Are we ready for the deadly heat waves of the future?

When days and nights get too hot, city dwellers are the first to run into trouble

hot city

HEAT ISLANDS  Heat claims more lives than floods, hurricanes and other weather-related disasters. How will cities cope as temperatures rise?


Some victims were found at home. An 84-year-old woman who’d spent over half her life in the same Sacramento, Calif., apartment died near her front door, gripping her keys. A World War II veteran succumbed in his bedroom. Many died outside, including a hiker who perished on the Pacific Crest Trail, his water bottles empty.

The killer? Heat. Hundreds of others lost their lives when a stifling air mass settled on California in July 2006. And this repeat offender’s rap sheet stretches on. In Chicago, a multiday scorcher in July 1995 killed nearly 700. Elderly, black residents and people in homes without air conditioning were hardest hit. Europe’s 2003 heat wave left more than 70,000 dead, almost 20,000 of them in France. Many elderly Parisians baked to death in upper-floor apartments while younger residents who might have checked in on their neighbors were on August vacation. In 2010, Russia lost at least 10,000 residents to heat. India, in 2015, reported more than 2,500 heat-related deaths.

Year in and year out, heat claims lives. Since 1986, the first year the National Weather Service reported data on heat-related deaths, more people in the United States have died from heat (3,979) than from any other weather-related disaster — more than floods (2,599), tornadoes (2,116) or hurricanes (1,391). Heat’s victim counts would be even higher, but unless the deceased are found with a fatal body temperature or in a hot room, the fact that heat might have been the cause is often left off of the death certificate, says Jonathan Patz, director of the Global Health Institute at the University of Wisconsin–Madison.

As greenhouse gases accumulate in the atmosphere, heat’s toll is expected to rise. Temperatures will probably keep smashing records as carbon dioxide, methane and other gases continue warming the planet. Heat waves (unusually hot weather lasting two or more days) will probably be longer, hotter and more frequent in the future.

Beyond deaths, researchers are beginning to document other losses: Heat appears to rob us of sleep, of smarts and of healthy births. “Heat has the ability to affect so many people,” says Rupa Basu, an epidemiologist with the California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment in Oakland. “Everybody’s vulnerable.”

Many people see heat as more of an annoyance than a threat, but climate change, extreme heat and human health are entwined. “There might not be a huge burden of disease from heat-related illness right now in your community,” says Jeremy Hess, an emergency medicine physician and public health researcher at the University of Washington in Seattle. “But give it another 20 years, and it might be a more significant issue.”

Adaptation has limits

The human body can’t tolerate excessive heat. The biological and chemical processes that keep us alive are best carried out at a core temperature of 36° to 37° Celsius (96.8° to 98.6° Fahrenheit), with slight variation from person to person. Beyond that, “the body’s primary response to heat is to try and get rid of it,” says Jonathan Samet, dean of the Colorado School of Public Health in Aurora. Blood vessels in the skin dilate and heart rate goes up to push blood flow to the skin, where the blood can release heat to cool down. Meanwhile, sweating kicks in to cool the skin.

With repeated exposure to high temperatures, the body can become more efficient at shedding excess heat. That’s why a person can move from cold Minneapolis to steamy Miami and get used to the higher heat and humidity. But there is a limit to how much a person can adjust, which depends on the person’s underlying health and the ambient temperature and humidity. If the outside is hotter than the body, blood at the skin surface won’t release heat. If humidity is high, sweating won’t cool the skin. Two scientists proposed in 2008 that humans cannot effectively dissipate heat with extended exposure to a wet-bulb temperature, which combines heat and humidity, that is greater than 35° C.

In New Delhi during a May 2015 heat wave, a man wipes his brow. That year, heat claimed more than 2,500 lives in the South Asian country.  Tsering Topgyal/AP Photo

Forced to regulate heat without a break, the body gets worn out. Heat exhaustion leads to weakness, dizziness and nausea. If a person doesn’t cool off, heat stroke is likely — and likely fatal. The ability to regulate heat breaks down and core body temperature reaches or exceeds 40° C. A person suffering heat stroke may have seizures, convulsions or go into a coma.

