Quick Key Facts
- The U.S. government defines extreme heat as a period of at least two or three days with temperatures above 90 degrees Fahrenheit.
- Extreme heat is the deadliest kind of extreme weather event in the U.S., killing an average of 618 people each year.
- A wet-bulb temperature measures heat and humidity. Scientists have estimated that humans cannot survive outdoors for longer than six hours when the wet-bulb temperature reaches 95 degrees Fahrenheit, though the real limit may actually be lower.
- More than 80 percent of the records for hottest day of the year set between 1961 and 2010 can be attributed to the climate crisis.
- The lower-income South Bronx is eight degrees Fahrenheit hotter than the wealthier Upper East and Upper West sides.
- Heat killed at least 436 U.S. workers between 2011 and 2021.
- If temperatures rise by 2 degrees Celsius, heat waves like the ones that broke records in China, Europe and North America in July 2023 would happen every two to five years.
- A single tree has the cooling impact of about two home AC units.
- One way to reduce the urban heat island effect is with “cool” roofs or pavement — coated in reflective material — or green roofs that incorporate plants into the built environment.
- Seville, Spain became the first city to establish a system for naming and ranking heat waves in 2022.
What Is ‘Extreme Heat’?
Extreme heat is a type of extreme weather event during which temperatures rise far above the average for a particular area, especially during its seasonal summer. The U.S. government defines “extreme heat” as at least two to three days with high heat and humidity and temperatures higher than 90 degrees Fahrenheit. However, what is considered normal temperatures will vary by region, so extreme heat in Seattle, where the average high in August is 76 degrees Fahrenheit, will look different from extreme heat in Phoenix, where the August average high is 104. Extreme heat is related to a heat wave, which is defined by the World Meteorological Organization as “a period of statistically unusual hot weather persisting for a number of days and nights.”
Why does this matter? First of all, while it often gets less attention than more dramatic weather events like hurricanes and tornadoes, extreme heat can be very dangerous. When it is hot, the body has to work harder to cool down, and it can stress itself out so much doing this that it leads to death. In fact, heat waves are the deadliest kind of extreme weather event in the U.S., killing around 618 people a year on average. And this isn’t just a U.S. problem. A 2003 heat wave in Europe killed more than 70,000 people, and more than 166,000 people died worldwide in heat waves between 1998 and 2017. Second, periods of extreme heat are becoming more frequent, longer, and even hotter because of the climate crisis caused primarily by the burning of fossil fuels. This in turn puts an even greater number of people at risk and makes it ever more urgent that communities understand and prepare for the threat.
What Is the Heat Index?
One thing that can make a period of extreme heat both more dangerous and feel more intense is humidity — the amount of moisture in the air. Because the human body cools itself by sweating, wet air can interfere with this process, making it harder to cool yourself down. That means that in humid climates, the temperature that shows up on the thermometer won’t necessarily correspond to how the outdoor air actually feels on your skin. This is where the heat index comes in.
The heat index describes what’s known as the “apparent temperature.” It combines the heat and humidity to tell you how it would actually feel in the shade. For example, if it is 90 degrees Fahrenheit, but 75 percent humidity, the heat index will be 109 degrees Fahrenheit. However, if it is 109 degrees Fahrenheit, but 15 percent humidity, the heat index will be around 107 degrees Fahrenheit. A heat index of 103 degrees Fahrenheit or higher is considered dangerous for human health, and moving into direct sunlight can add as much as 15 degrees to the heat index. The National Weather Service (NWS) allows you to calculate the heat index using this calculator.
What Is the Wet-Bulb Temperature, and Why Is It Important?
Another temperature measurement that takes humidity into account is the wet-bulb temperature. This is measured by wrapping a wet cloth around the bulb of a thermometer, mimicking how sweat would cool the human body. The more humid it is outside, the less the wet cloth is able to cool the thermometer, and the closer the thermometer’s measurement will rise to the outdoor temperature. This is an important measurement because, as the climate crisis intensifies, scientists are concerned that more places will see wet-bulb temperatures that the human body could not survive outside for more than six hours. This is usually defined as a wet-bulb temperature of 35 degrees Celsius, or 95 degrees Fahrenheit. That corresponds to a temperature of 40 degrees Celsius, or 104 degrees Fahrenheit, at 75 percent humidity. However, a study published in 2022 found that the critical threshold is likely to be “well below” that limit.
