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Our Cities Are Getting Hotter—And it’s Killing People

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Heat waves are causing more deaths in U.S. cities than all other disasters combined

People cool off at an open fire hydrant in the East Village neighborhood of Manhattan.
Nighttime temperatures made higher by urban heat island effect can be more dangerous than daytime highs as they don’t allow the human body to cool down.
AP Photo/Mary Altaffer

Among all the climate-related disasters that are confronting cities, heat waves are the deadliest. According to the Centers for Disease Control and Prevention, extreme heat now causes more deaths in U.S. cities than all other weather events combined. Longer, more frequent heat waves are expected in the future due to climate change, meaning summer’s death toll will rise.

Summers have been particularly brutal for cities in North America recently. In June 2018, dozens of people were killed across the U.S. and in Canada—including 28 people just in Montreal—after much of the country experienced multiple days that were 100 degrees Fahrenheit and higher. The following week, much of California experienced a heat wave with all-time record-setting temperatures, and another deadly heat wave swept through the Southwest later in July.

At one point, a punishing heat dome—a high-pressure system which locks in and intensifies hot air—had pushed temperatures above 90 degrees in 44 of 50 states.

Elsewhere in the Northern Hemisphere, other cities have seen high summer death tolls due to heat. Japan recorded an all-time high of 106 in 2018 as a total of 96 people were killed across the country. In Seoul, 29 people died during a two-week stretch of 95-degree days. Two separate deadly heat waves have struck Europe, where wildfires were burning inside the Arctic Circle.

Why are summer’s heat waves so deadly now?

The record-breaking heat set 2018 on track to be the fourth-hottest year on record. (The top three? 2016, 2017, 2015.) But it’s not just extreme temperatures, it’s these intensifying heat wave events—multiple high-temperature days in a row, with little nighttime relief—that are so devastating to cities.

Heat waves are especially deadly when nighttime temperatures don’t cool enough to offer urban residents relief. The human body isn’t able to recover from the effects of extreme heat if air temperatures don’t dip below 80 degrees Fahrenheit at night. This summer saw not only record-high temperatures, but record-high overnight lows in many parts of the country. That’s a bigger problem in cities, which retain their heat more than rural areas.

Extreme heat affects a city’s most vulnerable residents most. Older adults are most likely to suffer heat-related illness due to existing chronic medical conditions, as are residents of disadvantaged communities who may not have insulated homes or access to air conditioning. Workers who are outdoors may experience heat stress which can contribute to accidental deaths.

Researchers have only recently begun to understand the effects of extreme heat on urban residents. In 2003, during a heat wave that swept Europe, 35,000 people were estimated to have died at the time. Now scientists believe the death toll is closer to 70,000.

Is climate change causing deadly heat waves?

The increase in global average temperatures due to climate change makes extreme heat events more likely and “extreme” temperature more extreme, according to eight attribution studies by the Bulletin of the American Meteorological Society.

Extreme temperatures will also be most pronounced in cities that are further north, according to a study by Oxford University and the World Weather Attribution network. “What was once regarded as unusually warm weather will become commonplace,” said Dr. Friederike Otto, deputy director of the Environmental Change Institute at Oxford. “In some cases, it already has.”

This means the odds are increased that intense heat waves will happen both earlier and later in the year, according to the National Resources Defense Council (NRDC), lengthening the window for heat-related deaths to occur.

A 2017 NRDC study looked at how an increase in the number of “dangerous summer days” will increase the number of heat-related deaths in 45 U.S. cities. An estimated 150 Americans will die every summer day due to extreme heat by 2040, with almost 30,000 heat-related deaths annually. That’s twice the number of people who are killed by gun violence annually in the U.S. today.

How does urban design make extreme heat worse?

The way cities are designed—using heat-absorbing hardscape materials and eliminating cooling greenspace—means that urban areas can be several degrees hotter than the surrounding region due to what’s known as heat island effect. This is why cities are prioritizing heat-related design changes in an effort to cool down cities.

A healthy urban canopy is among the most effective ways for cities to combat extreme heat, so cities are finding creative ways to expand their tree-planting efforts. In Los Angeles, a recent study by the Los Angeles Urban Cooling Collaborative found that an increase in tree canopy paired with programs to cover roofs and pavements with more reflective surfaces could reduce heat-related deaths citywide by more than 25 percent.

Extreme heat paired with the way cities have grown in the last few decades have also put more people at risk for wildfires. At one point in 2018, more than 20 fires burned simultaneously in California before the traditional fire season had even started.

It’s estimated that the fire season has lengthened by 20 percent since 1979, due to climate change. At the same time, more Americans have moved to exurban fringes of cities that border forests or grasslands known as the wildland-urban interface (WUI). In the past 40 years, the number of Americans living in WUI lands has doubled. More than half of all new homes built since 1990 have been built on WUI lands. The deaths of 91 people in wildfires outside Athens in July are being partially blamed on the construction of over 3,000 illegal structures in hilly, forested areas.

Air-conditioning is another important tool to combat extreme heat, but plays a dubious role in our quest to cool cities. During heat waves, the immense amounts of energy air-conditioning units consume push aging power grids over capacity, causing major outages like one that affected 50,000 households in Los Angeles in July 2018. It’s estimated that air-conditioning generates half a billion metric tons of carbon dioxide emissions in the U.S. every year—a figure which will go up as cities get hotter. While air-conditioning might save lives, it’s also fueling another climate crisis.