Global warming made this summer's record heat across Southern Europe—with its wildfires and a heat wave so vicious it was nicknamed "Lucifer"—10 times more likely than it would have been in the early 1900s, scientists said today in a study published by the World Weather Attribution research group. If greenhouse gas emissions aren't cut soon, such heat waves will be the regional summer norm by 2050, the study concluded.
The scientists, from universities and research institutions in Europe and the United States, said they are more certain than ever that human-caused global warming is a key driver of the extreme heat.
As the average global temperature goes up, it becomes easier to pick out the climate change signal, said lead author Sarah Kew, a climate researcher with the Royal Netherlands Meteorological Institute.
The research is the newest in a series of climate attribution studies assessing how heat-trapping pollution affects recent extreme weather events like heat waves, droughts and extreme rainfall. The findings are crucial for governments that have to prepare for more extreme climate events ahead.
2003's Extreme Heat Set off Warning Bells
The urgency of improving understanding of the heat-related health risks from global warming was made clear in 2003, when the most extreme European heat wave on record killed more than 70,000 people. The summer of 2003 is still the hottest on record for the whole of Europe, although 2017 was hotter in the Mediterranean region.
A landmark climate attribution study in 2004 determined that the buildup of greenhouse gases from burning fossil fuels made the extreme temperatures of 2003 at least twice as likely as they would have been a world with no human-caused greenhouse gases.
Since then, the global average temperature has increased by another quarter degree Celsius and Southern Europe summers are warming at twice that rate, according to the European Environment Agency. Scientific understanding of the influence of climate change has also advanced.
This summer's heat wave started on the Iberian Peninsula in June—unusually early— and fueled deadly forest fires in Portugal. In August and early September, temperatures hit record highs and contributed to crop failures in the Balkans. The hot conditions also contribute to a water shortage and rationing in Rome.
2017's Heat 'Not All that Rare Anymore'
Attribution studies create digital models of the climate system to compare how it acts with and without the heat-trapping effect of greenhouse gases from human activities.
"We found that the 2017, heat was not all that rare anymore. Due to global warming, there's a 10 percent chance every year in many places," Kew said. The study's estimates of how global warming increases the likelihood of heat waves are conservative, she said.
In a world with no human-caused greenhouse gases, the chances of having a summer as warm as this one would approach zero, according to the study. With greenhouse gas emissions eventually raising temperatures 1.5 degrees Celsius warmer than pre-industrial times (about a half degree warmer than today), the chances increase to 24 percent. After 2 degrees Celsius of warming, the chances of a having summer like this rise to 42 percent.
French researcher Robert Vautard, who closely studied the deadly 2003 heat wave, said better climate simulations are making studies more accurate. The new attribution study on the 2017 heat wave confirms the trend climate scientists have been warning about: there will be more frequent and more intense heat waves in the decades ahead, sometimes in unexpected locations and at unanticipated times.
"The 2003 heat wave taught us that adaptation plans are necessary to protect vulnerable people," he said. "Now, we are also seeing mid-summer heat waves early and late, in June or September, which may require different adaptation measures."