Heat Waves and Climate Change

Across the globe, hot days are getting hotter and more frequent, while we’re experiencing fewer cold days. Over the past decade, daily record high temperatures have occurred twice as often as record lows across the continental United States, up from a near 1:1 ratio in the 1950s. Heat waves are becoming more common, especially in the U.S. West, although in many parts of the country the 1930s still holds the record for number of heat waves (caused by the Dust Bowl and other factors).

By midcentury, if greenhouse gas emissions are not significantly curtailed, scientists expect 20 record highs for every record low. In parts of the South, the frequency of days above 95 degrees Fahrenheit could triple, to over 75 days per year.


Increase in Total U.S. Heatwave Days

Threats Posed by Extreme Heat

Extreme heat can increase the risk of other types of disasters. Heat can exacerbate drought. This, in turn, can encourage more extreme heat, as the sun’s energy acts to heat the air and land surface, rather than to evaporate water. Hot, dry conditions also increase the risk of wildfires.

Human Health

Extreme heat is the deadliest natural disaster in the United States, killing more people on average (about 600 per year) than hurricanes, lightning, tornadoes, earthquakes, and floods combined. The Billion Dollar Weather Disasters database compiled by the National Oceanic and Atmospheric Administration lists heat waves as four of the top 10 deadliest U.S. disasters since 1980. Two heat waves in 1980 and 1988 were particularly deadly and account for the vast majority of extreme weather-related deaths in the database, with Hurricane Katrina (1,833 deaths) as the only non-heat wave event that caused more than 500 fatalities.

High humidity and elevated nighttime temperatures appear to be key ingredients in causing heat-related illness and mortality. Heat stress occurs in humans when the body is unable to cool itself effectively. Normally, the body can cool itself through sweating, but when humidity is high, sweat will not evaporate as quickly, potentially leading to heat stroke. When there’s no break from the heat at night, it can cause discomfort and lead to health problems, especially for the poor and elderly. Daily minimum temperatures in the U.S. are increasing slightly faster than daily maximum temperatures.


High temperatures at night can be particularly damaging to agriculture. Some crops require cool night temperatures, and heat stress for livestock rises when animals are unable to cool off at night. Heat-stressed cattle can experience declines in milk production, slower growth, and reduced conception rates.


Higher summer temperatures will increase electricity demand for cooling, as has been observed over the past 20 years. At the same time, higher temperatures lower the ability of transmission lines to carry power, possibly leading to electricity reliability issues during heat waves. Although warmer winters will reduce the need for heating, modeling suggests that total U.S. energy use will increase in a warmer future. In addition, as rivers and lakes warm, their capacity for absorbing waste heat from power plants declines. This can reduce the thermal efficiency of power production, make it difficult for power plants to comply with environmental regulations regarding their cooling water.

How to Build Resilience

Communities can bolster their resilience and reduce the impacts of extreme heat by:

  • Creating heat preparedness plans, identifying vulnerable populations, and opening cooling centers during extreme heat.
  • Using green roofs, improved building materials, and shaded building construction to reduce the urban heat island effect.
  • Pursuing energy efficiency to reduce demand on the electricity grid, especially during heat waves.
  • Shading and cooling livestock, breeding livestock selectively for heat tolerance, and switching to growing more heat-resistant crops.