Extreme weather is relative to “normal” conditions for a particular geography. Heat waves are a period of prolonged and abnormally high surface temperatures relative to those normally expected. There is no standard definition of extreme heat or heat waves; rather, it varies depending on what is considered average for a particular place at that time of year.

The National Weather Service (NWS) says a heat wave is “a period of abnormally hot weather generally lasting more than two days.” In the United Kingdom (U.K.), a heat wave threshold is met when a location records a period of “at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.”

Heat waves occur when high pressure in the atmosphere moves in and pushes warm air toward the ground, and it feels hotter, especially as the air warms up and is compressed. Heat waves are most common in summer when high pressure develops across an area.

Studies have found that heat waves are directly affected by climate change. One study concluded that heat waves are becoming as much as ten times more likely in some areas because of climate change. A study from the U.K.’s Met Office found that human-induced climate change made the 2018 heat wave in the country 30 times more likely than it would be naturally.

The U.S. Environmental Protection Agency (EPA) says, “As the Earth’s climate warms, however, hotter-than-usual days and nights are becoming more common (see the High and Low Temperatures indicator) and heat waves are expected to become more frequent and intense.”

Even small increases in extreme heat can result in increased deaths and illnesses. The World Health Organization (WHO) says exposure to heat causes severe symptoms, such as heat exhaustion and heat stroke, and heat can cause severe dehydration and contribute to blood clots. People with chronic diseases have a greater risk of complications and death during a heatwave, as do older people and children. Those dependent upon certain medications, including many psychiatric medicines, are also at increased risk of heat-related illnesses.

In June 2021, western North America experienced a record-breaking period of extreme heat. In British Columbia, Washington and Oregon, temperatures rose well above 100 degrees Fahrenheit (38 degrees Celsius), resulting in a spike in heat-related deaths and hospitalizations.

The early-summer heat wave became Washington state’s deadliest weather-related disaster in history, with at least 112 heat-related deaths counted. Evaluating deaths caused by heat can be challenging, and undercounts of heat-related deaths are likely. A spike in heat-related hospitalizations was also observed in the region. Researchers determined that only five other heatwaves globally were more extreme since 1960.

In March and April 2022, extreme heat hit South Asia, with India experiencing its hottest March since its records began in 1901, and April was the warmest in Pakistan. The heatwave was deadly and had cascading effects. At least 90 people died in India and Pakistan, flooding occurred in Pakistan after a glacier burst, wheat crops were scorched in India and a spike in electricity usage caused power shortages for millions. Climate scientists say the heat wave was made 30 times more likely because of climate change.

Key Facts

  • Heat waves are among the most dangerous natural hazards. In the U.S., extreme heat kills more people than all other natural hazard and extreme weather events. Heat waves may not produce immediate signs of death and destruction, unlike other natural hazards. However, they can be deadly and disrupt critical infrastructure. The European heat wave in July 2022 claimed the lives of at least 1,500 people. In North America, up to 20,000 deaths a year may be linked to hot temperatures. Heat waves can burden health and emergency services and increase strain on water, energy and transportation, resulting in power shortages. A heat wave in the U.K. in July 2022 melted airport runways and forced rail operators to put travel restrictions in place.
  • Population exposure to heat is increasing due to climate change, and the trend will continue. The World Metrological Organization (WMO) warns that heat waves will occur more and more frequently, with the increase linked to the planet’s warming caused by human activity. According to WHO, between 2000 and 2016, the number of people exposed to heat waves increased by around 125 million. In July 2022, authorities issued heat advisories and excessive heat warnings affecting more than 105 million people in 28 U.S. states. A heat wave in the U.K. exposed millions of Britons to the effects of extreme heat in July 2022, a country not accustomed to high temperatures. The WMO Secretary-General warned that the heat wave is the new normal.
  • Heat waves expose and exacerbate inequities, leaving some people more at risk than others. As with other natural hazards, the effects of extreme heat are felt unevenly across society. In U.S. cities, residents of low-income neighborhoods and communities of color endure far higher temperatures than people who live in whiter, wealthier areas. In New York, affluent neighborhoods are more likely to have households with air conditioning. Tree cover, which can defend people against the sun and decrease surface temperatures, is lower in minority neighborhoods in the U.S., and Black Americans are less likely to have access to air conditioning. Extreme heat heavily impacts laborers who must work to receive their daily wages, which decreases the number of workable hours and makes poor people poorer.
  • Extreme heat worsens drought conditions and wildfire risk. Heat waves, drought and wildfire are interconnected and can influence one another. Global warming is expected to increase the frequency of droughts and heat waves, and hot–dry events can cause wildfires, tree deaths and crop losses. A heat wave across Texas in the summer of 2022 exacerbated the extreme drought conditions impacting the state’s agriculture sector as soils lost moisture and retained heat. Drought is often linked to famine such as the current hunger crisis in the Horn of Africa or to increased costs in the food supply chain. In western Europe, the heat wave in July 2022 helped fuel fires in Spain, Portugal and France. Dry forests and hot winds complicated firefighting and forced evacuations in the region.
  • Urban areas are hotter than outlying areas. According to the U.S. EPA, “heat islands” are urbanized areas that experience higher temperatures than outlying areas, with daytime temperatures in urban areas about 1–7 degrees Fahrenheit higher than temperatures in outlying areas. Densely constructed buildings, more people and energy being burned off are causes of the heat island effect. As the climate warms, people who live in cities will face higher temperatures and stronger heat waves in the future. Increased demand for air conditioning to offset the effects of extreme heat may contribute to climate change when fossil fuels are burned to run air conditioners.

