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Warmth waves grow to be extra lethal when nights heat up quicker than days

A photographer takes a photo of the thermometer, which reads 55.5 degrees Celsius at the Furance Creek visitor station in Death Valley National Park, California on August 17, 2020.

David Becker | Reuters

The U.S. has experienced stifling hot temperatures this summer, breaking records and exposing millions of people to excessive heat warnings.

During a historic heat wave in the western United States, temperatures in California's Death Valley reached 130 degrees Fahrenheit, a potential record for the highest temperature reliably measured on Earth. Oakland, California hit 100 degrees for the first time in August and Phoenix had the highest temperature of the month at 117 degrees. And on Sunday, temperatures hit 121 degrees in part of Los Angeles as the state copes with ongoing wildfires.

The stifling heat is becoming more and more dangerous with climate change. One reason for this is that global warming is not happening uniformly: lower night temperatures, which normally provide critical relief from the hot days, are disappearing.

According to the National Oceanic and Atmospheric Administration, summer night temperatures are now warmer and warming up faster than daytime temperatures. This is a dangerous and potentially fatal combination of high day and night temperatures that does not give the human body a chance to cool off at night.

"Warm nights mean fewer chances to cool off, which increases exposure to high temperatures, especially for vulnerable people and places," said Kristie Ebi, professor of global health at the University of Washington.

In the US, heat kills more people than any other weather disaster, from floods to hurricanes.

"Climate change is dramatically increasing the intensity of heat extremes, and the consequences for human health are already evident in countries around the world," said Ben Zaitchik, associate professor at Johns Hopkins University. "If the body cannot cool down at night, the health effects of a hot period can be particularly severe."

Firefighter Sara Sweeney uses a backfire with a drip torch north of Monrovia, California on September 10, 2020 to protect mountain communities from the Bobcat fire in the Angeles National Forest.

David McNew | Getty Images

Climate change makes heat waves and droughts more frequent, more intense and more widespread. Dry and hot conditions exacerbate forest fires, which have become more devastating in recent years. Dozens of major fires are currently burning on the west coast of the US, destroying hundreds of homes and devastating entire neighborhoods in Oregon.

Climate change is also causing wetter heat waves. Hot and saturated air does not allow sweat to evaporate as quickly and heats the body even more, which can be fatal.

"The trend in California that we've seen since the 2006 heatwave is that heatwaves are also more humid and Californians in general are not used to high humidity and high temperatures – that combination is deadliest," said Rupa Basu, Chief of Air and Climate Epidemiology at the California Environmental Impact Assessment Office.

"Because of blackouts with blackouts, we can't just rely on (air conditioning) as a way to cool down," added Basu.

According to Randall Cerveny, a researcher at Arizona State University who studies temperature records for the World Meteorological Organization, daily lows in many parts of the world have risen faster than daily highs in recent decades.

Phoenix hit an alarming record this summer when there were 28 nights with temperatures never dropping below 90 degrees, beating the old record of 15 days in 2013 and 2003, according to Cerveny. The city also saw two separate weeklong streaks of nights that didn't drop below 90 degrees.

The temperature extremes are the result of global climate change as well as what is known as an urban heat island, which occurs when heat is absorbed by heat-absorbing asphalt or concrete during the day, causing hotter nights and early mornings.

"The extent of unwanted diseases and deaths will depend on proactive measures to raise awareness of the health risks of heat, reshape cities to reduce urban heat islands and reduce our greenhouse gas emissions," said Ebi.

The poor and minorities, who tend to live in areas without trees, are disproportionately affected by extreme heat. Blacks and Latinos in the US are more likely to live in hotter areas with more industrial activity and highways.

With higher night temperatures, people who cannot afford air conditioning in their homes are at increased risk of heat-related illnesses. And by sending hot air outside of homes, air conditioners can worsen the urban heat island.

"During the day you can seek relief by moving to cooler surroundings. At night, when you can't air-condition your house to a comfortable temperature, you're at the mercy of the background temperature," said Zaitchik.

Globally, every decade for the past 60 years has been hotter than the last. It is practically certain that 2020 will be one of the hottest years in history.

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