No one is immune to heat, but it hits some groups harder than others. The elderly, considered the most vulnerable, have fewer sweat glands and their bodies respond more slowly to rising temperatures. Children haven’t fully developed the ability to regulate heat, and pregnant women can struggle due to the demands of the fetus. People with chronic diseases like diabetes, cardiovascular disease and obesity can have trouble dissipating heat. And, of course, people living in poverty often lack air conditioning and other resources to withstand sweltering conditions.

Collateral damage

Researchers are discovering more ways that heat can hurt. Take sleep: The onset and duration of sleep is sensitive to temperature. The body cools down as it prepares to sleep; this decrease in core temperature is a signal to bring on the z’s. Body temperature stays low throughout the night, then rises just before awakening. A good night’s rest is a cornerstone of health.

Hot nights make for bad sleep, according to a study combining responses to a U.S. Centers for Disease Control and Prevention sleep survey of 765,000 U.S. residents from 2002 to 2011 with data on nighttime temperatures during that period. The higher the nighttime temperatures, the more nights respondents reported getting too little shut-eye. The effect hit low-income respondents and the elderly hardest, the researchers reported in May 2017 in Science Advances.

The ability to think and calculate may take a beating in the heat, according to a small study presented in January in Austin, Texas, at the American Meteorological Society’s annual meeting. Researchers from Harvard University tested undergraduate students for 12 days — the time before, during and after a heat wave. Twenty-four lived in buildings with air conditioning and 20 in buildings without. The researchers assessed how quickly and accurately students performed an addition and subtraction test and a test that asked for the color of a written word, rather than the word itself. During the heat wave, the students without air conditioning got about 6 percent fewer correct answers on the math problems and 10 percent fewer on the color problems than the students with air conditioning.

Heat may even increase the risk of stillbirth. Researchers with the National Institute of Child Health and Human Development in Bethesda, Md., analyzed weather data and more than 223,000 U.S. births from 2002 to 2008. During the warm months of the year, a 1 degree C increase in temperature during the week before birth was associated with about four additional stillbirths per 10,000 births, the researchers reported in June 2017 in Environmental Health Perspectives.

In July 2014, a worker in Las Vegas cools off with a wet cloth during a break from her outdoor advertising job. Hot days are expected to become more common. AP Photo/John Locher

As heat gets vicious, it threatens to disrupt the fabric of society. Extreme heat — beyond a wet-bulb temperature of 35° C — could become more regular in South Asia and the Persian Gulf, rendering parts of those areas uninhabitable, according to studies in the August 2017 Science Advances (SN: 9/2/17, p. 10) and the February 2016 Nature Climate Change. It’s not hard to imagine that there will be profound societal and political instability “in a world where tens of millions of people have to move and are looking for cooler places to live,” says Howard Frumkin, a physician epidemiologist specializing in environmental health at the University of Washington.

Emerald cities

Fifty-four percent of the world’s population — and around 80 percent of U.S. residents — live in urban areas. Cities are where some action to combat heat can be taken now, says Brian Stone Jr., an environmental planner and member of the Urban Climate Lab at Georgia Tech in Atlanta. “If we’re waiting for the national government to signal it’s time to do this, we’re going to wait too long,” he says. “We are well into a world that’s been altered by climate change.”

Heat thrives in cities. All of the nonreflective roofs, walls, roads and other surfaces absorb and retain heat during the day. Waste heat, emitted from air conditioners and vehicles, concentrates in cities too. Together, these factors contribute to what’s called an urban heat island, an amplification of heat that occurs within cities. On average, a city with at least a million residents can be 1 to 3 degrees C hotter than surrounding areas. At night, the temperature differences widen. Cities may be as much as 12 degrees C hotter than surrounding areas in the evening hours, because cities release built-up heat back out among buildings and avenues.


