German and Spanish researchers writing in the journal Climate Change report that on average, there are now five times as many record-breaking hot months worldwide than could be expected without long-term global warming.
In parts of Europe, Africa and southern Asia, the figure is even higher with the number of monthly records increasing by a factor of 10, they said.
The increase was revealed by analysis of 131 years of monthly temperature data for more than 12,000 grid points around the world provided by NASA, the researchers from Potsdam Institute for Climate Impact Research and Complutense University of Madrid said.
"The last decade brought unprecedented heat waves; for instance in the United States in 2012, in Russia in 2010, in Australia in 2009, and in Europe in 2003," study lead author Dim Coumou says. "Heat extremes are causing many deaths, major forest fires and harvest losses -- societies and ecosystems are not adapted to ever new record-breaking temperatures."
The surge in the number of records has been particularly steep over the last 40 years, due to a steep global-warming trend during this time, the researchers said.
"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," Potsdam researcher Stefan Rahmstorf said. "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."