Extreme Heat

• How Will Climate Change Affect Extreme Heat Across the United States?
• Impacts of Extreme Heat
• Resources
• References

---

There are several ways to define extreme heat. It is frequently based on how unusually hot and humid it is for an area.¹ While summertime temperatures of 100˚F might be normal for Phoenix, Arizona, they would be considered extreme for Seattle, Washington. Extreme heat can also be based on whether a certain temperature is reached. For example, the following figure counts extremely hot days as days where the temperature equals or exceeds 95˚F.

As average temperatures rise due to climate change, the risk of extreme temperatures, heat waves, and record-breaking temperatures increases. While warming temperatures are expected across the United States, the severity of warming in a given location depends on geographic factors such as latitude, elevation, and proximity to the ocean. Explore the following figure to learn how the number of extremely hot days is projected to change with different global temperature increases.

Source: National Climate Assessment Interactive Atlas Explorer

How Will Climate Change Affect Extreme Heat Across the United States?

• The United States is expected to continue to warm faster than other parts of the world. As average temperatures increase, the risk of extreme heat goes up. Across the contiguous United States, average temperatures have already risen about 60% more than the global average since 1970.² This is expected to continue as global temperatures rise due to climate change. The northern and western parts of the country are projected to warm even more.³
• Extremely hot days are projected to occur more often. The number of days above 95˚F is expected to increase across the United States.⁴
• Heat waves are projected to get worse. Multiday heat waves are expected to be longer, affect more of the country, and become more severe.⁵ In recent decades, heat waves have already become hotter, occur more often, and are larger and longer-lasting.⁶
• Warm nights are expected to become more common. Many parts of the country are projected to experience more warm nights where temperatures do not drop below 70˚F.⁷ Elevated nighttime temperatures mean that people and infrastructure do not have a chance to cool down overnight.

Impacts of Extreme Heat

Continued warming poses risks to many aspects of life, including:

• Human Health
• Daily Activities
• Agriculture and Food Supply
• Infrastructure and Transportation Systems

Human Health

• Heat is already the leading weather-related cause of death in the United States.⁸ The risk of heat-related death and illness is expected to increase as extreme heat events become more frequent and intense.⁹
  • Extreme heat threatens people’s health when the body is unable to cool down. One way to cool down is for sweat to evaporate off a person’s body. When humidity is high, it is harder for sweat to evaporate and keep a person cool. Heat, especially when made worse by humidity, can cause heat exhaustion and heat stroke, which can lead to hospitalization or death.¹⁰
  • Heat can worsen pre-existing health conditions. Higher temperatures are associated with an increase in emergency room visits and hospitalizations related to conditions such as cardiovascular diseases, respiratory diseases, and diabetes.¹¹ Heat can also cause adverse pregnancy and birth outcomes.
  • Extreme heat can have negative impacts on mental health and has been linked to increased rates of suicide and interpersonal violence.¹²
• While anyone can be impacted by extreme heat, certain groups are more at risk than others.¹³ Hotter summer temperatures will increase that risk.¹⁴
  • Children, adults over 65, pregnant people, people with pre-existing medical conditions or who are taking medications, and people with mental health or substance use disorders face a higher risk of heat-related health impacts.¹⁵
  • Heat waves disproportionately impact communities and people who have been marginalized, including people of color and low-income individuals.¹⁶ This is in part due to systemic discriminatory development practices, such as redlining. These practices have left some communities with less access to cooling resources and fewer strategies to reduce heat, such as green space and urban trees.¹⁷
  • People living in cities, especially in low-income and high-density neighborhoods, will experience more extreme heat because of the heat island effect.¹⁸
• People who work outdoors are more exposed to extreme heat. This can include agriculture, fishing, construction, transportation, utility, or delivery workers.¹⁹ Workers in buildings that are not air-conditioned or climate controlled are also at risk.²⁰

  

