21st century warm streak ‘almost certainly’ human caused

Chances of record-breaking heat happening without emissions from burning fossil fuels are 1 in 5000 or less, scientists calculate

(Flickr/Toshiyuki IMAI)

(Flickr/Toshiyuki IMAI)

By Megan Darby

The chances of natural variability causing the record warm streak this century are vanishingly small.

Greenhouse gases from burning fossil fuels are “almost certainly” behind the surge in global temperatures, according to a study published in Nature.

Without human activities, scientists calculate the odds are between 1 in 5000 and 1 in 170000 of the pattern seen between 2000 and 2014: thirteen of the 15 warmest years on record.

“2015 is again the warmest year on record, and this can hardly be by chance,” said co-author Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research.

Media coverage has played up the role of El Nino in the last two years of extreme warmth, but that does not explain the medium term trend.

“Natural climate variability causes temperatures to wax and wane over a period of several years, rather than varying erratically from one year to the next,” said lead author Michael Mann, director of the Earth System Science Center, Penn State University.

“That makes it more challenging to accurately assess the chance likelihood of temperature records. Given the recent press interest, it just seemed like it was important to do this right, and address, in a defensible way, the interesting and worthwhile question of how unlikely it is that the recent run of record temperatures might have arisen by chance alone.”

Read more on: Climate science