When heat waves gripped much of the planet last summer, sweltering temperatures stuck around for days or even weeks in many places. As climate change warms the planet, heat waves are moving slower and lasting longer, according to a study published Friday.
The study found that every decade between 1979 and 2020, the speed at which heat waves pushed by air circulation slowed by about 5 miles per day. Heatwaves now last an average of four days longer.
“This has really strong implications for public health,” said climate scientist Wei Zhang of Utah State University, who co-authored the study in the journal Science Advances.
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The longer heat waves remain in one place, the longer people are exposed to life-threatening temperatures. As workers slow down in extreme heat, economic productivity also decreases. Heat waves also dry out soil and vegetation, damage crops and increase the risk of wildfires.
These changes in heatwave behavior have been more pronounced since the late 1990s, Zhang said. He attributes the changes largely to human-induced climate change and partly to natural climate variability.
The study is among the first to track how heat waves move in both space and time.
Rachel White, an atmospheric scientist at the University of British Columbia who was not involved in the paper, said she expects to see research like this.
“We know that climate change increases the severity 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 this 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 at least 95 percent of the time in local history. maximum temperature (basically, huge bubbles of unusually hot air). Heat waves were also supposed to last at least three days. The researchers then measured how far these giant air masses moved over time and calculated their speed.
Over the years they studied, heat waves slowed by about 8 kilometers (or about 5 miles) per day per decade.
The life expectancy of heatwaves has also increased: They lasted an average of 12 days from 2016-20, compared with eight days from 1979 to 1983. These longer-lasting heat waves also travel farther, increasing the distance they travel. It travels approximately 226 kilometers every ten years.
The researchers also found that heat waves are becoming more frequent, increasing 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 are traveling further afield, especially in South America.
To examine the role of climate change, researchers used models to simulate temperatures in scenarios with and without warming from human greenhouse gas emissions. They found that the scenario involving these emissions was the best match for what actually happened to heatwave behavior; This suggests that climate change is a significant force behind these trends.
Scientists began to detect that, at least during the summer months, air circulation at higher latitudes in the Northern Hemisphere is broader and upper atmospheric winds, including jet streams, are weakened. This could cause any extreme weather events to stall and outstay their welcome.
“It makes sense that this would slow the pace of heat waves,” said Wisconsin state climatologist Stephen Vavrus. 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. But White thinks more research is needed to determine whether the jet stream is actually the cause.
Whatever the exact reasons for the slowdown, the detrimental effects remain.
“It’s a combination of factors,” Vavrus said. If heat waves become more frequent, more intense, last longer and cover a larger area, “that really increases our concern about their impact,” he said.
Zhang is particularly concerned about cities that are often hotter than their surroundings due to the urban heat island effect. “If these heat waves last much longer in the city than before, it will cause a very dangerous situation,” he said.
Besides 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 wait in the sun during the increasingly hot summer months. He suggested cities build more cooling centers, especially for people experiencing homelessness.
“There are some things a community can do,” he said.
While he expects international leaders to make progress on reducing greenhouse gas emissions and halting climate change, local adaptation efforts are important to help keep people safer, Zhang said.
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