  • Without proper training and monitoring, as well as hydration, breaks, and shade,²¹ more frequent and intense heat waves could increase health risks for outdoor workers.
  • Farmworkers are more at risk of heat stress and death than other U.S. workers. The number of unsafe working days for farmworkers due to heat is projected to double by the middle of the century.²²

Daily Activities

• Extreme heat will impact schools, recreation, homes, workplaces, and other parts of daily life for children and adults.
  • High temperatures at home or at school can make it harder to learn and are associated with learning losses for children.²³ Many schools in the United States don’t have air conditioning or are in need of system upgrades.²⁴
  • Hotter temperatures can make outdoor recreational activities unsafe, particularly during midday when the sun is at its highest angle and is most intense.
  • Outdoor workers could lose up to 34 labor hours per year per person due to high temperature days with continued climate change.²⁵ They could also face lower or more irregular pay. Across the whole economy, this could contribute to up to $46 billion in lost wages by the middle of the century.²⁶
  • As climate change leads to an increase in unusually hot days, Americans are expected to spend more money on electricity for cooling.²⁷ Energy costs, which can already be a burden to families, may become increasingly unaffordable. Living in older or poorly insulated homes that are not energy-efficient adds to this concern.

Agriculture and Food Supply

• Hotter temperatures are expected to reduce agricultural food supply, disrupt subsistence activities, cause stress to livestock, and impact water availability. 
  • Drought and changes to the natural timing of seasonal plant growth lead to decreased crop yields.²⁸
  • Warming temperatures can increase the range of pests and non-native species that can harm crops and livestock.²⁹
  • Some crops, such as corn, wheat, and rice, are predicted to be up to 30 percent more expensive by the middle of the century because of reduced production.³⁰
  • Extreme heat can place strain on livestock and limit their production.³¹ Dairy cows are particularly sensitive to heat stress, which can reduce how much and the quality of milk they produce.^32,33
  • Indigenous people and others who practice foraging, hunting, fishing, and farming for subsistence, cultural traditions, and medicinal purposes face can be particularly vulnerable to climate impacts.³⁴

Infrastructure and Transportation Systems

• Rising temperatures and extreme heat can damage energy infrastructure and transportation systems, causing safety hazards, increased need for road maintenance, and airport and train delays.³⁵
  • Extreme heat can cause powerlines to sag. This reduces the efficiency of the grid during a time when demand may already be high due to increased use of cooling systems.³⁶
  • Extreme heat can contribute to electricity infrastructure, including transformers and transmission lines, deteriorating more quickly.³⁷
  • Many types of vehicles can overheat due to high temperatures. Tires can also deteriorate more quickly.³⁸
  • Extreme heat can cause roadways, airport runways, and rail lines to expand and buckle, leading to delays and requiring more frequent repairs or speed restrictions.³⁹

Resources

• Fifth National Climate Assessment: Human Health
• Fifth National Climate Assessment: Climate Trends
• Climate Change and Social Vulnerability in the United States
• Climate Change and Children’s Health and Well-Being in the United States
• National Integrated Heat Health Information System Heat.gov

References

¹ EPA. (2016). Climate Change and Extreme Heat: What You Can Do to Prepare (pdf) (5 MB). EPA 430-R-16-061.

² Marvel, K., et al. (2023). Ch. 2. Climate trends. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH2 p. 2-4.

³ Ibid, p. 2-21.

⁴ Ibid, p. 2-24.

⁵ Ibid, p. 2-24.

⁶ Ibid, p. 2-24.

⁷ Ibid, p. 2-18.

⁸ NOAA (National Oceanic and Atmospheric Administration). (2023). Weather related fatality and injury statistics. Retrieved November 26, 2024, from www.weather.gov/hazstat

⁹ Hayden, M.H., et al. (2023). Ch. 15. Human health. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC.  https://doi.org/10.7930/NCA5.2023.CH15 p. 15-6.

