2015 is the warmest year on record and this can hardly be by chance, say researchers, adding that record warm years are almost a certainty in this century owing to human-made climate change.
Without greenhouse-gas emissions from burning coal and oil, the odds are vanishingly small that 13 out of the 15 warmest years ever measured would all have happened in the current, still young century, said the researchers from Germany-based Potsdam Institute for Climate Impact Research.
To understand this, the scientists performed a sophisticated statistical analysis, combining observational data and comprehensive computer simulations of the climate system.
Their new approach allowed them to better separate natural climate variability from human-caused climate change.
"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, Earth System Science Centre at Pennsylvania State University.
What is more, the anomalous global average warmth comes with substantial impacts.
"It has led to unprecedented local heat waves across the world - sadly resulting in loss of life and aggravating droughts and wildfires," Rahmstorf pointed out.
The risk of heat extremes has been multiplied due to our interference with the Earth system, as our data analysis shows, he added in a paper published in the journal Scientific Reports.
That makes it more challenging to accurately assess the chance likelihood of temperature records.
Taking human-caused global warming into account makes the recent record temperatures quite likely, the study further shows.
"Natural climate variations just can't explain the observed recent global heat records, but man-made global warming can," Rahmstorf noted.
