Is climate change to blame for the extreme heat wave in Europe?

The extreme heat wave that's been scorching Europe in triple-digit temperatures may be up to 100 times more likely to occur today than it was in 1900, according to a new study from team of scientists at World Weather Attribution. Additionally, the analysis finds the heat wave was 7 degrees Fahrenheit hotter than it would have been in the early 20th century.

A climate attribution study is an analysis specifically designed to determine the likelihood of an extreme weather event, especially as it relates to the contribution from human-caused global heating. 

On Monday the heat wave began to ease, but not before shattering several remarkable records. In France, the city of Gallargues-le-Montueux, near the city of Nîmes, hit 114.6 F — breaking the country's all-time heat record, set in 2003, by an astounding 3 degrees F.  In fact, an impressive 14  weather stations surpassed or tied the country's all-time heat record. 

France was not alone. Germany, Switzerland, Lichtenstein, the Czech Republic, Luxembourg and Andorra all set new all-time June heat records. On Sunday the mercury soared to 103.3 F in Bernburg, Germany — the highest temperature recorded anywhere in that country in the month of June. In Austria and the Netherlands, it was the warmest June ever recorded. Furthermore, hundreds of individual towns and cities across Europe broke their June or all-time heat records.

Of course, summer heat waves are nothing new, but a growing body of scientific evidence shows they're becoming more frequent and more extreme.

To conduct the analysis on what role climate change may have played, the team defined the heat wave as a three-day average of daily mean temperatures over the whole country of France and also one individual city, Toulouse. They performed a statistical analysis on the observation records and a study using climate models.

The observational analysis concludes that the June 2019 heat wave was around 100 times more likely now than it would have been in the year 1901. Also, the temperatures of the heat wave would have been around 7 degrees cooler at that time.

Next, to determine the contribution from heat-trapping greenhouse gasses in the environment, the team compared the observations with results from various climate models. The study concludes, "All models and observations qualitatively agree on a strong human influence in increasing heatwave risk."

Specifically, the model average shows, at minimum, humans have made this heat wave 5 times more likely —  but some models show more than a 100-fold increase. The models also show that human-caused climate change contributed an extra 3 to 4 degrees F to the heat wave temperatures, as compared to 1900.

A factor of 5 times greater "is most likely a conservative estimate, while the best estimate would likely be substantially higher," one of the authors of the report, Sonia I. Seneviratne of the Institute for Atmospheric and Climate Science at ETH Zürich, told CBS News.

Another of the authors, climate scientist Geert Jan van Oldenborgh, explained that climate models tend to have biases in representing heat waves at these small space and time scales, consequently showing fewer severe heat waves. "The big problem is that the models simulate heat waves that look different from observed ones. This can be due to other effects influencing the observations or to the models misrepresenting some physics that is needed for these extreme events," he said.

Oldenborgh said the models "do fine on monthly mean temperatures," which is a larger time frame, "just not on 1-day or 3-day extreme heat waves."

CBS News also asked Oldenborgh whether his analysis found any sign that changes in atmospheric steering patterns, or wavier jet streams caused by abnormally fast warming in the Arctic, contributed to this heat wave. In response, he said, "We found no change in the statistics of the weather pattern that caused this heat wave, so no connection with wavy or resonant jet streams for this event."

Jet stream changes due to warming is a growing area of climate research which is hotly debated among climate scientists. CBS News reached out to Professor Michael Mann from Penn State University, who studies this phenomenon, for his opinion on the influence of steering patterns.

Mann feels the methods used in this study do not sufficiently detect the changing steering patterns. "The approach used by this group is deficient in its inability to account for such factors. As a result, they systematically underestimate the role that climate change is playing with these events," he said.

Mann continues, "The problem with their attribution analysis is that they can only attribute mechanisms resolved by their models. As we showed in our Science Advances article last year, the models fail to resolve the jet stream dynamics behind many of these events."

Although the two scientists may disagree on some of the specifics, they do agree that society's dependence on the burning of fossil fuels, and the heating it causes, are fueling more severe heat waves and extreme weather in general.

2 July 2019