Global warming to blame for 75 percent of hottest days

http://www.cbsnews.com/news/global-warming-to-blame-for-75-percent-of-very-hot-days/

April 27, 2015

If you find yourself sweating out a day that is monstrously hot, chances are you can blame humanity. A new report links three out of four such days to man's effects on climate.

And as climate change worsens around mid-century, that percentage of extremely hot days being caused by man-made greenhouse gases will push past 95 percent, according to the new study published Monday in the journal Nature Climate Change.

Humans have not had as great an effect on heavy downpours, though. The Swiss scientists who did the study calculated that 18 percent of extreme rain events are caused by global warming. But if the world warms another two degrees Fahrenheit (1.1 degrees Celsius) - expected to happen around mid-century - about 39 percent of the downpours would be attributed to humanity's influence, according to the study. That influence comes from greenhouse gases, mostly carbon dioxide from the burning of coal, oil and gas.

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Lead author Erich Fischer, a climate scientist at ETH Zurich, a Swiss university, and colleague Reto Knutti examined just the hottest of hot days, the hottest one-tenth of one percent. Using 25 different computer models. Fischer and Knutti simulated a world without human-caused greenhouse gas emissions and found those hot days happened once every three years.

Then they calculated how many times they happen with the current level of heat-trapping gases and the number increases to four days. So three of the four are human caused, the team said.

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The figures that Fischer and Knutti calculated are global estimates. The margins of error, plus or minus about 13 percent with current hot days, grow larger when smaller regions are considered. However, they found Africa and South America now have the highest percentages of unusual hot days that could be blamed on human influence, 89 percent and 88 percent respectively. Europe, at 63 percent, and North America, with 67 percent, come in at the lowest. By mid-century, if emissions continue at current pace, all continents will be able blame at least 93 percent of super hot days on humans.

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When people ask if a single weird weather event is due to human activity or just natural variation, that's the wrong question because both factors are always involved, said Princeton University climate scientist Michael Oppenheimer, who wasn't part of the study but praised it heavily. This study, he said, asks the right question: "How much of the change is due to human activity and how much is natural variation?"

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