A cartoon of people standing in flames looking happy and a devil looking confused. The caption reads: "Ah, what a relief! The heat waves on Earth were becoming unbearable."

Extreme heat may not trigger the same visceral fear as a tornado, but it causes nearly twice as many fatalities in the United States each year — more than any other weather hazard. As the climate continues to warm, that number could rise dramatically in the U.S. and around the world.

Since the late 1800s, human-caused climate change has warmed the Earth’s average temperature by around 1 degree Celsius (1.8 degrees Fahrenheit). That doesn’t sound like much, but a relatively small warming of the average temperature has a resulted in a large jump in extreme heat.

Is there a link between climate change and recent heat waves?

The American Meteorological Society defines a heat wave as “a period of abnormally and uncomfortably hot and usually humid weather.” A number of notable heat waves have occurred in recent years with the summer of 2023 providing many examples of how climate change drives heat waves. 

The summer of 2023 was the hottest on record by a wide margin, according to satellite data analyzed by Europe’s Copernicus Climate Change Service. And an analysis from Climate Central’s Climate Shift Index tool shows that the fingerprints of climate change are all over this unusually hot season. Over 6.5 billion people — 81% of the global population — experienced at least one hot day in July 2023 that was at least three times more likely because of climate change. Of those, at least 2 billion people saw climate-induced high temperatures on every single July 2023 day.

Heat waves were seen in nearly every corner of the globe in July 2023, and climate change made them more likely according to World Weather Attribution, a research group based in the U.K.

“Without human induced climate change these heat events would however have been extremely rare,” states a report published on the World Weather Attribution website. “Maximum heat like in July 2023 would have been virtually impossible to occur in the US/Mexico region and Southern Europe if humans had not warmed the planet by burning fossil fuels.”

What can we expect from heat waves in the future?

Our recent past is merely a paltry precursor to a much hotter future. In a 2019 study, researchers at Princeton University found that as global temperatures increase, heat waves will become more frequent and the time between them will become shorter.

An investigation of future regional heat waves finds that the number of heat-wave days may increase by 4 to 34 days per season for every one degree Celsius (1.8 degrees Fahrenheit) of increased global warming. 

In a 2016 post, climate scientist James Hansen warned, “the tropics and the Middle East in summer are in danger of becoming practically uninhabitable by the end of the century if business-as-usual fossil fuel emissions continue.”

A study released by the Union of Concerned Scientists in July 2019 found a dramatic increase in dangerous heat index days because of combined higher temperatures and humidity. The researchers found that by mid-century, in the United States, days with 105-degree heat index temperatures will triple. 

Research released around the same time by the Swiss Federal Institute of Technology in Zurich, Switzerland, concluded that in general, cities’ climates will shift 600 miles southward by mid-century. By 2050, Minneapolis’ warmest month will increase from 80 F to 90+ F.

Dallas, Texas, currently has just a few days above 105 degrees Fahrenheit per year. By 2050, that number is expected to jump to nearly 30 — and by 2100 it may catapult to 60 or more days, according to the nonprofit Climate Central. 

What are the consequences of extreme heat?

A large increase in extreme heat will come with consequences. Here are a few examples:

  • Intense marine heat waves may wipe out the majority of global coral reefs by mid-century, along with the vulnerable creatures that rely on them and the benefits to people the reefs provide.
  • Hotter days evaporate more moisture from the ground, drying out vegetation. The result: losses in agriculture, more intense large wildfires, and a longer fire season.
  • Workplaces may get less safe and productive  as increases in heat and humidity push the limits of human tolerance, making it difficult for workers to stay cool. Outdoor workers are at heightened risk of heat-related illnesses and fatalities.
  • More heat-related illnesses and deaths will occur. A 2017 study concludes: “It’s the sustained nature of heat waves that impose more devastating impacts than extreme temperatures on a single day. Excessive human morbidity and mortality rates are clearly associated with sustained extreme temperatures.”
  • The 2018 National Climate Assessment estimates that in 49 large U.S. cities alone, changes in extreme temperatures are projected to result in more than 9,000 additional premature deaths by the end of the century, assuming people continue releasing heat-trapping gases at current rates. That’s a conservative estimate compared to an analysis from the Natural Resources Defense Council, which found that an increase in dangerously hot days could lead summertime deaths across large U.S. urban areas to grow from the 1975-2010 annual average of about 1,360 to 13,860 by the mid-2040s — and to 29,850 by late century.
  • Humidity may become literally unbearable by the end of this century for people living in regions like the Persian Gulf and South China. According to Tom Matthews of Loughborough University in the United Kingdom, once the wet bulb temperature (the temperature of the air factoring in cooling as a result of evaporation) reaches 95 degrees F, the body can no longer cool itself through evaporation of sweat, which can lead to heat stroke and death. Wet-bulb temperatures this high are projected to be more common later this century.
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What can we do?

These are frightening numbers, but there’s a lot that can still be done now to minimize the impacts. Society could invest in aggressive steps to reduce carbon pollution and other heat-trapping gases and to increase our abilities to withstand harsher climate conditions.

The National Climate Assessment estimates half of the projected deaths in the U.S. can be avoided if the global community takes swift climate action. But because some of the projected heating is already inevitable, we will also have to adapt to a warmer world, for example by planting shade trees and ensuring people have access to cooling centers. Such actions would also help protect people’s lives.

This article was revised and updated September 6, 2023.