¹⁰ Sarofim, M. C., Saha, S., Hawkins, M. D., Mills, D. M., Hess, J., Horton, R., Kinney, P., Schwartz, J., & St. Juliana, A. (2016). Chapter 2: Temperature-related death and illness. In USGCRP (U.S. Global Change Research Program), The impacts of climate change on human health in the United States: A scientific assessment (pp. 43–69). http://dx.doi.org/10.7930/J0MG7MDX

¹¹ Hayden, M.H., et al. (2023). Ch. 15. Human health. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH15.p. 15-6.

¹² Ibid, p. 15-10.

¹³ Ibid, p. 15-6.

¹⁴ EPA. 2021. Climate Change and Social Vulnerability in the United States: A Focus on Six Impacts. U.S. Environmental Protection Agency, EPA 430-R-21-003.

¹⁵ Hayden, M.H., et al. (2023). Ch. 15. Human health. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH15. p. 15-6.

¹⁶ EPA. 2021. Climate Change and Social Vulnerability in the United States: A Focus on Six Impacts. U.S. Environmental Protection Agency, EPA 430-R-21-003.

¹⁷ Chu, E.K., et al. (2023). Ch. 12. Built environment, urban systems, and cities. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH12. p. 12-13.

¹⁸ Ibid. pp. 12-12-13.

¹⁹ Hayden, M.H., et al. (2023). Ch. 15. Human health. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH15. p. 15-12.

²⁰ Ibid, p. 15-12.

²¹ EPA. (2024). Preventing Heat Stress in Agriculture. Retrieved 11/11/2024.

²² Bolster, C.H., et al. (2023). Ch. 11. Agriculture, food systems, and rural communities. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH11. p. 11-16.

²³ EPA. 2023. Climate Change and Children’s Health and Well-Being in the United States. U.S. Environmental Protection Agency, EPA 430-R-23-001.

²⁴ United States Government Accountability Office. (2020). School Districts Frequently Identified Multiple Building Systems Needing Updates or Replacement. GAO-20-494.

²⁵ EPA. (2021). Climate Change and Social Vulnerability in the United States: A Focus on Six Impacts. U.S. Environmental Protection Agency, EPA 430-R-21-003. p. 39.

²⁶ Hayden, M.H., et al. (2023). Ch. 15. Human health. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH15. p. 15-12.

²⁷ EPA. (2024). Climate Change Indicators: Residential Energy Use. Retrieved 11/11/2024.  

²⁸ Bolster, C.H., et al. (2023). Ch. 11. Agriculture, food systems, and rural communities. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH11. p. 11-4. 

²⁹  Ibid. p. 11-24.

³⁰  Ibid. p. 11-17.

³¹ Wilson, A.B., et al. (2023). Ch. 24. Midwest. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH24. p. 24-8.

³² Ibid. p. 24-8.

³³ Key, N., Sneeringer, S. and Marquardt, D. (2014). Climate Change, Heat Stress, and U.S. Dairy Production. ERR-175, U.S. Department of Agriculture, Economic Research Service.

³⁴Bolster, C.H., et al. (2023). Ch. 11. Agriculture, food systems, and rural communities. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington DC. https://doi.org/10.7930/NCA5.2023.CH11. p. 11-18.

³⁵ Liban, C.B., et al. (2023). Ch. 13. Transportation. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH13. p. 13-8.

³⁶ Zamuda, C.D., et al. (2023). Ch. 5. Energy supply, delivery, and demand. In: Fifth National Climate Assessment. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH5. p. 5-7.

³⁷ Ibid, p. 5-7.

³⁸Jacobs, J.M, et al. (2018). Ch. 12. Transportation. In: Fourth National Climate Assessment, Volume II. U.S. Global Change Research Program, Washington, DC. https://doi.org/10.7930/NCA5.2023.CH12. p. 489.

³⁹ Ibid, p. 489.