Heat waves and climate change

Top Line

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

Facts for Any Story

• 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.¹National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS), U.S. Natural Hazards Statistics, 2018. View Source
• Heat waves are occurring three times more often than they did in the 1960s—about six per year compared to two per year.²U.S. Global Change Research Program Indicator Platform, Heat Waves. View Source Some recent evidence suggests the increase has been even greater.³Diffenbaugh, N. S., (2020) Verification of extreme event attribution: Using out-of-sample observations to assess changes in probabilities of unprecedented events. Science Advances, 6, eaay2368. View Source
• The average number of days between May and September with at least one large heat wave in the northern mid- to high latitudes doubled between the 1980s and the 2010s, from 73 to 152. By the 2010s, all but 10 days during those five months had two or more simultaneous heat waves within those latitudes—a seven-fold increase in such days since the 1980s and noteworthy since simultaneous heat waves severely increase stresses on ecosystems, agriculture, and utilities. During the same period, the geographic expanse of coinciding heat waves grew by 46% and the hottest day of the year with coinciding heat waves became 17% hotter.⁴Rogers, Cassandra D.W. et al, (2022) Sixfold Increase in Historical Northern Hemisphere Concurrent Large Heatwaves Driven by Warming and Changing Atmospheric Circulations. Journal of Climate, 1063–1078. View Source
• The heat wave that struck the Pacific Northwest in the last week of June 2021 broke heat records in dozens of places. In Quillayute, Washington, the temperature hit 110 degrees F, exceeding the previous record by 11 degrees.⁵Di Liberto, Tom, (2021) Astounding heat obliterates all-time records across the Pacific Northwest and Western Canada in June 2021. National Oceanic and Atmospheric Administration Event Tracker. View Source According to one analysis, that heat wave would have been virtually impossible in the absence of human-caused global warming.⁶Philip, Sjoukje Y. et al, (2022) Rapid attribution analysis of the extraordinary heat wave on the Pacific coast of the US and Canada in June 2021. Earth System Dynamics, 13, 1689–1713, 2022. View Source
• Globally, there has been a 90-fold increase in the frequency of monthly heat extremes in 2011-2020 compared to 1951-1980: Extreme heat events that deviate strongly from the norm in a given region (so-called 3-sigma events) now affect about 9% of all land area at any time, on average. In the last decade, record-breaking hot months occurred eight times more often than would be expected without global warming.⁷Robinson, Alexander et al, (2021) Increasing heat and rainfall extremes now far outside the historical climate. npj Climate and Atmospheric Science 4, Article number: 45 (2021). View Source With continued climate change, the frequency of these intense heat events is likely to increase further,⁸Mann, M.E., et al., (2018), Projected changes in persistent extreme summer weather events: The role of quasi-resonant amplification. Science Advances 4, eaat3272. View Source with particular risks for some important food-producing regions.⁹Kornhuber. K. et al., (2019) Amplified Rossby waves enhance risk of concurrent heatwaves in major breadbasket regions, Nature Climate Change, 10, 48-53. View Source
• Summer nights have warmed at nearly twice the rate of summer days in the United States¹⁰NOAA data show that summer nighttime lows have increased by 1.46 degrees F while summer daytime highs have increased 0.77 degrees F per century from 1895 when national records began to 2018. View Source—a shift that exacerbates health impacts because hot nights reduce the body’s ability to recover from hot days.¹¹U.S. Centers for Disease Control and Prevention (CDC), National Environmental Public Health Tracking Network. View Source The percentage of the U.S. continental land area experiencing abnormally hot nighttime temperatures increased from 5 percent to 40 percent over the four decades between 1970 and 2010.¹²NOAA/National Centers for Environmental Information, Climate Extremes Index. (Specific Data.) View Source
• In addition to more extreme heat, humidity is also rising in some regions like the eastern United States.¹³Seager, R. et al. (2015), Journal of Applied Meteorology and Climatology, 54, 1121. View Source¹⁴Raymond, C., et al., (2020), The emergence of heat and humidity too severe for human tolerance, Science Advances 6 (19), eaaw1838 View Source This takes an extra toll on health because humidity interferes with the body’s ability to cool itself through the evaporation of sweat.¹⁵NOAA, NWS, What is the Heat Index? View Source A temperature of 90 degrees F with 80 percent humidity feels like 113 degrees F.¹⁶Ibid. View Source
• 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).¹⁷CDC, Natural Disasters and Severe Weather. View Source
• 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.¹⁸Horton, D. E., Diffenbaugh. A. et al.(2015), Nature, 522, 465. View Source¹⁹Mann, Michael E. et al.(2017), Scientific Reports, 7, 45242. View Source

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.