These Landsat satellite images show urban Atlanta on September 28, 2000. The core urban area is at the center of the images. The left side shows areas of vegetation (green), bare ground (brown) and roads and dense development (gray). The heat map on the right shows the areas of densest development also have the hottest land surface temperatures (red), near 30 degrees Celsius. The areas of heaviest vegetation are the coolest (yellow) due to evaporation of water and shade.

Marit Jentoft-Nilsen/NASA

City planners can rid their locales of some of this heat with several strategies. One is to plant more trees to create shade for residents and structures. Trees also lower the air temperature by transferring water from the soil through the tree to the air. The surrounding air is cooled as the water changes from a liquid to a vapor. The process is “much like the way sweating works for our bodies,” says George Ban-Weiss, an environmental engineer at the University of Southern California in Los Angeles.

Another strategy is to reduce the amount of sunlight that city surfaces absorb by using “cool” materials on exposed surfaces. The best known are cool roofs, which “reflect more sunlight than usual,” says Ronnen Levinson of Lawrence Berkeley National Laboratory in Berkeley, Calif., who studies cool surfaces and urban heat islands. In general, to make a surface cool, you make it lighter, with coatings or other light-colored materials. For example, a white roof that reflects 80 percent of the sun’s light on a typical summer afternoon will stay about 31 degrees C cooler than a gray roof that reflects only 20 percent.

Giving buildings cool-surface makeovers counters the urban heat island effect and reduces the temperature inside a building. “In disadvantaged communities, people simply may not have air conditioning to help them ride out hot summers,” Levinson says. Cooling off the insides of buildings is “where I think the greatest potential benefits are for improving human comfort and health,” he says.

Stone has estimated how many heat-related deaths could be avoided by reducing urban heat island effects. In 2016, he and colleagues produced a report for the city of Louisville, Ky., that analyzed the impact of adding 450,000 trees, converting 168 square kilometers of surfaces to cool materials and more. The researchers estimated that areas of the city could reduce average summertime temperatures by as much as 1.7 degrees C or more. And based on the 53 deaths Stone attributed to the city’s unusually warm summer of 2012, there could be 11 fewer deaths from heat, a reduction of 21 percent. “When we get a big heat wave,” Stone says, “that could really translate into hundreds of lives.”

Many cities in the United States and abroad are working on tempering their urban heat islands with a variety of strategies, including programs to install cool roofs or plant more trees. The city of Los Angeles now requires that new or replaced roofs for homes and other residential buildings meet a solar reflectance index value — a measure of a materials’ ability to stay cool in the sun between zero (black surface) and 100 (white) — of at least 75 for flatter roofs and 16 for steeper ones. Through a provision in California’s building energy efficiency code, cities throughout the state have been converting flat, commercial roofs, like those on big-box stores, to light-colored cool roofs when a new topper is needed.

New York City has planted a million new trees since 2007 and committed additional funds to adding even more to streets and parks. The city also has coated 0.62 square kilometers of roof surfaces white since 2009. The city of Ahmedabad, India, where about 25 percent of the residents live in slum communities, announced a heat action plan in 2017 that includes a cool roofs initiative to paint or otherwise convert at least 500 slum household roofs and to improve the reflectivity of roofs on government buildings and schools.

Measures that tackle the urban heat island effect also make cities more energy efficient (by reducing the cooling needs inside buildings) and more comfortable (by shading city residents). Individual cities need to implement strategies that make sense for their landscapes, their water resources, their usual climate and their populations, Ban-Weiss says.

But ameliorating urban heat can only do so much. There will still need to be a worldwide push to reduce emissions of greenhouse gases. Ban-Weiss and colleagues estimated how much cool roofs could counter warming from climate change in Southern California. Assuming that greenhouse gas emissions continue to increase, the widespread adoption of cool roofs in the Los Angeles metropolitan area would offset some of the warming expected by midcentury, the team reported in 2016 in Environmental Research Letters. But by the end of the century, Ban-Weiss says, the cool roof benefits “become mostly dwarfed by climate change.”

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