Blame global warming for about 75 percent of the world’s unusually hot days and 18 percent of its extreme snow or rain, according to a new paper in the journal Nature Climate Change.

Heat waves and heavy storms are occurring at least four times more often than they did before carbon pollution started driving up thermometers. Global average temperatures are now about 0.85 degrees Celsius (1.4 Fahrenheit) higher than before industrialization.

Additional heat and precipitation are expensive. Severe weather costs the U.S. economy as much as $33 billion a year, according to a U.S. Energy Department report released April 21.


And those figures will increase as the planet continues to warm, as climate change may not be smooth or gradual, according to the new paper. At 2 degrees Celsius — United Nations climate negotiators’ avowed upper limit — extremely hot days may be twice as likely as at 1.5 degrees Celsius of warming. After 2 degrees Celsius, the odds of high-heat days may be five times greater than today.

“What used to be a one-in-a-thousand day, a one-in-three-year event, actually occurs four times in three years,” Erich Fischer, a researcher at the Institute for Atmospheric & Climate Science in Zurich and one of the study’s authors, said in a telephone interview. “Weather extremes have always been occurring, before any human influence, but that doesn’t mean that there can’t be human influence on the extremes.”

Scientific Consensus

There is widespread scientific consensus that human activity is causing climate change, which President Barack Obama said “can no longer be denied” while visiting the Florida Everglades last week.

Fischer and colleague Reto Knutti examined more than a century’s worth of data using more than two dozen climate models, running simulations that account for rising greenhouse-gas emission levels through the year 2100. Fischer was quick to emphasize the global nature of their results. The study is confined to heat and precipitation. How might evolving conditions affect meteorological events like tornadoes or hail? “We do not agree how they change,” he said.

Global warming’s responsibility for the world’s heavy rainfall may increase from 18 percent today to 40 percent if temperatures rise by 2 degrees Celsius.

The authors said that their results may be of use to policy makers, by sharpening their understanding of risk as climate change worsens. “With every degree of warming,” they wrote, “it is the rarest and the most extreme events — and thereby the ones with typically the highest socioeconomic impacts — for which the largest fraction is due to human-induced greenhouse gas emissions.”

Translation: The worse things get, the more we’re bringing it on ourselves.