Between 2001 and 2010, over 136,000 people died because of heat waves, representing an increase of more than 2,000% compared to the previous decade. And as Earth continues to warm, scientists say extreme heat waves will only get worse.

Understanding the causes of heat waves is a key factor in our ability to predict them, which in turn enables the prompt implementation of emergency response plans. Understanding these causes, especially those that are related to land conditions, could potentially help us lessen the severity of heat waves in the future as well.

Heat waves were exacerbated by advected heat from upwind drought-wracked regions. Researchers from Ghent University in Belgium and Wageningen University in the Netherlands have identified an important new contributor to heat wave severity. Using data from the “mega–heat waves” in western Europe in 2003 and in Russia in 2010, researchers determined that these heat waves were exacerbated by advected heat from upwind drought-wracked regions.

Dominik Schumacher, the lead author of the study, explained to Eos that there are three main sources of heat during a heat wave. First, heat can come from below, rising up from the Sun-warmed ground. Second, heat can come from above, from a layer of air called the free troposphere directly above the planetary boundary layer. Third, said Schumacher, “you can also have heat coming in from the side, or horizontally. This is the part that we investigated more closely. Up until now, this wasn’t really looked at too much.”

Previous research had already shown that local soil conditions play an important role in mediating the first source of heat, the heat coming from the ground. “When the soils dry out, even more energy goes into heating the air above, and less energy goes into evaporation,” said Schumacher.

Researchers wanted to know if dry soils also played a role in the third contributor to heat waves, the horizontal transportation of heat from other places on Earth. By using a Lagrangian trajectory model, Schumacher said, “we can trace the air back in time, we can find out where the air came from that is residing over our heat wave region now. Where was that air 5 days ago? Even more importantly, we can also analyze if the air gains or loses heat on the way to our heat wave region.”

In the case of the 2010 heat wave in western Russia, unusual weather patterns brought warm air from the east and southeast into the heat wave region. These regions were in the middle of a drought and had extremely low soil moisture. Researchers estimate that this soil dryness was responsible for 30% of the heat that was transported from the drought region to the heat wave region in western Russia. Thus, drought and soil conditions in far-off places can substantially worsen heat waves.

Physical geographer David Keellings of the University of Alabama, who was not involved with the new research, said that heat waves in which a substantial amount of heat is advected from another location happen not just in Europe but across North America and Asia as well. Thus, this research could help us understand heat waves that occur across a substantial part of the globe.

Mitigating Heat Waves

Understanding how land conditions like soil moisture contribute to heat waves could also provide important clues about how to mitigate heat waves in the future.

Schumacher is involved in a project called Dry-2-Dry, which is headed by Ghent University’s Diego Miralles and is in the process of investigating how land management practices could ameliorate droughts and heat waves. In particular, Schumacher said that the positioning of irrigated cropland upwind of certain areas could help to lessen some types of heat waves.

Keellings said that understanding global (in additional to local) contributors to heat waves is important for improving seasonal heat wave predictions, that is, the likelihood of heat waves in a given summer, not just whether one will occur in the next week or so.

“We’re looking at things that go on around the world, whether it’s sea surface temperature or broad-scale movements in the atmosphere or soil moisture and drought or land cover change or influences of urbanization—all of these things are going on at large scales—and trying to see what is their relationship to the chance of a heat wave happening,” he said.

Keellings said that not only are heat waves becoming more frequent and lasting longer but preliminary work from his lab suggests that they are also covering larger areas than they used to. And these worsening heat waves will have serious consequences. Although the number of deaths caused by heat waves is certainly the most troubling, heat waves have also grounded airplanes in the United States, disrupted train travel in the United Kingdom, and even interfered with electricity output from nuclear reactors in France and Germany.

Keellings said that the human element of heat waves is a major motivator for his research. “What drives me to investigate heat waves is not just that they’re fascinating events from a climate science perspective…but also what ultimately drives this is that heat waves are hugely linked to human health.”

—Hannah Thomasy (@hannahthomasy), Freelance Science Writer