Occasional heat waves have always been an aspect of summer weather in most of the United States. But as climate change makes heat waves more intense and more frequent, we need to be cognizant of the dangers and health risks that come with it.
Over the coming decades, every region of the US is expected to experience hotter temperatures and more frequent and intense heat waves. This worsening heat poses serious health risks, especially for the very young and elderly, construction and agricultural workers, and those living in the core of urban areas.
These two companion fact sheets summarize:
- The latest science on how extreme heat has changed and is likely to keep changing as global temperatures rise
- The current and potential impacts of extreme heat on people
- How these changes could be curtailed if we take action to reduce the heat-trapping emissions we're putting into the atmosphere
Extreme heat basics
“Heat wave,” “excessive heat event,” “heat advisory,” “hot spell”—extreme heat has many different names but generally refers to temperatures that are either exceptionally high relative to typical local conditions or reach levels that may be harmful to human health or infrastructure. When extreme daytime temperatures persist over a prolonged period (usually at least two days), it is often referred to as a heat wave.
The human body sweats to cool itself when exposed to high temperatures. In humid conditions, sweat does not evaporate as quickly and the body’s ability to cool itself is compromised. Heat stress can result and, if not addressed, can lead to heat-related illnesses like heat exhaustion and heat stroke. The impacts of temperature extremes on human health and well-being are therefore generally considered in concert with humidity to measure heat stress conditions: those in which the human body has difficulty cooling itself.
Extreme heat now and in the recent past
Extreme heat events can be measured in a few different ways: the maximum temperatures hit (intensity), how often the events occur (frequency), or how long they last (duration). The Dust Bowl era of the 1930s holds the record for peak frequency, intensity, and duration of heat waves across much of the United States to this day—but the frequency and intensity of heat waves have increased in the last several decades in many regions.
In addition to changes in extreme temperatures alone, studies of heat stress trends over the late 20th to early 21st century found increases across much of the country in the conditions that cause heat stress for humans. In a study of changes in extreme heat stress conditions at 187 weather stations across the US between 1949 and 2005, 20 percent of the stations had recorded a substantial increase in the number of one-day, extreme heat stress events (relative to local conditions between 1961 and 1990). On average, these stations recorded 12 more days with extreme heat stress per year in 2005 than they did in 1949.
The future of extreme heat
In the next few decades, if carbon emissions continue to grow, most of the country could see 20 to 30 more days annually with maximum temperatures above 90 degrees Fahrenheit. The Southeast could be hit even harder, potentially enduring 40 to 50 more such days.
By 2100, all parts of the United States are likely to experience more heat waves, with the Southeast, Southwest, and Alaska likely to see the biggest increases. Not only will these heat waves be more frequent, they will also be hotter than what we experience today. If carbon emissions continue to increase substantially, the hottest daily temperatures that occur in a given year in the United States are likely to increase by at least 10°F as compared to the end of the 20th century.
The health effects of extreme heat
Heat is among the deadliest extreme weather hazards in the United States. When exposure to heat is high enough to raise the body's core temperature, heat stress illness—which encompasses heat cramps, heat exhaustion, and heat stroke—can occur.
With heat cramps, an individual experiences muscle pain or spasms. Heat exhaustion can cause dizziness, a weak pulse, nausea, and fainting. The most severe heat-related illness, heat stroke, occurs when a person’s temperature increases above 103°F. Increased daily air temperatures or periods of extended high temperatures have also been shown to increase cardiovascular mortality, respiratory mortality, and heart attacks.
Some US residents are at greater risk of adverse health effects and death from extreme heat. Low-income residents, the young and elderly, construction and agricultural workers, individuals with pre-existing medical conditions, and people living in the center of urban areas can be more vulnerable to physical harm from heat.
The impacts of extreme heat on daily life
Extreme heat can affect many aspects of daily life and routines, including:
- Going to school—The effect of hot conditions on kids diminishes their ability to learn and lowers their performance on tests. Low-income and minority students are disproportionately affected, and research suggests that this is in part because they are more likely to attend schools and live in homes without air conditioning.
- Working outside—Millions of people living in the United States work primarily outdoors—construction workers, police officers, farm workers, military personnel, roofers, postal workers, landscapers, and others—and are at risk of heat stress when temperatures soar.
- Living in the city—Urban residents face unique heat-related risks in a changing climate. In addition to experiencing the effects of global temperature increases, cities experience temperature increases locally due to the urban heat-island effect. Cities are hotter primarily because they contain an abundance of heat-retaining materials and surfaces, such as asphalt, pavement, and cement.
- Living in rural areas—Heat stress also has major implications for rural residents’ livelihoods. For example, heat stress can have negative effects on crop production, as it can adversely affect several processes, including flowering and photosynthesis. Heat stress is also costly to livestock farmers.
- Traveling—Many types of infrastructure are affected by extreme heat, including our roads, rails, and air travel. A 2017 heat wave in Phoenix, for example, led to dozens of flight cancellations when temperatures increased to 119°F—above the operable limit of several types of aircraft
What can we do? Preventing the worst consequences of extreme heat
By stabilizing global carbon emissions in the next few decades (so that atmospheric carbon dioxide remains below 550 parts per million by 2100), the frequency of heat waves the US is likely to see around mid-century would be reduced by approximately 50 percent, compared to a scenario in which carbon emissions continue to increase rapidly through to 2100.
If the global community pursues aggressive emissions reductions in line with the Paris Agreement goal of keeping warming below 3.6°F, the hottest daily temperatures that occur each year in many parts of the country are likely to increase by just 3.6°F instead of by 10°F, as compared to the higher-emissions scenario.
Individuals and communities need policies and infrastructure that take better account of more frequent, intense, and longer-lasting extreme heat. For example, in the coming decades increased reliance on air conditioning to cope with extreme heat is likely to contribute to increased global warming emissions, worsening air quality, and an increase in air pollution–related mortality unless we more aggressively invest in clean energy technology, energy conservation, and energy efficiency measures.
Existing policies also need to be leveraged and new policies enacted to better help people—particularly outdoor workers, children, low-income and minority groups, elderly people, and athletes—to cope with extreme heat. Policies that ensure the safety of all outdoor workers, documented or otherwise, and expand access to and awareness of public cooling facilities are just two examples.