As countries across Europe continue to shatter temperature records — Germany hit 104.5 degrees Fahrenheit on Sunday, an all-time high since records began in 1881 — scientists around the world are beginning to wonder: what role is climate change playing in this most recent heat wave?
The answer, according to an international group of scientists at the World Weather Attribution Center, is “a significant” one. According to an analysis conducted last week, “it is virtually certain that climate change increased the likelihood of the ongoing heat wave stretching across much of Europe.”
The scientists looked at temperature records from a few different European cities — Madrid, Zürich, and Paris, among others — and, using a combination of observed and forecasted data, compared the temperatures seen this summer to temperatures from the beginning of the century, “before global warming played a significant role in our climate.” They found that in every case, a three-day period of temperatures as high as they have been this month would have been quite rare at the beginning of the 20th century — but is much more common now. In Madrid, for instance, a three-day period with temperatures like the ones seen during this heat wave would have been “exceptionally rare in the 1920s” but is now “likely to happen roughly 1 in 40 years.”
The kinds of conditions that are conducive for heat waves to occur, those are most likely in the future with higher temperatures
As part of the analysis, researchers at Oxford also simulated the likelihood of seeing temperatures as high as have been seen recently in Europe both with and without climate change. Of the five cities analyzed in their simulation, they found that the current heat wave conditions were at least twice as likely to occur due to climate change.
For some, that’s a more difficult connection to make than saying that climate change made the heat wave more likely, or made the heat wave worse. Scientists agree that human activities — such as the burning of fossil fuels — have altered the composition of the atmosphere, creating a new kind of climate backdrop against which weather events occur. But they’re more hesitant to say that climate change caused a particular weather event to occur in the first place.
“I don’t think you can ever say that,” Kevin Trenberth, a senior scientist with the National Center for Atmospheric Research, told ThinkProgress. “Even without climate change, there has always been heat waves. It just relates to the right meteorological setup.”
That doesn’t mean that scientists never confidently attribute weather events like heat waves to climate change. In 2014, five research groups, each using different methods, concluded that the Australian heat wave of 2013 was almost certainly a result of man-made climate change. The study, which was published in Bulletin of the American Meteorological Society, was touted as one of the most conclusive links between an extreme weather event and climate change, because it involved several different models coming to the same conclusion.
In the long-term, climate models confidently show the potential for heat waves increasing. Two meteorological factors in particular influence the formation of heat waves: anticyclonic conditions (high pressure weather systems that create settled weather conditions) and relative dryness. Warmer temperatures are expected to lead to an intensification in the hydrologic cycle, meaning that any rain that falls will evaporate more quickly, leading to more surface dryness that in turn can lead to anticyclonic conditions.
Once the stage is set, exactly how glamorous the show is — that’s the thing that is determined by … climate change
“The kinds of conditions that are conducive for heat waves to occur, those are most likely in the future with higher temperatures,” Moetasim Ashfaq, a scientist at the Climate Change Science Institute at the Oak Ridge National Laboratory, told ThinkProgress. “When we talk about the present, what’s happening today, that may be because of climate variability.”
One type of climate variability that can lead to prolonged heat waves is known as blocking. It’s when a high pressure system remains stagnant over a region, blocked by two areas of low pressure. Blocking was what caused the 2003 heat wave across Europe that killed more than 70,000; it was present during the 2010 heat wave in Russia that killed more than 50,000 people; and, according to Ashfaq, it’s what scientists expect caused the recent deadly heatwaves in India and Pakistan.
Are blocking patterns becoming more common because of global warming? Ashfaq wagers that they are, but cautions that, in the short term, it’s difficult to say with certainty whether a particular event was caused by climate change, or climate variability. Andrew Freedman, at Mashable, uses the analogy of a baseball player on steroids hitting a home run — do you know for certain that the player was able to propel the ball over the fence because of the steroids? Was it a single instance of good contact, or did the steroids make it more likely that he could hit the ball harder, and farther? If you’re just looking at one run — one single event — it’s hard to tell. But over the long-term, the data will show that the steroids make the baseball player more likely to hit a home run.
Trenberth phrases it another way: at this point, with existing climate models, it’s very difficult to know whether a single event was caused by climate change, or natural climate variability, like an El Niño pattern. What’s not hard is figuring out whether climate change made a weather event worse.
“It’s always the natural variability — that is, the weather — that sets the stage,” he said. “But then once the stage is set, exactly how glamorous the show is — that’s the thing that is determined by the climate change aspect.”