It’s not your imagination: Hot summer days really are getting hotter.

In fact, the majority of Americans now face extreme summer heat much more frequently than in previous decades, according to a new analysis and map released Tuesday by the Natural Resources Defense Council. The study, which compared historical temperature data to present temperature data, found that nearly 210 million Americans — two-thirds of the U.S. population — live in counties that see more than nine extreme heat summer days annually, a marked increase from half a century ago.

“Scientific studies have shown that as temperatures rise, so do the number of heat-related illnesses and premature deaths,” Kim Knowlton, senior scientist and deputy director for NRDC’s Science Center, said on a press call Tuesday. “Heat is the number one extreme weather killer in the U.S.”

The report links the rise in extremely hot summer days to global climate change, arguing that as greenhouse gas emissions have contributed to a rise in global temperature, they have also contributed to more frequent and dangerous heat waves and extreme heat. The NRDC report specifically looked at two different sets of temperature data: summer temperatures between 1961 and 1990, and summer temperatures between 2007 and 2016. Researchers calculated “extreme heat days” by finding the number of days between 2007 and 2016 where temperatures exceeded the 90th percentile of daily maximum temperatures measured from 1961 to 1990.

The report adds to a growing body of science directly linking climate change with increased extreme heat: In 2015, the Bulletin of the American Meteorological Society’s annual attribution report found that nearly every extreme heat event that year was made more likely or more extreme by climate change. Earlier this summer, the New York Times also published a report based on analysis from retired NASA scientist James Hansen, detailing how summer temperatures have shifted “drastically,” becoming much hotter than in previous decades.


“It’s our hope that the map will provide some insights into what people know from their own experience, which is that temperatures locally are really changing and that the summers of the past no longer speak to the summers of the present,” Knowlton said.

Extreme heat was on full display around the world this summer, from record-breaking heat waves in the U.S. West and Southwest to a heat wave in Europe so hot that locals named it “Lucifer.” In Arizona, which saw temperatures rise near 120 degrees Fahrenheit towards the end of June, commercial airlines were grounded because they could not fly in such extreme temperatures. Record-heat also fueled several massive fires across the western United States, contributing to the most expensive fire season on record. And while the NRDC study looked only at summer temperatures — measurements taken between the months of June and August — climate change also contributes to unseasonably warm temperatures year round. This week in southern California, for instance, temperatures have climbed to the triple digits, breaking records and contributing to “the most dangerous fire weather conditions that Southwest California has seen in the past few years.”

Extreme heat has also been linked to stronger or more dangerous storms, like Hurricane Harvey, which dumped more five feet of rain over Houston and parts of the Texas Gulf Coast in August. The storm was fueled by above-average water temperatures in the Gulf of Mexico, as well as a warmer atmosphere, which allowed the storm to carry more moisture.

“Just in the last month, we’ve seen the profound impact of hurricanes fueled by warm water temperatures and wildfires fueled by warm temperatures,” Linda Rudolph, director of the Center for Climate Change and Health at the Public Health Institute said during Tuesday’s press call. “If we keep pouring climate pollutants into the atmosphere, we will face a level of global warming in which our best efforts will be inadequate to prevent the worst adverse impacts of heat on our health.”


Aside from fueling wildfires and storms, extreme heat poses a slew of public health risks, from heat-related illness like heat stroke to the exacerbation of existing health conditions like heart disease or asthma. Heat can be especially dangerous for low-income communities, which lack access to potentially life-saving technology like air conditioning units. Low-income communities can also lack information or access to cooling centers, which can be located miles away from vulnerable communities. This is especially true for the elderly, who may not be able to walk or access public transportation in order to get to cooling centers. Low-income families may also not have the funds to retrofit their homes to become more energy efficient — or less likely to let in heat during extremely hot months — and the Trump administration has proposed cutting federal programs that help improve the energy efficiency of homes for low income families.

Urban areas are especially vulnerable to extreme heat, due to the high density of paved surfaces that absorb and radiate heat, combined with a lack of cooling green spaces or trees — a phenomenon known as “urban heat island effect.” A study published by NRDC earlier this year found that 45 major urban areas in the United States could see 28,000 more deaths annually by the end of the century due to extremely hot days.

“Extreme heat events already cause more deaths in a typical year than any other extreme event,” Rudolph said. “When air conditioning can literally save lives, we need to make sure that people know about and access the help they need with energy costs.”