A 2018 study found that deadly “wet-bulb” heat waves could occur in the Persian Gulf, Southeast Asia and, most concerningly, China’s north plain breadbasket by the end of the century if nothing is done to curb emissions. However, a later study in 2020 found that the 95 degree wet-bulb threshold had likely already been breached in locations in South Asia and along the Persian Gulf and Gulfs of California and Mexico. These breaches have so far only lasted for an hour or two, but the study also found that high humidity and heat conditions overall had more than doubled since 1979.
What Is the Wet Bulb Globe Temperature?
In addition to the heat index and the wet-bulb temperature, the NWS is trialing something called the Wet Bulb Globe Temperature, or WBGT. The WBGT is intended to tell people like outdoor workers or athletes how they will experience temperatures while under direct sunlight. To do this, it combines a “dry-bulb” thermometer reading to indicate what your average household thermometer would measure in the shade, a wet-bulb reading to assess humidity and a “black-bulb” reading to assess the impact of sunlight and wind.
What Causes Extreme Heat?
Extreme heat has both short-term, weather-related causes and longer-term causes related to climatic shifts. At the same time, the way that humans construct their communities and organize their societies can make the impacts of extreme heat even more intense.
In the immediate term, extreme heat is caused by high-pressure systems that settle over a given area. High pressure does two things: It forces air — which becomes warmer as it descends — toward the ground, and it shoves clouds out of the way, allowing the sun free access to heat the area. If this high-pressure system gets stuck in place for at least a week, it becomes what’s known as a heat dome. Heat domes are more likely to form during a hemisphere’s summer, because during that period, the hemisphere is angled towards the sun, which heats the atmosphere and makes the baseline temperature warmer. At the same time, the temperature contrast between the poll and the equator weakens, which causes the jet stream to become more wavy. Since high- and low-pressure systems ride in the crests and troughs of these waves, a wobbly jet stream means a greater chance an area will come under a heat dome. In July 2023, for example, three record-breaking heat domes formed over the U.S. and Mexico, China and Southern Europe. Phoenix saw a record stretch of 31 days at 110 degrees Fahrenheit or higher, China saw its highest temperature on record at 126 degrees Fahrenheit and the city of Rome broke its record at 109.
Because of human activities like the burning of fossil fuels and the clearing of natural carbon sinks like forests, the Earth has warmed by 1.1 degrees Celsius above the 1850 to 1900 average. The Intergovernmental Panel on Climate Change said that it was ““virtually certain” that this warming has led to more frequent and intense heat extremes and heat waves over most land areas on the globe since the middle of the 20th century. This makes sense, because as the average temperature warms, it approaches levels that were considered extreme in the past, and pushes the new extremes even higher. It’s estimated that the climate crisis was responsible for more than 80 percent of the records for hottest day of the year set between 1961 and 2010, and the trend is only escalating. June 2023 was the hottest June on record, and July 2023 was the hottest month on record overall. Of the three major July 2023 heat domes mentioned earlier, the one over China was made 50 times more likely by the climate crisis, and the ones over North America and Southern Europe would have been “virtually impossible” without it.
Beyond burning fossil fuels, there are other things humans can do that make heat extremes worse, and one of them is to build extensively over natural areas. Buildings and pavement tend to absorb sunlight more than trees and water and then re-release it into the surrounding area as heat. For this reason, urban areas — where human infrastructure is densely packed — tend to be hotter. What’s more, all the people and activity in cities burns energy that gets released as something called “waste heat.” For this reason, urban areas tend to be one to seven degrees Fahrenheit hotter during the day — and two to five degrees Fahrenheit hotter at night — than their surroundings. This is what’s known as the urban heat island effect.
Another thing that humans do to make heat worse is to structure deeply unequal societies that discriminate on the basis of arbitrary factors like race or class. In the United States in the 1930s, this took the form of “redlining” — labeling neighborhoods where people of color were more likely to live as higher risk for loans. While that practice is now illegal, its legacy of under-investment lingers. A study of Baltimore, Dallas and Kansas City found that once-redlined neighborhoods still had less vegetation, hotter temperatures and more low-income residents. In New York City, the lower-income South Bronx is eight degrees Fahrenheit hotter than the wealthy Upper East and Upper West sides, and half of the people who died from heat-related causes between 2000 and 2012 were Black and half lived in less wealthy zip codes. Globally, poorer nations closer to the equator are at greater risk from the rising temperatures they did very little to cause.
