying to lower the odds that the one day that you get into an auto accident, you will survive."

"My immediate reaction to the calculation is that they are very optimistic," said Inez Fung of the University of California, Berkeley, referring to the 50-50 chance of stabilizing temperature at around 4 degrees Fahrenheit warmer.

One thing the model does not include is the melting of the large ice sheets in Greenland and Antarctica, she noted, which would accelerate temperature increase for a given set of emissions.

Nonetheless, she said: "I fully agree with their conclusion. We have to act now. Whether the cup is half empty or the cup is half full, we have to act now."