Without greenhouse-gas emissions from burning coal and oil, the odds are slim that 13 out of the 15 warmest years measured in the last 150 years would all have happened in the current century, reported a group of scientists from the United States and Germany Monday in the journal Scientific Reports.
"Natural climate variations just can't explain the observed recent global heat records, but man-made global warming can," Stefan Rahmstorf, a co-author from the Potsdam Institute of Climate Impact in Germany, said in a statement.
The natural climate variations the team took into account are volcanic eruptions and shifts in the sun's output, which can affect global temperatures.
By running statistical analyses of real-world measurements and comprehensive computer simulations of the climate system, they were able to better separate natural climate variability from human-caused climate change.
As a result, they found that the odds are between one in 770 and one in 10,000 that the record global temperatures between 2000 and 2014 resulted from natural forces. When they included data for 2015, which came in after the study was completed and showed that the year was the hottest since records began in the 19th century, the odds were even slimmer: between one in 1,250 and one in 13,000.
“While considerably greater than cited in some media reports, the odds are low enough to suggest that recent observed runs of record temperatures are extremely unlikely to have occurred in the absence of human-caused global warming,” the authors wrote.
Previous research that was reported had determined the that the odds of random causes behind global warming were one in 27 million to one in 650 million.
The scientists tried to account for the fact that heat from one warm year spills over into the next, which means a record broken one year is related to the previous year before.
“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 Michael Mann, a meteorology professor at Penn State university, in an announcement.
“That makes it more challenging to accurately assess the likelihood of temperature records. Given the press interest, it seemed important to do this right, and address the interesting and worthwhile question of how unlikely it is that the recent run of record temperatures might have arisen by chance alone,” he said.
The UN World Meteorological Organization confirmed on Monday that the global average surface temperature in 2015 shattered all previous records, with 15 of the 16 hottest years on record occurring since 2000.
“We have reached for the first time the threshold of 1C above pre-industrial temperatures. It is a sobering moment in the history of our planet,” said WMO secretary-general Petteri Taalas.
This report contains material from Reuters.