Much of Europe—and particularly France—has been sweating through an incredible heat wave in recent days, with temperature records falling left and right. Despite it being only June (albeit the hottest June on record in Europe), a station in Gallargues-le-Montueux actually broke France's all-time high by more than 1.5°C, reaching a sweltering 45.9°C (114.6 °F).
A team of climate scientists with an established method of rapidly analyzing extreme weather events like this has already taken a look at this heat wave. (The study has yet to be peer-reviewed but follows a protocol which has.) The team's results give a good idea of the role of climate change in this heat wave.
The first question is how to define this weather event. The scientists decided to go with a human-health-relevant definition of the three-day mean temperature rather than a single daily high. They focused on June temperatures for the whole of France, as well as performing a local-scale analysis for just the city of Toulouse—where much of the team coincidentally happened to be attending a conference on weather extremes at the time.
The analyses look at both changes in past weather data and a host of climate-model simulations. In this case, the data shows a very large increase in heat waves since the start of the 20th century. Based on the most recent data, this heat wave looks like it is approximately a 30-year event (meaning it has a probability of about 1 in 30 of occurring in a given year).
Around 1900, however, this would have been a much rarer event. The difference means it's now roughly 100 times more likely to happen in our current, warmer climate. Put another way, the current 30-year heat wave event is a whopping 4°C or so hotter than what would have been a 30-year heat wave at the start of last century. These numbers came out pretty much the same for Toulouse and for France as a whole.
The climate-model simulations proved a little murkier. To start with, the models' results were compared to data to see how accurately they simulate this particular weather pattern in this particular place. In this case, the model simulations produced fewer and weaker heat waves than have actually occurred, so the models have to be taken with a grain of salt.
Nevertheless, they do all simulate increases in heat waves in France due to human-caused warming (that relationship is relatively straightforward). But the increase in probability was mainly in the 5- to 100-times-greater range. As for the increase in the temperature of a 30-year heat wave, it was closer to 1-2°C than the 4°C change seen in the actual data.
Taken together, the researchers conclude that climate change made the recent heat wave at least five times more likely, though that's the lower limit of their results. This isn't really a surprise, especially given that past analyses by this team have found similar answers for other heat waves around Europe.
Fortunately, the researchers also note that past heat waves have led to improvements in preparedness. The extreme heat was forecast days in advance, and France put its emergency plans into motion—very likely saving lives.