What Are the Effects of Extreme Heat?
On Human Health
High heat is dangerous for human health, but what exactly does it do? The most dangerous heat-related illness is heat stroke. This is what happens when the body can no longer internally regulate its own temperature, which can spike to as high as 106 degrees Fahrenheit in just 10 to 15 minutes. It can be fatal if left untreated. Other heat-related illnesses include heat exhaustion, when the body sweats away too much water and salt; rhabdomyolysis — when muscles break down after working too hard in high heat; heat rash, caused by too much sweating; and heat cramps, also caused by sweating helpful salts out of muscles. Certain groups of people are more vulnerable to heat-related illnesses, especially the elderly, young children and people with mental illnesses or chronic ailments. High heat can also be especially dangerous for people who work outdoors, such as farmworkers or construction workers, who are more likely to suffer from dehydration or heat-related illnesses as they exert themselves. Heat killed at least 436 U.S. workers between 2011 and 2021.
On Power Grids
High heat puts an additional strain on energy systems as two problems intersect: On the one hand, as more people crank up the air conditioning to cool down, power demand goes up; on the other hand, power lines can’t transmit as much energy when temperatures are high. If a heat wave coincides with a drought, this can also reduce hydropower resources, as happened in China in August 2022, when a depleted Yangtze River forced businesses to open for reduced hours to accommodate power rationing. Loss of power can make heat waves more deadly, as blackouts mean air conditioners or fans will not run. A 2023 study found that a blackout during a historical heat wave in Phoenix would kill more than 13,000 people and force more than half of the population to visit the emergency room.
Humans aren’t the only living things that get stressed in the heat. Plants and animals do too. So when high temperatures persist, they can also put a strain on crops and livestock. For example, an early Kansas heat wave in June 2022 killed at least 2,000 cattle who were caught out in high temperatures, high humidity and low wind. The summer 2022 heat wave in Spain cut its olive crop in half the next year, and a 2023 heat wave means that the 2023 to 2024 crop would also be diminished. A 2021 study found that heat waves could lead to 10 times more crop damage than previously predicted, because extended periods of high heat damage plants more than single high-temperature days.
On Other Extreme Weather Events
Periods of high heat can also make other weather extremes more likely. High temperatures cause moisture to evaporate from both soil and bodies of water, worsening droughts. Drier soils warm the air above them as well, leading to a drought-heat wave feedback loop. Higher temperatures also suck the moisture out of vegetation, which is then more likely to ignite and spread wildfires.
What Can Be Done About Extreme Heat?
There are two main ways to deal with extreme heat: trying to prevent it from getting worse (mitigation) and trying to make it less dangerous when it does occur (adaptation).
Stop Burning Fossil Fuels
Since the longer, more frequent and more intense heat waves of the present and recent past are primarily caused by the climate crisis, the No. 1 way to halt and reverse this trend is to stop burning fossil fuels as soon as possible and transition to renewable energy. To limit temperature rise to 1.5 degrees Celsius above pre-industrial levels, the IPCC says that we must cut greenhouse gas emissions almost in half by 2030 and reach net zero by 2050, but fossil fuel infrastructure currently in use would overshoot the 1.5 target if left to run as usual. If temperatures are allowed to reach even two degrees Celsius above pre-industrial levels, heat waves like the three that baked China, North America and Europe in 2023 would make people sweat every two to five years. Switching to battery-stored renewable energy also has an added benefit of making power grids more resilient during heat waves. For example, during the record-breaking 2023 summer heat wave in North America, Texas kept the lights on with the help of newly added solar power capacity.
Deforestation and land-use change are also important contributors to climate change, so preserving forested areas and other natural spaces will also help keep the planet from overheating. What’s more, trees and forests themselves have a cooling effect independent of their carbon uptake because they move water from the soil to their leaves. One tree essentially has the cooling impact of two home AC units. This means that clearing a forest can raise the surrounding temperature by as high as 4.5 degrees Celsius. Parts of North America, Europe and Asia that experienced moderate deforestation since the start of the industrial revolution are now 0.3 degrees Celsius warmer than they would have been if the trees had not been felled, and every 10 percent decrease in forest on these continents pushes annual maximum temperatures up by 0.12 degrees Celsius.
