Heat Waves Are Moving Slower and Staying Longer, Study Finds

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

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

“This has strong implications for 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, damage crops and increase the risk of wildfires.

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

The study is one of 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 was not 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 that climate change is increasing the frequency of heat waves,” said White. “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 that reached 1 million square kilometers (247 million acres) or more, where temperatures rose to the 95th percentile of least of the local history. maximum temperature (basically, huge blobs of unusually hot air). The heat waves also had to last for at least three days. The researchers then measured how far these huge air masses moved over time to calculate their speed.

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

The average lifespan of heat waves has also stretched: From 2016-20, they lasted an average of 12 days, compared to eight days from 1979 to 1983. These longer-lasting heat waves are also traveling farther, increasing their length. travel 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 last longer especially in Eurasia and North America. And they are traveling further, especially in South America.

To examine the role of climate change, the researchers used models to simulate temperatures in scenarios with and without warming from human greenhouse gas emissions. They found that the scenario with these emissions best matches what actually happened to the behavior of the heat waves, indicating that climate change is the main force behind the trends this.

Scientists have begun to detect a larger pattern of upper atmosphere air circulation and winds, including the weakening of the jet stream, at least during the summer at higher latitudes in the Northern Hemisphere. This could cause extreme weather events of all kinds to wear out their welcome.

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

The new study found a correlation between a weaker jet stream and slower heat waves. White, however, thinks that more research is needed to determine if the jet stream is really the cause.

Whatever the precise causes of the slowdown, the adverse effects remain.

“It’s kind of multiple factors conspiring together,” Vavrus said. If heat waves become more frequent, more intense, last longer and cover a larger area, he said, “that increases our concern about their consequences.”

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

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

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

As he waits for international leaders to make progress on cutting greenhouse gas emissions and halting climate change, Zhang said, local adaptation efforts are important to help keep people safer.

c.2024 The New York Times Company

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