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Hundreds of Seattleites are projected to die in severe heat waves as the globe warms.

How much humans limit greenhouse gas emissions will have a significant impact on just how many people perish, according to a new study published earlier last week in the journal Science Advances.

Researchers used climate-warming scenarios and heat-mortality data to predict the severity of future heat waves for 15 U.S. cities, including Seattle.

Here, about 725 people are projected to die in each extreme heat event if global temperatures rise about 5.4 degrees Fahrenheit since pre-industrial times. In that scenario, Seattle could see daily mean temperatures (averaging day and night measures) of about 97 degrees Fahrenheit at their highest.

If countries meet the greenhouse gas reduction levels they pledged in the 2016 Paris Agreement, global temperatures could reach or approach that mark by about 2100, according to previous research cited in the study.

President Donald Trump vowed in 2017 to withdraw from that deal. Meanwhile, global carbon dioxide emissions hit record highs last year, according to scientists' projections. Washington state's most recent tally of its greenhouse gas emissions showed a rise.

Scientists have warned for years that climate change would cause killer heat waves, but the Science Advances paper estimates specific impacts for individual cities like Seattle and makes clear how many lives are at stake.

"These are the first estimates of what that increased burden from extreme heat waves would be if we don't cut emissions," said Kristie Ebi, a professor of global health at the University of Washington who was one of the study's authors.

That could help clarify the climate threat to the public, scientists said.

"This puts numbers on it to communicate that risk a little better to people," said Karin Bumbaco, Washington's assistant state climatologist, who was not involved in the paper.

Heat waves would increase in frequency as well as severity, Ebi said.

The researchers studied the impacts of one-in-30-year heat waves, or events that would have a 3.3% chance of happening each year. But as the world warms, the odds shift.

"In an average lifetime, this is the kind of heat wave you'd expect to experience maybe two to three times," Ebi said. "Of course, with warming, that 1-in-30 event becomes much more frequent."

Limiting warming to 3.6 degrees Fahrenheit would save 279 lives in each extreme Seattle heat wave. Keeping temperatures to an increase of 2.7 degrees Fahrenheit would avoid another 105 deaths.

The highest mean temperature (averaging day and night measures) in Seattle was about 83.5 degrees Fahrenheit from 1987 to 2000, according to the study data. But, if global temperatures rise 5.4 degrees, models predict Seattle could see that figure rise to about 97 degrees Fahrenheit.

By that time, mean temperatures are expected to exceed 83.5 degrees more than five days a year, on average.

"No one needs to die in a heat wave. All those deaths are preventable," Ebi said. "Rapidly reducing our greenhouse gas emissions will save lives in Seattle."

The numbers reflect averages and estimates. The study also did not consider adaptations people might make, like increased use of air conditioning. Nor do they consider population or demographic changes.

"There would be the same number living in Seattle and they'd all be the same age as they are today," Ebi said of the scientists' models. "There are ways the study is overestimating risk and ways the study is underestimating risk. It's not possible to tell how that would balance out."

About as many people in Seattle would die in each severe heat wave as in Houston, a much warmer city with a population more than double Seattle's.

"Seattle doesn't get very many heat waves. Houston gets more heat waves but they're better prepared for them—everything from air conditioning to how the city is built," Ebi said.

Of the 15 cities studied, Seattle ranked fourth-highest in the number of deaths projected from heat waves for every 100,000 residents.

"We're not a city that's adapted well to higher temperatures," Ebi said. "We need to take more action intermediately to adapt."

Heat kills by myriad methods.

People can simply overheat, which can progress, if not treated, to heat stroke, said Jeff Duchin, health officer for Public Health—Seattle & King County.

"When temperature regulation of the body can't cope, organs basically start to fry," Duchin said. Heat can also exacerbate symptoms from underlying ailments like cardiac disease, diabetes or kidney problems.

The elderly, the very young and pregnant women are most at risk, Duchin said.

Wealth can also be a factor, said Mark Stephan, an associate professor in political science at Washington State University Vancouver.

"Heat waves have a bias in terms of who is affected, partly based on age, but also based on income," he said. "Climate change only exacerbates this, arguably."

People's ability to afford air conditioning, live in a home with effective air circulation, or leave town for places where it's cooler, gives them advantages.

"Think about suburban single-family homes with a lot of leafy coverage of trees, houses with flow of air, versus someone who lives in an apartment complex," he said.

Heat often brings other threats.

"Heat waves are just one part of the package of health threats we're going to see with climate change," Duchin said. "It comes along with extreme weather events, wildfires, poor air quality and the cascading effects those events cause. It's hard to look at it in isolation."

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