Thousands of Americans are expected to perish in cities during future heat waves

Heat waves kill more Americans than any other weather event.

The torrid episodes are especially problematic in cities, where the concrete and asphalt sprawl traps heat, boosting already unusually hot temperatures by some 2 to 5 degrees Fahrenheit.

Yet, as average temperatures continue their relentless rise, the U.S. government expects urban dwellers to experience more heat extremes, some unprecedented. Scientists, however, found that limiting Earth's warming this century to 2.7 degrees F (1.5 degrees Celsius) above pre-industrial temperatures will spare thousands of Americans during the hottest heat waves (events that on average hit once every 30 years). The new research, published Wednesday in the journal Science Advances, found that 2720 deaths could be avoided in the most populous metropolis, New York City.

"We all know high temperatures kill," said Eunice Lo, lead author of the study and a research associate at the University of Bristol. "We have all read news about people that have died from extremely high temperatures. But I don’t think we are particularly aware of how many deaths could occur from extremely high temperatures."

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The research is especially salient because it's now nearly impossible for global society to curb the planet's warming at 1.5 degrees C, or even 2 degrees C. Emissions of the heat trapping gas carbon dioxide will likely keep rising for another decade. Already, the rate of CO2 increase in the atmosphere is unprecedented in the historical and geologic record, and extraordinary transformation is now required to curb the planet's warming at levels that would limit the worst consequences of climate change.

This simply doesn't bode well for the denizens of U.S. cities, where populations are expected to boom. But the big picture is clear: Curbing temperatures will save lives, particularly in the 15 U.S. cities considered for this research.

"The main result is true for all cities," said Lo. "Heat related deaths could be avoided if we limit global warming."

Lo and her team looked at daily deaths from 15 U.S. cities between 1987 to 2000 to see how many people died from heat-related events. Then, accounting for population increases, they simulated heat waves that would occur in a world that warmed by 1.5 C, 2 C, and 3 C. Compared to 3 C (5.4 degrees Fahrenheit) -- which is where Lo says we're likely headed if nations fail to commit to ambitious carbon cuts -- many lives will likely be spared under the cooler alternatives.

Los Angeles is expected to avoid some 1085 deaths during an extreme heat wave under a 1.5 C scenario, and 759 deaths under a 2 degree C scenario. Chicago would avoid around 875 (1.5 C) and 636 (2 C) deaths.

In short, there's a lot of lives either spared or dead, between a world with 1.5 or 2 C versus 3 C of warming. "It's easy to think that one degree doesn't make a difference," noted Lo. "But there's a lot of difference."

That so many lives would be spared from extreme heat is little surprise.

"We know that severe heat can have severe health effects," said Noah Diffenbaugh, a climate scientist at Stanford University who had no role in study.

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Diffenbaugh's research has found that, if carbon emissions keep increasing, much of the globe will experience "the permanent emergence of unprecedented summer heat" in the coming decades. This means, by mid-century, even "cool" summers will be hotter than the hottest summers we experience today.

But, he emphasized, the consequences of climate change aren't a future idea. They're here. "It's already clear that we are experiencing impacts from the global warming that's already happened," he said.

Greenland is experiencing unprecedented melting. Western U.S. states are prepping for long-term droughts. The West is has entered a new, potent fire regime. Massive glaciers are vanishing in front of scientists' eyes. Sea creatures are fleeing their warming homes. The cost of beer is projected to rise as extreme weather ravages barley yields. The only thing really keeping the climate from completely going off the rails is the ocean, which naturally absorbs bounties of carbon dioxide. But the seas can only soak up so much of the gas.

Although this research improves our understanding of how heat waves will impact the U.S. populace, there are still some weighty questions, noted Justin Mankin, who researches climate change at Dartmouth College and had no involvement in the study.

Of note, there's a tremendous amount of difference between how each city's inhabitants react to heat waves, with people in Phoenix experiencing a far lower mortality rate than folks in New York City, Mankin noted. Indeed, Arizonians may be better adapted to handle high temperatures, or have buildings better designed to keep cool. In short, things in New York City -- which is projected to have the largest losses -- could change in time for better, or for worse. Without understanding how future New Yorkers will respond to more extreme heat, it's difficult to know if Lo's death projections are too high, or too low.

"So it’s pretty tough to say whether the estimates here are conservative or aggressive," said Mankin.

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Also, a big factor in any heat wave is humidity, which makes it challenging for people to cool off. The scope of this study looked only at temperature, but it's really the "heat-humidity" combo that makes a sweltering New York all the more dangerous than a bone dry Phoenix. Unfortunately for the denizens of U.S. cities, humidity is expected to increase as global temperatures rise (the air holds more moisture as the climate warms).

But the big picture is clear, noted Mankin. Weighing temperature alone, the fate of many people in the U.S. differs substantially between a climate stabilized at 1.5 C or 2 C of warming, versus an extreme 3 C.

Lo thinks Americans should take notice, specifically because the federal government is currently led by the Trump administration, which has repeatedly proved hostile to climate science. Incredibly, one of the president's science advisors is adamant that the planet is in dire need of more carbon dioxide.

"If I were a person in the U.S. and knew people in my own city could be affected by adverse temperatures, a reasonable thing would be to vote for a party that cares about climate," said Lo.