Protecting Your Health
If you receive a heat alert or warning on your phone, or if your weather station predicts a spate of hotter-than-average days, there are things you can do immediately to prepare and to protect yourself, your home and your loved ones. Make both a go-kit with at least three days of supplies and a stay-at-home kit with two weeks’ worth in case of a power outage, including a gallon of water per person in your home. Also make a note of air conditioned places you can go, like public libraries. If you have AC, double-check that it is working, close the drapes over east- and west-facing windows, and make reflectors out of cardboard and foil to bounce light away from your home. Make sure you drink lots of water — around three quarters of a gallon per day. If you sweat a lot, have a drink with electrolytes or something to eat. Learn the signs of heat-related illnesses and how to provide first aid so that you can recognize them in yourself and others.
Protecting Your Community
Some people are more vulnerable to heat than others. Make plans to check on people in your neighborhood who are older or dealing with chronic illness, especially if they live alone. Communities, like cities or towns, can help keep everyone safe by designating cooling centers where people without AC can go, and covering sidewalks as “cool routes.” Cities can make plans ahead of heat waves and publicize them so that residents, medical professionals and emergency workers know what to do. They can also make it easier for neighbors to check in on each other, as New York has done with its Be A Buddy NYC program. Finally, governments can protect outdoor workers by passing regulations to ensure they can take the water and shade breaks they need. Currently, the U.S. Occupational Safety and Health Act doesn’t list specific rules for heat safety, though the Biden administration is working to change that.
Reducing the Urban Heat Island Effect
Currently, more than 55% of people on Earth live in cities, and that number is projected to jump to 68% by 2050 even as temperatures continue to climb. So how to keep all those city dwellers cool? Luckily, there are ways to reduce the urban heat island effect. A big one is to plant trees or other greenery. Another solution is to change the material used for roofs and pavement. “Cool” roofs or pavements are made with material for reflecting light rather than absorbing it, and the idea is so “cool” it’s united the East and West Coasts, with New York adapting a CoolRoofs Project and LA coating its streets in white hues. Finally, urban planners can combine the two ideas by weaving plants into the built environment through green roofs or using reinforced grass for a kind of permeable pavement.
Naming Heat Waves
One way to help inform people of the dangers of heat waves and make sure they take them seriously is to name major heat waves the way meteorologists do tropical storms. In the summer of 2022, Seville, Spain, became the first city to launch a naming and ranking system for heat waves. In summer 2023, an Italian website christened two major heat waves Cerberus and Charon, while meteorologist Guy Walton used the concept to raise awareness about the causes of the climate crisis by naming U.S. heat waves after major oil and gas companies, beginning with Amoco, BP and Chevron.
What to Do About AC?
One immediate solution for high heat — cranking up the AC — also poses problems. Depending on how a grid is powered, expanding the use of AC can lead to the burning of more fossil fuels and therefore further heat the atmosphere in the long term. Further, many ACs use refrigerants that are themselves greenhouse gases. At the same time, AC can be life saving — the nine people who died inside during Phoenix’s 2023 heat wave as of July 30 didn’t have it — and unequal access is a major environmental justice issue. Only eight percent of the people in the hottest parts of the planet actually have AC access, and in the U.S. low-income people and people of color are less likely to have AC. Solutions include making ACs more efficient, using more environmentally friendly refrigerants and decarbonizing the grids that power them. Another promising technology is the heat pump, which works as both a furnace and an air conditioner by pumping hot air in or out depending on the season and is more efficient than both. In the U.S., their purchase is also subsidized by the Inflation Reduction Act, making them a more affordable bet for lower-income households in need of cooling technology.
The world is getting hotter. The 10 warmest years on record all took place since 2010, and 2023 is likely to be the hottest yet. Heat waves all over the globe are lasting longer and spiking higher, and people are suffering. That’s why it’s important to understand extreme heat — both so we know how to protect ourselves and our communities from it now and so we can push for the energy policies and urban planning decisions that will keep us cool in the future.
Phoenix baked by Heatwave Chevron during summer 2023 opened a window into the hottest timeline, with people rushing to the hospital with dangerous burns simply from falling on the sidewalk and a total of 180 dying from the heat in Maricopa County as of the end of August. But the future doesn’t have to look this way. We could instead be living in cities powered with renewable energy and cooled with heat pumps, with trees in every neighborhood, roofs and streets painted white or planted with green things and systems in place to make sure everyone is taken care of during the hotter months. Because of the climate crisis, we are all feeling the heat right now, but that doesn’t mean we have to let it burn.