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It’s virtually certain that 2020 will be on the top five list of hottest years on record for the planet, according to atmospheric scientists at the National Oceanic and Atmospheric Administration. In a briefing, NOAA officials announced their three-month outlook for this summer, with above average temperatures expected across almost all of the United States. The likelihood of excessive heat is highest in the West and Northeast.

What’s also likely is that this heat will contribute to thousands of deaths—and that this impact could be made worse by the pandemic. More people will stay at home this summer rather than cool off in public spaces, even if they lack air conditioning. To prepare, researchers say we need to understand that heat waves are a serious natural disaster made worse by climate change, and it’s on us to make cities safer.

Heat claims more lives than any other weather-related cause, including tornadoes, hurricanes, and cold temperatures. It’s perhaps easy, especially from the perspective of someone with air conditioning or living in a cooler climate, to think of heat as merely a nuisance. But for vulnerable populations, including those over 65 and people with underlying health conditions, heat can kill swiftly. While 90 percent of Americans have some form of air conditioning, new research finds that thousands still die from heat-related causes each year. That may be in part because having A/C doesn’t mean it necessarily works, that the person can afford to run it, or that they can stay inside during hot days.

That threat is worse for disadvantaged communities. In a recent study in Environmental Research Letters analyzing energy use in 180,476 Southern California homes, residents in low-income census tracts were less likely to use air conditioners when temperatures got hot. This is due to lack of cooling units, or, where air conditioning was installed, an avoidance of costly energy bills. As extremely hot days grow more common due to climate change, those communities are especially vulnerable to heat risks.

Many urban areas also face the heat island effect, in which dark pavement and roofs absorb heat, enough to take temperatures from merely warm to deadly. Areas with lots of concrete and pavement can be much hotter than reported for a given city in its weather forecast. The worst heat islands are disproportionately communities of color, including areas that have endured historic housing discrimination. “We need to be mindful that if a weather forecast says it’s going to be 85 degrees, a person who’s at risk might be living where it’s 95,” says Aaron Bernstein, director of the Center for Climate, Health, and the Global Environment at the Harvard T.H. Chan School of Public Health. “It’s really important that people, particularly the medical community, and cities recognize that these disparities in heat exposure are huge.”

The number of annual deaths from heat vary among estimates, but some researchers think that those published by the federal government are too low—perhaps by an order of magnitude. According to CDC data released last week, 702 people died from heat-related causes, on average, between 2004 and 2018. But two other recent studies suggest that is an underestimate. In a statistical analysis of 297 counties in Environmental Epidemiology, which covered 61.9 percent of the U.S. population, researchers found that heat contributed to the deaths of an average of 5,608 people annually between 1997 and 2006. In another recent study in the journal Geohealth, researchers found heat caused about 12,000 deaths per year across the country in the last decade.

This roughly 10-fold variation between the CDC’s and other estimates is based on an important methodology difference. CDC numbers are based on death certificate data, compiling instances in which heat illness was explicitly noted. The problem is, many heat-caused deaths aren’t reported as such. Instead, heat aggravates an existing condition, especially chronic cardiovascular and respiratory illnesses, which then leads to death. “A number of epidemiologic studies have shown that deaths from cardiovascular and respiratory disease are elevated when it’s really hot outside,” says Kate Weinberger, epidemiologist at the University of British Columbia and lead author of the study in Environmental Epidemiology. “Those don’t get captured on the death records.”

Researchers can account for these less-apparent heat deaths by doing epidemiological studies that look for increases in deaths when temperatures are above normal. “This is a call to action for investing in heatwave early warning systems and thinking about the design of our cities to make sure that we can keep residents cool,” says Kristie Ebi, an author of the Geohealth study and an epidemiologist at the University of Washington. “Every heat-related death is preventable.”

Many cities try to protect their citizens by warning them of hot days and encouraging them to visit cool areas—air-conditioned libraries, shopping malls, or community centers. Officials sometimes even make public transit free to help people move to safety.

But now, with the threat of coronavirus looming, people are probably more likely to stay at home, and cities might not be able to safely provide the same cooling centers. Some are now relying on “buddy systems.” In Boston, Bernstein says that the city reached out to community organizations and asked members to create plans identifying who they know that’s at risk of heat illness, and assigning a buddy to check on that person when a heat wave is pending. Such a network could help ensure individuals at risk—especially those who are isolated—have support in protecting themselves from the heat. In New York City, officials are planning to distribute air conditioning units for free to low-income seniors and offer assistance with energy bills.

Nationally, the CARES Act also offers some assistance in paying energy costs. Mo Chen, lead author of the Environmental Research Letters study mentioned above and an environmental engineer at the University of Southern California, says that part of the problem is that people may not know what assistance is available to them. Using utility and census data to map areas of vulnerability, as he did for Southern California, could help officials know which people to contact about such programs.

However, there’s an inherent tension between helping people in the short term by increasing air conditioning and further perpetuating the problem by burning more fossil fuels to run A/C. The good news is that there are plenty of long-term strategies that don’t face that trade-off.

Numerous opportunities exist to cool cities down. As Bernstein explains, the main heat threat to people living in cities isn’t that our global climate has warmed by about one degree Celsius, it’s that urban areas are paved over with heat-absorbing materials. Moving forward, city planners can build heat-reflective roofs and roads. Planting trees and expanding green spaces can also help. And, as Bernstein adds, it’s a compounding benefit. Green spaces have been shown in numerous studies to have mental health benefits. “You can easily justify the investment alone for heat [management],” he says of implementing these measures. “But then when you add on these other dimensions, how can we afford not to do this?”

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