Long-term global warming has brought the world five times as many record-breaking hot months worldwide - ten times as many in parts of Europe, Africa and southern Asia. And it's only going to get worse.
"Statistics alone cannot tell us what the cause of any single heat wave is, but they show a large and systematic increase in the number of heat records due to global warming," says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research (PIK).
"Today, this increase is already so large that by far most monthly heat records are due to climate change. The science is clear that only a small fraction would have occurred naturally."
The new study is based on 131 years of monthly temperature data from NASA, covering more than 12,000 grid points around the world. The team says the surge in the number of record-setting hot months has been particularly steep over the last 40 years.
While there is an identifiable effect from natural variability, with especially high numbers of heat records during El Niño years, this isn't nearly enough to explain the overall development of record events, says the team.
If global warming continues, they say, the number of new monthly records will be 12 times as high in 30 years as it would be without climate change.
"Now this doesn't mean there will be 12 times more hot summers in Europe than today - it actually is worse," points out PIK's Dim Comou.
"To count as new records, they actually have to beat heat records set in the 2020s and 2030s, which will already be hotter than anything we have experienced to date. And this is just the global average - in some continental regions, the increase in new records will be even greater."