In predicting heatwaves in a warmer world, climate models vary a lot in their results—from mildly worse (top) to real scorchers (bottom). Image courtesy of the researchers.

After the United Nation’s climate change conference in Copenhagen at the end of last year, most of the world’s big carbon dioxide emitters—including the United States, the European Union, and China—signed the “Copenhagen Accord,” saying they would not allow Earth to warm any more than an average of 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial temperatures. While they didn’t say how they’d achieve it, they felt staying below this target would help the Earth avoid “dangerous” changes in the climate.

But even if climate change is limited to 2 degrees Celsius on average, some parts of the world will likely be hit with scorching heat waves unlike any they suffer today, according to a new study to be published in Geophysical Research Letters, a journal of the American Geophysical Union. As dire as that may sound, there’s a lot of uncertainty about how bad these heat waves might actually be: there’s a very small chance they’ll be mild, but there’s a much bigger chance that they could be really extreme.

Researchers Robin Clark, James Murphy and Simon Brown at the Hadley Centre, part of the United Kingdom’s Met Office and located in Devon, England, used a streamlined version of a very successful computer model of the climate, known as HadCM3, to develop the findings. They simplified the ocean, so that they could run scenarios more quickly, and surveyed climate scientists to establish a plausible range of numbers for other parameters that capture aspects of the world that they can’t simulate in detail, such as how deep plants’ roots go into the ground, or the ups and downs of craggy mountains.

The researchers plugged the different parameters into the model, and ran it over and over—224 times—to get a plausible range of outcomes of the future. They looked at all the models that warmed by about 2 degrees Celsius overall, and measured the changes in the heat waves in these warmer worlds — such as how much hotter the hottest day would get. And they found that some places would suffer far worse heat waves than others.

“Even with a global two-degree target, in reality, some places could wind up with a six-degree change in the extremes,” says lead author Robin Clark. “For unlucky parts of the world, such as southern Europe and much of North America, such large changes could be dangerous.”

They also found there was a large spread in what the models predicted would happen for a particular region, even though the models agreed on the average warming worldwide. In a band across New England and the Great Lakes region, for example, some of the models predicted that heat waves would be about 2 degrees to 4 degrees Celsius hotter, whereas other models predicted the worst heat waves would peak at about 8 degrees to 10 degrees Celsius above those of today.

Clark says he hopes the findings will alert people about what could happen. “If we’re not careful, we could wind up with surprises along the way,” he says. One surprise could be that some places will cross a threshold, and then warm much more rapidly. In the models, Clark and colleagues found that a key factor limiting severe heat spikes was how much water is in the soil. In areas where the soil is moist, the stored water evaporates during heat waves, providing a natural buffer that keeps the land from getting as hot as it otherwise would.

“But with climate change, many places would dry out, and you wouldn’t have that soil- moisture cooling effect,” Clark says. Then, “when you cross over that threshold, you can have a very big change in temperature.”

That’s what happened in the European heat wave of 2003, which killed more than 30,000 people. The summer was preceded by a dry winter and spring, “so when summer came, there wasn’t that cooling effect” from the soil, Clark says.

It will be a major task to limit warming to 2 degrees Celsius, since it would require the world to completely change how it generates and uses energy. The pledges already on the books—such as Norway’s pledge to go carbon neutral by 2030, and the European Union’s goal of getting down to around 5 to 20 percent of its 1990 emissions by mid-century—most likely won’t be enough to meet the 2-degree target, according to a recent analysis. (See the Climate Action Tracker.)

But, as this study finds, even if we were to meet this target, it most likely wouldn’t be enough to protect large parts of the planet. “Even with just 2 degrees” of warming, Clark says, “a lot of damage could occur.”

— Mason Inman, contributing science writer