Climate Change Will Expose Millions of People in U.S. to 'Off-the-Charts' Extreme Heat

In the coming decades, climate change will lead to a significant increase in the frequency and severity of extreme heat across the contiguous United States, the Union of Concerned Scientists (UCS) has warned.

The organization has conducted an analysis of current heat trends to make predictions about the future climate for a new report and accompanying study published in the journal Environmental Research Communications.

"Extreme heat is dangerous, and it can be deadly—it is currently one of the top weather-related causes of death in the United States," Rachel Licker, Senior Climate Scientist, at UCS told Newsweek.

"Extreme heat is projected to increase as a result of global warming, but we noticed that there was a lack of national-scale information on how extreme heat is likely to change in the most commonly reported term our country uses—the heat index, or, "feels like" temperature which results from the combination of temperature and humidity," she said. "Extreme heat affects all U.S. residents unlike other climate impacts that are more localized, such as hurricanes and large wildfires."

According to the research, climate change will lead to a spike in the number of days per year when the heat index—or "feels like" temperature—exceeds 105 degrees Fahrenheit, unless drastic action is taken to curb greenhouse gas emissions.

"Our results show that failing to reduce heat-trapping emissions would lead to a staggering expansion of dangerous heat across the U.S.," Licker said. "By midcentury, the number of days with a heat index above 105F would quadruple such that more than 150 cities across the country would experience an average of 30 or more days per year with a heat index above 105."

"By late century, about 120 million people across the U.S.—more than one-third of today's population—would experience the equivalent of a week or more of conditions so hot they exceed the National Weather Service's current heat index scale," she said.

Currently, the only place that experiences these "off-the-charts" days—when the heat index is 127 degrees Fahrenheit or higher—is the Sonoran Desert on the border of southern California and Arizona.

Because this area is sparsely populated, only about 2,000 people are exposed to these extreme days for a week or more every year on average. But worryingly, the researchers predict that this figure is set to rise significantly.

In fact, the analysis suggests that there will be few areas of the country that will escape these extreme heat events, save for some high-altitude mountainous regions.

"Our analysis shows a hotter future that's hard to imagine today," Kristina Dahl, senior climate scientist at UCS and co-author of the report, said in a statement. "Nearly everywhere, people will experience more days of dangerous heat even in the next few decades."

"By the end of the century, with no action to reduce global emissions, parts of Florida and Texas would experience the equivalent of at least five months per year on average when the 'feels like' temperature exceeds 100 degrees Fahrenheit, with most of these days even surpassing 105 degrees," she said.

An increase in days with extreme heat—particularly those which exceed the upper limit of the heat index—could pose unprecedented health risks which the country is not currently prepared to deal with.

"Extreme heat is already one of the top weather-related causes of death in the United States and contributes to many more deaths for a variety of reasons, including the fact that it exacerbates common health conditions like cardiovascular and respiratory diseases," Licker said.

"There are many segments of the U.S. that are uniquely vulnerable to extreme heat, including the elderly, children whose bodies are less efficient at cooling than adults, and low-income residents who may not be able to afford the cost of air conditioning units or the energy to run them for prolonged periods of time during an extreme heat event," she said.

But while the report paints a gloomy picture of future temperature trends, Licker notes that unlike many of the effects of climate change, extreme heat responds pretty rapidly to emission reductions.

"We need to take action on two fronts—making deep cuts in our heat-trapping emissions so that we can stave off the most dangerous increases in extreme heat and investing in measures to keep people safe when extreme heat hits," she said.

"To reduce heat-trapping emissions, we need to continue to implement and strengthen the Paris climate agreement by doing things like transitioning our energy system to low-carbon energy sources, ramping up energy efficiency, and electrifying as many energy systems as possible across the transportation, buildings, and industrial sectors," Licker said.

To keep people safe from extreme heats events, the researchers recommend that a variety of measures should be taken.

"We need to do things like develop next-generation heat warning systems and protective measures based on public health guidance, invest in heat-smart infrastructure so that buildings, public housing, energy, and transportation infrastructure can withstand future heat, and invest in low-carbon, energy-efficient power systems, appliances, and design features that can help cut heat-trapping emissions and other pollutants while providing access to affordable, reliable ways to help cool homes and businesses," Licker said.

This article was updated to include additional comments from Rachel Licker.

Sonoran desert
As seen from the air, the vast Sonoran Desert stretches into the distance on December 9, 2010 in the Tohono O'odham Reservation, Arizona. John Moore/Getty Images

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