How to Help

  • Support the development and application of heat mitigation and response plans. We know that heat waves will continue to increase in frequency and intensity and occur in under-prepared regions. Community-based plans are needed that identify heat-related risks and the populations exposed, particularly groups of people and geographies most likely to be severely and inequitably impacted. Plans should ensure the meaningful participation of at-risk populations in the planning process. The Indian city of Ahmedabad created the country’s first heat action plan in 2013, and plans in India are now embracing a more proactive than reactive approach.
  • Invest in solutions aimed at people disproportionately impacted by heat waves. Extreme heat is worse for low-income, non-white Americans. Studies show that people with lower incomes are exposed to heat waves for longer periods compared to their higher-income counterparts because of a combination of location and access to cooling solutions like air conditioning. Expanding access to sustainable air conditioning and community cooling centers and increasing vegetation such as trees in neighborhoods that lack them are needed. Direct cash payments can also support people whose lives and income opportunities are disrupted. Energy assistance plans can help low-income tenants and homeowners cover the cost of running air conditioning.
  • Fund climate change adaptation. Heat waves are directly affected by climate change. A recent study found that an average of 37% of heat-related deaths are connected to human-influenced climate change. Further, researchers discovered that there is excess mortality from heat on every continent, making this an issue of global concern. Ultimately lowering carbon emissions is required to reverse the damaging effects of a warming planet. Creative and localized solutions are also needed so communities can adapt to more periods of extreme heat. Green spaces and corridors, cool roofs, community pools, public climate shelters, mobile cooling stations and checking on at-risk neighbors are examples of adaptation measures that funders can support.

What Funders Are Doing

  • The Kresge Foundation awarded $600,000 to West Harlem Environmental Action in 2021 to implement a community-driven plan to address the health hazards of extreme heat for low-income communities of color in northern Manhattan and throughout New York City. This is not a new area for Kresge. In 2019, they provided the same organization with a $150,000 grant to support an interdisciplinary, community-driven planning process to address climate-change-related extreme heat impacts for vulnerable residents of northern Manhattan. In 2015, they provided a grant to the Los Angeles Alliance for a New Economy. The $660,000 grant was made to ensure that climate-resilience investments made by the City of Los Angeles are maximized and reach those communities most impacted by climate stressors such as extreme heat and drought.
  • The Robert Wood Johnson Foundation awarded $584,886 to the City of Tempe in 2020 to build community resilience to extreme heat and empower youths, city staff and regional partners to create people-centered thermal performance metrics to track the efficacy of new policy and investments in front-line infrastructures.
  • The Barr Foundation awarded $350,000 to Mystic River Watershed Association, Inc. and Metropolitan Area Planning Council in 2020 to jointly manage an Extreme Heat Task Force to implement COVID-19 safe cooling strategies in the most COVID-impacted communities of Greater Boston. They also provided a $600,000 grant to Boston University to support research on extreme heat impacts and interventions in Chelsea and East Boston.

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(Photo Source: Graeme Maclean; CC BY 2.0 )