Heat Waves and Climate Change



Top Line

Human-caused global warming has increased the frequency, size, and duration of extreme heat events. What were once very rare events are now becoming more common.

The Essentials

  • Extreme heat is the deadliest form of extreme weather in the United States, causing more deaths than hurricanes and floods combined; more than twice as many deaths as tornadoes; and more than four times as many as from extreme cold. 1
  • Heat waves i are occurring three times more often than they did in the 1960s—about six per year compared to two per year. 2
  • Record-breaking hot months are occurring five times more often than would be expected without global warming, suggesting that 80 percent of such monthly heat records are due to human-caused climate change. 3
  • Summer nights have warmed at nearly twice the rate of summer days in the United States 4 —a shift that exacerbates health impacts because hot nights reduce the body’s ability to recover from hot days. 5 The percentage of the U.S. continental land area experiencing abnormally hot nighttime temperatures ii increased from 5 percent to 40 percent over the four decades between 1970 and 2010. 6
  • In addition to more extreme heat, humidity is also rising in some regions like the eastern United States. 7 This takes an extra toll on health because humidity interferes with the body’s ability to cool itself through the evaporation of sweat. 8 A temperature of 90 degrees F with 80 percent humidity feels like 113 degrees F. 9
  • Those most vulnerable to heat-related illness and death include young children, the elderly, those with chronic illnesses, student athletes who practice outside, outdoor workers, city dwellers, and those without air conditioning (or who lose it during power outages). 10
  • Heat waves iii are not just more frequent than in the past but on average affect a 25% larger land area in the Northern Hemisphere than they did in 1980; including ocean areas, heat waves grew 50% larger. 11 The Northern Hemisphere-wide string of extreme heat events in the summer of 2018 was the largest ever recorded and a new analysis concluded that it would not have occurred without human-caused global warming. 12
  • Recent evidence suggests that climate change is altering atmospheric circulation, such as the jet stream, causing persistent weather patterns to get stuck in place, increasing the duration and damaging effects of heat waves. 13, 14

Pitfalls to Avoid

It’s no longer true that “no single extreme weather event can be attributed to human-caused climate change”—a common refrain in past coverage of heat waves and other weather extremes. With more than 200 peer-reviewed studies published to date, attribution science—which can indicate the contribution of human-caused climate change to individual heat waves and other extremes—is increasingly credible and deserving of careful coverage.

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i. Defined for the purposes of the report cited to mean two or more days in which the coolest temperature (adjusted for humidity) at night is warmer than that of 85 percent of July and August nights averaged over a 30-year period.

ii. Defined for the purposes of the study cited as the warmest ten percent of such nighttime temperatures since 1910.

iii. Defined for the purposes of the study cited as three or more consecutive days with temperatures at least 90 percent higher than the historical maximum.


Last Updated July 16, 2019



This tip sheet for reporters is one in a series produced jointly by two philanthropically supported, editorially independent services for journalists: SciLine, based at the American Association for the Advancement of Science (AAAS), and Climate Communication.

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