Heat waves move slower and last longer, research shows

By | April 1, 2024

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

Between 1979 and 2020, the speed at which heat waves travel, driven by atmospheric circulation, decreased by about 8 kilometers per day every decade, the study found. Heat waves now also last about four days longer on average.

“This has really major 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 linger in one place, the longer people are exposed to life-threatening temperatures. As workers slow down during extreme heat, economic productivity also increases. Heat waves also dry out soils and vegetation, damaging crops and increasing the risk of forest fires.

These changes in heat wave behavior have been more noticeable since the late 1990s, Zhang said. He attributes the changes largely to human-induced 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 for this kind of research.

“We know that climate change is increasing the intensity of heat waves. We know that climate change is increasing the frequency of heat waves,” White said. “But this study really helps us better understand how that happens.”

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

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

The average lifespan of heat waves has also gotten longer: from 2016 to 2020, they lasted an average of 12 days, compared to eight days from 1979 to 1983. These longer-lived heat waves also travel farther, increasing the distance they travel. traveling at about 140 miles per decade.

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

There are some regional differences. Heat waves last longer, especially in Eurasia and North America. And they travel further, especially in South America.

To investigate the role of climate change, the researchers used models to simulate temperatures in scenarios with and without warming caused by human greenhouse gas emissions. They found that the scenario with these emissions best matched what actually happened with heatwave behavior, indicating that climate change is a major factor behind these trends.

Scientists have begun to detect a larger pattern of air circulation and upper atmosphere winds, including the jet streams becoming weaker, at least during the summer at higher latitudes in the Northern Hemisphere. This could mean that all kinds of extreme weather events are no longer welcome.

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

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

Whatever the exact reasons for the delay, the damaging effects remain.

“It’s kind of multiple factors conspiring,” Vavrus said. If heat waves become more frequent and intense, last longer and cover a larger area, he says, “then the concern we have about their impacts really increases.”

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

In addition to 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 cities build more cooling centers, especially for people experiencing homelessness.

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

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

c.2024 The New York Times Company

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