Kathrin Wehrli was in Switzerland when the 2015 European heat wave shattered heat records across the continent. (Subsequent heat waves in 2018 and 2019 also set new records.) “I remember getting home and it was just too hot for everything,” she said. “I mean, you would eat mostly salad and cold stuff because you couldn’t stand cooking something in your flat.”
Since then, Wehrli has used computer climate simulations to extensively study the key environmental processes—atmospheric circulation, soil moisture, sea surface temperatures, and the presence of greenhouse gases—that contributed to the heat wave.
Her results reveal that atmospheric wind circulation and dry soil played similarly important roles in driving major heat waves.
“I was actually surprised to find such an important role for soil moisture,” said Wehrli, a graduate researcher at the Swiss Federal Institute of Technology (ETH) Zurich. Most climate scientists probably think that atmospheric “circulation defines it all,” said Wehrli, who was lead author of the study, but these results suggest that land surface conditions are equally important.
Simulating Heat Waves
The study closely analyzes five major heat waves that occurred between 2010 and 2016 in different parts of the world: Russia, the U.S. Midwest, Australia, Europe, and South Africa. “Our main criterion in the end was that these were impactful events,” said Sonia Seneviratne, a climate scientist at ETH Zurich and the senior author of the study. Each event received widespread media attention due to its large-scale agricultural, economic, and human health effects.
The researchers simulated global climate patterns using the Community Earth System Model. However, “normally in a climate model, everything is interacting and computing all the winds and all soil moisture conditions,” Seneviratne said. “It doesn’t represent the weather of a given year because the weather is, to some extent, random.”
To isolate the role each individual factor played in a given heat wave, researchers input the observed data for that variable and ran the climate simulation to quantify its impact on the temperature. In one set of experiments, they “nudged” the model to reflect observed atmospheric wind circulation patterns to analyze the impact of wind patterns on the heat waves. In a second set of experiments, the researchers prescribed how dry the soil was in the model to reflect on-the-ground measurements to see how soil moisture influenced temperature.
These simulations were computationally intensive, generating over 80 terabytes of data in all. “I need 6 hours to run one simulation year,” Wehrli said. For the 1979–2016 time period simulated in the computer model, each simulation would take over 9 days.
Unsurprisingly, the data that are output are intricate. “You can think of it like a very complex spreadsheet, but you cannot read it in Excel,” she said. The “four-dimensional cube of data” gives numbers about the location and times of atmospheric data and the state of the land surface.
“There have been studies that look at each [factor] individually, but to do it in a way that the authors did it here is very difficult,” said Dann Mitchell, an associate professor of atmospheric science at the University of Bristol in the United Kingdom who was not involved in the study. “So that’s not been done before.”
One limitation of the study is that only one computer model was used. It would be interesting to see the results of testing a few different climate models, Mitchell said. However, he didn’t expect there would be significant changes to the overall conclusions.
Wehrli and her coauthors published their results in the Journal of Geophysical Research: Atmospheres.
Soil, Wind, and Climate Change
The results of the study indicated that both atmospheric circulation dynamics and dry soil made significant contributions to the heat waves, each accounting for between 20% and 70% of each weather anomaly. “This paper is really nice because it’s actually quantifying the importance of both,” said Mitchell. “But there’s never a heat wave where both things aren’t important.”
“In the past, I think that the role of soil moisture has been underestimated,” said Seneviratne. Understanding its importance could lead to better forecasting for future heat waves, she said. Mitchell agreed: “We need more model development, especially on the soil moisture side.”
Recent climate change also explained between 10% and 40% of these heat waves, with the largest contribution observed during the 2015 European heat wave. Thinking back on her experience, “it was really interesting to look at this from a scientific perspective, and then really saying, ‘Wow, I know why it happened: It was climate change!’” Wehrli said.
It may be possible to indirectly combat the worsening heat waves by increasing soil moisture through increased irrigation, Seneviratne said. There is evidence, for example, that agricultural irrigation has mitigated high temperature extremes in the Great Plains of the United States.
But such solutions would be short-term at best.
“As we always say, in these things the best solution is cutting carbon emissions straight off,” Mitchell said.
—Richard J. Sima (@richardsima), Science Writer