Warming’s role in extreme weather quantified

Scientists have long suspected that some surges in extreme weather — from devastating droughts to thrashing superstorms — are caused by global warming. And now scientists have numbers to support that idea.

About 75 percent of extreme heat spikes and 18 percent of extreme precipitation over land worldwide can be blamed on this largely human-driven climate change, researchers report April 27 in Nature Climate Change. And if the planet’s temperature rises 2 degrees Celsius over preindustrial conditions, global warming will be responsible for nearly all heat extremes and about 40 percent of heavy precipitation over land, the authors report. The current era is experiencing warming of 0.85 degrees C above preindustrial levels.

“This is a considerable fraction” of extreme weather events, says climate scientist Peter Stott of the Met Office Hadley Centre in Exeter, England, who was not involved with the study. Overall, the study shows that “relatively small rises in global temperature translate into large increases in the likelihood of extremes,” he says.

Researchers have long thought that global warming would increase some extreme weather. But much of the research has focused on understanding climate change’s role in favoring specific events, such as headline-grabbing superstorms. In the new study, the researchers took a wider view.

“This is a global statement,” says climate scientist Erich Fischer of ETH Zurich in Switzerland. Over the whole planet, there will be more heat and precipitation extremes due to global warming. Each region of the world might experience that differently, he says — areas such as the tropics and high latitudes may experience more or less of those extremes.

To come up with the worldwide numbers, Fischer and climate scientist Reto Knutti, also at ETH Zurich, analyzed 25 different computer simulations of global climate. The simulations ran through different climate periods, from preindustrial times into the current era and on to periods of 2-degree and 3-degree warming.

The researchers used the averages from the 25 simulations to find the likelihood of extreme heat and precipitation during each of those climate periods. Specifically, the researchers looked for instances of daily high temperatures or heavy rainfall that would occur only once every 1,000 days in preindustrial times.

Such rare events were not so rare in warmer periods, Fischer and Knutti found. They then computed how much of the increase in extremes could be directly linked to global warming, which most experts have concluded is largely driven by humans. Some extremes might also be caused by other alterations in climate or natural extremes.

“You can still get the biggest heat that you have ever seen without any human changes,” says climate scientist Sebastian Sippel of Max Planck Institute for Biogeochemistry in Jena, Germany. But the study clearly shows that warming is having an effect, Sippel says.

Of course, the new results can’t identify which extreme temperatures or storms are directly due to human-caused climate change, Sippel says. It’s similar to a doctor not being able to tell exactly which cigarette caused a smoker’s lung cancer.

Despite the lack of such fine-grained detail, Fischer is hopeful that the new calculations will guide decision-makers and spur researchers to the next step of calculating which populations will suffer the brunt of the extra extremes.