Heat Waves Are Moving Slower and Staying Longer, Study Finds

Restaurants patrons use water misters to keep cool during a heat wave, in the Piazza della Signoria in Florence, Italy on July 13, 2023. (Francesca Volpi/The New York Times)
Restaurants patrons use water misters to keep cool during a heat wave, in the Piazza della Signoria in Florence, Italy on July 13, 2023. (Francesca Volpi/The New York Times)

When heat waves swept across large parts of the planet last summer, in many places the oppressive temperatures loitered for days or weeks at a time. As climate change warms the planet, heat waves are increasingly moving sluggishly and lasting longer, according to a study published Friday.

Each decade between 1979 and 2020, the rate at which heat waves travel, pushed along by air circulation, slowed by about 5 miles per day, the study found. Heat waves also now last about four days longer on average.

“This really has strong impacts on public health,” said Wei Zhang, a climate scientist at Utah State University and one of the authors of the study, which appeared in the journal Science Advances.

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The longer heat waves stick around in one place, the longer people are exposed to life-threatening temperatures. As workers slow down during extreme heat, so does economic productivity. Heat waves also dry out soil and vegetation, harming crops and raising the risk of wildfires.

These changes to heat wave behavior have been more noticeable since the late 1990s, Zhang said. He attributes the changes in large part to human-caused climate change, but also in part to natural climate variability.

The study is among the first to track how heat waves move through both space and time.

Rachel White, an atmospheric scientist at the University of British Columbia who wasn’t involved in the paper, said she had been waiting to see research like this.

“We know that climate change is increasing the intensity of heat waves. We know climate change is increasing the frequency of heat waves,” White said. “But this study really helps us understand more about how that’s happening.”

Zhang and his colleagues analyzed temperatures around the world between 1979 and 2020. They defined heat waves as contiguous areas reaching a total of 1 million square kilometers (247 million acres) or more, where temperatures rose to at least the 95th percentile of the local historical maximum temperature (basically, enormous blobs of unusually hot air). The heat waves also had to last for at least three days. The researchers then measured how far these giant air masses moved over time to calculate their speed.

Over all the years they studied, heat waves slowed down by about 8 kilometers (or nearly 5 miles) per day each decade.

The average life span of heat waves has also stretched out: From 2016-20, they persisted for an average of 12 days, compared with eight days from 1979 to 1983. These longer-lived heat waves are also traveling farther, increasing the distance they travel by about 226 kilometers per decade.

The researchers also found that heat waves are becoming more frequent, to an average of 98 per year between 2016 and 2020, from 75 per year between 1979 and 1983.

There are some regional differences. Heat waves are lasting longer particularly in Eurasia and North America. And they are traveling farther particularly in South America.

To examine the role of climate change, the researchers used models to simulate temperatures in scenarios with and without the warming from human greenhouse gas emissions. They found that the scenario with these emissions was the best match for what has actually happened to heat wave behavior, indicating that climate change is a major force behind these trends.

Scientists have started to detect a larger pattern of air circulation and upper atmosphere winds including the jet streams getting weaker, at least during the summer at higher latitudes in the Northern Hemisphere. This could cause extreme weather events of all kinds to stall and overstay their welcome.

“It stands to reason that that would slow down the speed of heat waves,” said Stephen Vavrus, the state climatologist for Wisconsin. Vavrus studies atmospheric circulation but wasn’t involved in this research.

The new study did find a correlation between a weaker jet stream and slower heat waves. White, however, thinks more research is needed to determine whether the jet stream is truly the cause.

No matter the exact reasons for the slowdown, the harmful effects remain.

“It’s sort of multiple factors conspiring together,” Vavrus said. If heat waves become more frequent, more intense, last longer and cover a greater area, he said, “that really increases the concern we have for their impacts.”

Zhang is especially concerned about cities, which are often hotter than their surrounding areas because of the urban heat island effect. “If those heat waves last in the city for much longer than before, that would cause a very dangerous situation,” he said.

Alongside his atmospheric research, Zhang is helping with local efforts to plant more trees and grasses around bus stops in Salt Lake City, where people have to wait in the sun during increasingly hot summers. He suggested that cities build more cooling centers, especially for people experiencing homelessness.

“There are some things a community can do,” he said.

While waiting for international leaders to make progress on cutting greenhouse gas emissions and stopping climate change, Zhang said, local adaptation efforts are important to help keep people safer.

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