By 2050, 95 percent of very hot days will be caused by human-influenced global warming

Computer models show potential impact of greenhouse gas emissions.
By Laurel Kornfeld | Apr 28, 2015
Using computer models based on current global temperature data, a team of Swiss scientists determined that 95 percent of extremely hot days worldwide in 2050 will result directly from global warming caused by human activity emitting greenhouse gases.

Zurich Erich Fischer,ETH at Zurich climate scientist and fellow researcherReto Knutti used 25 separate computer models to examine the hottest one tenth of one percent of current hot days around the globe.

In computer simulations of a world without human emissions of greenhouse gases, such days occurred only once every three years.

Simulations of a world with current levels of man-made greenhouse gas emissions resulted in these hot days occurring four times every three years.

That means three of the four super hot days are caused by human activity, they noted.

Projecting forward to 2050 based on current levels of man-made greenhouse gas emissions, the computer model showed 26 super hot days occurring every three years.

Fischer and Knutti acknowledge the study's margins of error is plus or minus 13 percent and grows larger when focused on smaller regions.

Africa and South America currently have the highest percentage of very hot days attributable to human-caused global warming, Eighty-eight and 89 percent of their very hot days can be blamed on human activity, in contrast to 67 percent for North America and 63 percent for Europe.

If human emissions continue at their current rate, by 2050, at least 93 percent of very hot days in 2050 will be directly attributable to man-made global warming.

The studies show less of a human effect on extreme rainfall, attributing 18 percent of today's severe downpours to global warming.

Current predictions estimate the world will warm another two degrees Fahrenheit by 2050 (1.1 degrees Celsius) if no reductions in greenhouse gas emissions are made.

That level of warming would increase the percentage of extreme rain events worldwide attributable to human activity to 39 percent.

Many scientists not involved with the study, published Monday in the journal Nature Climate Change, commended it.

One of them, Princeton University climate scientist Michael Oppenheimer, said the questions that should be asked about extreme weather events are not whether individual ones are caused by human activity but, "How much of the change is due to human activity and how much is natural variation?"

Drew University climate scientist Drew Shindell said knowing the percentage of extreme weather events caused by human activity will enable governments to assign a dollar amount to carbon dioxide emissions as part of a policy aimed at reducing them.

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