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Extreme Heat and Climate Change

Extreme heat—periods of unusually high temperatures that threaten human health, ecosystems, and infrastructure—is becoming more frequent and intense as the climate warms. Communities across the United States are already experiencing longer heat seasons, more dangerous heat index days, and greater risks to vulnerable populations.

Historical Conditions and Future Projections for Heat

Across the globe, hot days are getting hotter and occurring more frequently. The year 2024 was the hottest year on record, with global average temperatures more than 1.5 °C above the 1850-1900 average. The past 10 years, from 2015 to 2024, were the hottest on record globally. In cities across the United States, the average rate of extreme heat events increased from two per year in the 1960s to ten per year between 2010 and 2020. Additionally, as of 2024, the average length of heat-wave season in the U.S. has increased by 46 days since the 1960s.

Looking ahead, the Fifth National Climate Assessment estimates that most areas of the United States will experience 15–30 more days over 95°F per year with 2°C of global warming. Some places, like Florida, could experience up to 50 more days over 95°F per year under this scenario, while the Northeast may only experience an increase of 5–15 days over 95°F per year.

Heat waves are more dangerous when combined with high humidity. The heat index measures the combination of temperature and humidity. A 2019 study projected that nationwide over the coming century, the annual number of days with a heat index above 100 degrees F will double and days with a heat index above 105 degrees F will triple, compared to the end of the 20th century.

Notes

These maps show changes in the (a) projected number of hot days with a maximum temperature at or above 95°F, (b) cold days (minimum temperature at or below 32°F), and (c) warm nights (minimum temperature at or above 70°F) at a global warming level of 2.0°C. Changes are relative to the period 1991–2020. Based on LOCA2/STAR. Values for Alaska, Hawai‘i, and Puerto Rico are averages from STAR downscaling of 44, 15, and 31 stations, respectively. The Hawai‘i value for cold days is for Mauna Loa, the only network station where freezing temperatures occur. Areas of no change (shown in white) generally will not experience temperatures exceeding those thresholds. Data were not available for the US-Affiliated Pacific Islands and the US Virgin Islands. Figure credit: NOAA NCEI and CISESS NC.

Source

Fifth National Climate Assessment, Climate Trends

Impacts of Extreme Heat

Rising temperatures across the country pose a threat to people, ecosystems, and the economy through impacts on public health, worker productivity, built infrastructure, agriculture, and energy systems. Extreme heat also interacts with other climate hazards, amplifying risks across sectors.

Extreme heat can increase the risk of other types of disasters and exacerbate the consequences of existing environmental disparities. Heat can worsen drought conditions, and hot, dry weather can, in turn, increase wildfire risk. Buildings, roads, and infrastructure absorb heat, leading to temperatures that can be 1 to 7 degrees F hotter in urban areas than in outlying areas – a phenomenon known as the urban heat island effect, which disproportionately impacts frontline communities.

Public Safety and Health Impacts

Extreme heat is the leading cause of weather-related deaths in the United States, resulting in an average of 238 fatalities a year between 1995 and 2024. Between 2000 and 2025, heat deaths in the United States increased by more than 50%.

Hot days are associated with increases in heat-related illnesses, including cardiovascular and respiratory complications and kidney disease. Heat stress occurs in humans when the body is unable to cool itself effectively. Usually, the body cools itself through sweating, but when humidity is high, sweat evaporates more slowly, potentially leading to heat stroke. High humidity and elevated nighttime temperatures are likely key ingredients in causing heat-related illness and mortality. When extreme heat persists into the night, it can cause discomfort and lead to health problems, especially for people who lack access to cooling, often in low-income communities. Other groups that are particularly vulnerable to heat stress include older adults, infants and children, people with chronic health conditions, and outdoor workers.

Our blog article “Equity in Resilience: Addressing the unequal health impacts of extreme heat” further details how extreme heat disproportionately impacts specific populations and suggests steps state and local governments can take to respond to heat in their communities equitably.

Extreme temperatures can also negatively impact air quality. Hot, sunny days can increase ground-level ozone production, a harmful pollutant that is the main component of smog and can damage the respiratory system. Exposure to elevated ozone levels is particularly harmful to people with asthma. In addition, greater use of air conditioning requires more electricity, which, depending on the source, can emit other pollutants, including particulates that affect air quality. These increases in ozone and particulate matter can pose serious risks to people, particularly the same vulnerable groups directly impacted by heat mentioned above.

Agriculture Impacts

High temperatures can be damaging to agriculture. High daytime temperatures negatively impact plant growth, and some crops require cool night temperatures. Heat waves also increase the chances of livestock experiencing heat stress, especially when nighttime temperatures remain high, and animals are unable to cool off. Heat-stressed cattle can experience declines in milk production, slower growth, and reduced conception rates.

Heat waves can exacerbate droughts and wildfires, leading to negative impacts on the agriculture sector. For example, the 2021 drought in the West forced North Dakota cattle ranchers to sell their stock due to a lack of winter feed. Wildfires in California have burned agricultural land and are raising insurance costs for farms and wineries.

Overall, rising temperatures and more frequent extreme heat events threaten both crop yields and livestock productivity, especially when combined with other climate‑driven hazards like drought and wildfire.

Energy Impacts

Warmer temperatures affect many aspects of the U.S. energy system, including production, transmission, and demand. While higher summer temperatures increase electricity demand for cooling, they also reduce the capacity of transmission lines to carry power, potentially leading to reliability issues such as rolling blackouts during heat waves. Although warmer winters may reduce heating demand, modeling suggests that total U.S. energy use will still 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 thermal efficiency, making it difficult for power plants to comply with environmental regulations on cooling-water temperatures. These challenges could lead to temporary plant shutdowns during periods of extreme heat.

Overall, extreme heat intensifies stress on the energy system by increasing demand, reducing transmission efficiency, and straining power‑plant operations—raising the risk of outages during the times communities need cooling the most.

How to Build Resilience to Extreme Heat

Our publication, “Resilience Strategies for Extreme Heat,” lays out strategies for building resilience to extreme heat, such as:

  • Identifying vulnerable populations and creating heat preparedness plans with all residents in mind, which may include steps such as opening cooling centers during extreme heat and adopting workplace heat-stress standards.
  • Installing cool and green roofs and cool pavement to reduce the urban heat island effect.
  • Planting trees to provide shade and to cool the air through evapotranspiration.
  • Pursuing energy efficiency to reduce demand on the electricity grid, especially during heat waves.

We also explore near-term local policy actions to address extreme heat in our blog article, “Targeted policies to address extreme heat in the United States.”

Other Tools and Resources:

The Climate Mapping for Resilience and Adaptation portal helps communities understand and plan for their climate risks today and in the future, including a real-time map of wildfire, drought, flooding, and extreme heat across the United States.

Learn more about resilience through our Climate Resilience Portal.

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