Extreme Heat and Climate Change

Extreme Heat and Climate Change

Updated July 30, 2026

Scientific consensus supports a causal relationship between

increasing global temperatures and the incidence of extreme heat.

Extreme heat can have a range of serious consequences, with

record-breaking temperatures and heatwaves occurring in recent

years. For example, global temperature datasets from the

National Aeronautics and Space Administration (NASA) and

National Oceanic and Atmospheric Administration (NOAA)

show 2024 was the warmest year on record and 2015-2024 was

the warmest decade on record since records began in 1880. Both

the year and the decade are part of a global warming trend

(Figure 1). In addition, the United States experienced recordbreaking heatwaves in 2021 and 2023. These heatwaves brought

extreme temperatures and life-threatening conditions in some

areas. Historical studies of the United States have found

increases in heatwaves regionally during the period 1981-2015

and for the continental United States (CONUS) during the period

1981-2018.

This In Focus describes some of the health, infrastructure, and

productivity impacts of extreme heat; the relationship between

extreme heat and human-caused climate change; and related

policy considerations. Although the terms extreme heat or

heatwave lack a consensus definition, some studies have

characterized extreme heat events by their frequency, intensity,

and duration.

Extreme Heat Effects

A study published in the Journal of the American Medical

Association found that 1,722 heat-related deaths occurred in the

United States in 2022. The Centers for Disease Control and

Prevention (CDC) states that extreme heat can cause various

effects, including heat stroke, heat exhaustion, fainting, heat

cramps, and heat rash. Extreme heat has affected U.S.

communities, causing heat-related deaths and increases in heatrelated medical conditions. These extreme heat events can place

a strain on health care systems.

Extreme heat can affect the health, safety, and productivity of

workers. According to the CDC, extreme heat can put workers at

increasing risk of workplace injuries and other issues.

Extreme heat can strain the power grid, potentially leading to

high demand for air conditioning, refrigeration, and other power

needs. It has been estimated that during summer 2023, about

two-thirds of North America faced the potential for insufficient

operating reserves of electricity from extreme heat. Extreme heat

may accelerate the degradation of roads, bridges, and railroad

tracks. Increases in heat and humidity may decrease aircraft

performance, and extreme heat may reduce the operational

capacity of military aircraft by reducing payload and range.

According to some sources, past extreme heat events caused

flight delays and cancellations in Seattle, Portland, London, and

Phoenix.

Extreme heat can stress plants, livestock, and poultry, reducing

agricultural yields. Extreme heat exposure during critical growth

stages may reduce yields in crops, such as corn and soybeans.

Extreme heat events can stress livestock and may reduce meat

and milk production.

Figure 1. Global Average Temperature Departures, 1880-2025

Source: NOAA, National Centers for Environmental information, “Climate at a Glance: Global Time Series.”

Note: Global average temperature departures calculated as deviations from the 1901-2000 baseline average.

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Extreme Heat and Climate Change

Extreme Heat and Human-Induced Climate

Change

According to the U.S. Global Change Research Program

(USGCRP), human activity is warming the planet, leading to

more frequent extreme high temperatures and heatwaves. The

2023 USGCRP Fifth National Climate Assessment (NCA5)

stated the following:

The connection between warming and heatwaves is well

understood: at the very basic level, as average temperatures

warm, the risk of extreme temperatures and record-breaking

temperatures goes up.

The Intergovernmental Panel on Climate Change Sixth

Assessment Report stated that “human-induced greenhouse gas

forcing is the main driver of the observed changes in hot and cold

extremes on the global scale.”

In addition to studies on the general connection between

warming and extreme heat, climate attribution studies examine

the influence of human-caused climate change on individual

extreme heat events. While not every extreme heat event is

caused by climate change, some studies have found that humancaused climate change has increased the intensity of certain

extreme heat events in the United States and other countries.

The NCA5 stated, based on modeling, that it is very likely that

heatwaves will increase in frequency, severity, duration, and

spatial extent as global warming continues. The NCA5 included

estimates of the increase in annual days above 95°F for the

CONUS, Hawaii, and Puerto Rico, for global warming of 2.0°C

(3.6oF) above preindustrial temperatures (Figure 2). A

Department of Defense indicator of changes in heat risk exposure

is the average number of days with maximum temperatures

above 95°F. The NCA5 estimated that the number of days above

95oF (i.e., one way to define extreme heat) is projected to

change, depending on the region, by up to 50 additional days

with 2°C of global warming. The largest increases are projected

to occur in southern regions.

Considerations for Congress

If Congress chooses to address extreme heat, it can do so in a

variety of ways; the following discussion is not comprehensive.

For example, Congress has oversight of federal activities

authorized in congressional legislation, including those for

research, preparation, response, and mitigation of extreme heat

risks. Congress may consider the level of funding for research on

extreme heat. For example, NOAA has ongoing research to

forecast extreme heat. Some scientists have suggested the need to

better account for vegetation dynamics and aerosol effects in

order to improve modeling of heatwaves. Others have proposed

interdisciplinary research on reducing vulnerabilities to extreme

heat risk.

Federal actions to prepare and respond to heat extremes are

subject to congressional oversight and include the National

Integrated Heat Health Information System (NIHHIS). News

sources have reported staff and program cuts at NIHHIS, and

Congress may consider reviewing this interagency initiative.

In August 2024, the Occupational Safety and Health

Administration (OSHA) published a Notice of Proposed

Rulemaking for Heat Injury and Illness Prevention in Outdoor

and Indoor Work Settings. The post-hearing comment period for

this rulemaking ended in October 2025. OSHA does not have

specific heat exposure standards, and this rulemaking would

develop a heat-specific workplace standard. Congress may

engage in review of the proposed rule and its potential effects.

Some Members of Congress introduced legislation in the 119th

Congress to prepare for and mitigate against the effects of

extreme heat, including health and financial risks. Examples in

the 119th Congress include studying the financial costs of

extreme heat (S. 1743) and coordinating federal extreme heat

planning and response (H.R. 3704).

Figure 2. Projected Changes in Annual Days with Temperatures at or Above 95°F at 2°C of Global Warming

Source: U.S. Global Change Research Program Fifth National Climate Assessment (NCA5); adapted by CRS from NCA5 Figure 2-11.

Notes: No change is projected for Alaska (not shown). Data were not available for the U.S.-affiliated Pacific Islands and the U.S. Virgin Islands. Changes are relative to

the period 1991–2020.

Jonathan D. Haskett, Analyst in Environmental Policy

IF12733

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Extreme Heat and Climate Change

Disclaimer

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