What is a heat wave? Extreme temperatures and climate change, explained

It’s gearing up to be another scorching year.

Countries like Brazil, Thailand, Japan, Kenya, Nigeria, Australia, and Spain already experienced record warm temperatures this year, and in the past few days, heat has killed dozens in India and Mexico. Now states like California, Nevada, Arizona, and Texas are getting ready to roast as a massive heat wave settles in. It’s likely to push temperatures well into triple digits. And summer hasn’t even officially started yet.

It’s an alarming echo of 2023, which was the hottest year on record, but this year could be hotter still. Though the Pacific Ocean is shifting into its La Niña phase, which typically brings cooler global weather, the extraordinary warmth over the past year is still baked in. Scientists say these record highs align with their expectations for climate change, and warn that more scorchers are coming.

There’s more to heat waves like this than high temperatures, though. The forces behind them are complex and changing. They’re a public health threat that can exacerbate inequality, cause infrastructure to collapse, and amplify other problems stemming from warming. But with global average temperatures continuing to rise, more records will fall.

Extreme heat might not seem as dramatic as hurricanes or floods, but the National Weather Service has deemed it the deadliest weather phenomenon in the US over the past 30 years, on average.

What counts as a heat wave is typically defined relative to local weather conditions, with sustained temperatures in the 90th to 95th percentile of the average in a given area. So the threshold for a heat wave in Tucson is higher than the threshold in Seattle.

During the summer in the Northern Hemisphere, the northern half of the planet is tilted toward the sun, which increases daylight hours and warms the hemisphere. The impact of this additional exposure to solar radiation is cumulative, which is why temperatures generally peak weeks after the longest day of the year.

Amid the increase in temperatures in the summer, meteorology can push those numbers to extremes.

Heat waves typically begin with a high-pressure system (also known as an anticyclone), where atmospheric pressure above an area builds up. That creates a sinking column of air that compresses, heats up, and oftentimes dries out. The sinking air can act as a cap or heat dome, trapping the latent heat already absorbed by the landscape. The high-pressure system also pushes out cooler, fast-moving air currents and squeezes clouds away, which gives the sun an unobstructed line of sight to the ground.

The ground — soil, sand, concrete, and asphalt — then bakes in the sunlight, and in the long days and short nights of summer, heat energy quickly accumulates and temperatures rise.

Heat waves are especially common in areas that are already arid, like the desert Southwest, and at high altitudes where high-pressure systems readily form. Moisture in the ground can blunt the effects of heat, the way evaporating sweat can cool the body. But when there’s little water in the ground, in waterways, and in vegetation, there isn’t as much to soak up the heat besides the air itself.

“It compounds on itself,” said Jonathan Martin, a professor of atmospheric science at the University of Wisconsin Madison. “When you’re dry, you get warm. When you’re excessively warm, you tend to build and strengthen the anticyclone, which encourages continuation of clear skies, which in turn encourages a lack of precipitation, which makes it drier, which makes the incoming solar radiation more able to heat the ground.”

Power lines out of the Handley Generating Station along Rosedale Street in Fort Worth.

Heat waves threaten the power grid just as people need electricity the most.
Yffy Yossifor/Fort Worth Star-Telegram/Tribune News Service via Getty Images

But extreme heat can also build up in places that have a lot of moisture. In fact, for every degree Celsius the air warms (1.8 degrees Fahrenheit), it can absorb about 7 percent more water, which can create a dangerous combination of heat and humidity (more on that below).

Urban areas further exacerbate this warming. As roads, parking lots, and buildings cover natural landscapes, cities like Los Angeles and Dallas end up absorbing more heat than their surroundings and can become as much as 20°F warmer. This is a phenomenon known as the urban heat island effect.

Heat waves typically last around five days but can linger longer if the high-pressure system is locked in place. “In some cases, you actually can get these kinds of patterns getting stuck, and that can lead to heat waves lasting much longer,” said Karen McKinnon, an assistant professor of environment and sustainability at the University of California Los Angeles.

Eventually, the high-pressure system will start to weaken, allowing in cooler air and precipitation that can bring the heat wave to an end. However, as the warm season continues, more high-pressure systems can settle in and restart the heating process.

How climate change worsens heat waves

It can be tricky to tease out how a specific weather event was influenced by climate change, but scientists in recent years have been developing models and experiments to figure out just how much humanity’s hunger for fossil fuels is making individual disasters worse. It’s part of a subfield of climatology known as attribution science, and extreme heat is the classical example.

“Heat waves were actually the extreme events that attribution science were pioneered around,” said Jane W. Baldwin, a postdoctoral fellow at the Lamont-Doherty Earth Observatory at Columbia University. “Almost any kind of metric related to heat waves you can imagine is getting worse and is projected to get worse.”

An “Extreme Heat Danger” sign at the Badwater Basin in Death Valley, California, US, on June 17, 2021. Two people are visible through a heat haze.

Climate change is making extreme heat events more severe.
Kyle Grillot/Bloomberg via Getty Images

Climate change caused by greenhouse gases from burning fossil fuels is poised to make heat waves longer, more intense, and more frequent. It takes time for the dust to settle on the heat waves of a given moment, to allow scientists to evaluate just how much humans have contributed to the problem.

But researchers looking at past events and other parts of the world have already found that humans share a huge portion of the blame. After a summer 2019 heat wave was blamed for 2,500 deaths in Western Europe, a study found that climate change made the heat five times as likely as it would have been in a world that hadn’t warmed. Heat waves in the ocean have become 20 times as likely as average temperatures have risen. And researchers reported that the 2020 heat wave in Siberia was 600 times as likely due to climate change than not.

The mechanism is simple: The burning of fossil fuels adds greenhouse gases to the atmosphere, which traps more heat energy and pushes up average temperatures — which, in turn, also pushes up extreme temperatures.

That heat isn’t distributed evenly, however. Nighttime temperatures are rising faster than daytime temperatures. “In general, since records began in 1895, summer overnight low temperatures are warming at a rate nearly twice as fast as afternoon high temperatures for the U.S. and the 10 warmest summer minimum temperatures have all occurred since 2002,” according to the National Oceanic and Atmospheric Administration. This can seriously impair how people cope with high heat.

The effects of warming can vary by latitude, too. Polar regions are warming up to three times as fast as the planetary average, fueling heat waves in the Arctic. In fact, cooler parts of the planet are heating up faster than places closer to the equator, so people living in temperate climates may experience some of the biggest increases in extreme heat events. Already hot parts of the world also get hotter, pushing them beyond the realm of habitability at certain times of the year.

And as human-generated greenhouse gas emissions continue to flood the atmosphere — atmospheric carbon dioxide concentrations recently peaked at 420 parts per million — heat waves are projected to become more frequent and more extreme.

Heat wave impacts vary based on a person’s location, health, and even income

While there is some debate about whether extreme heat or extreme cold has a larger public health impact overall, it’s clear that high temperatures exact a huge toll in terms of health and the economy. Here’s how the impacts of scalding temperatures ripple throughout the world and how they’re shifting as the planet warms.

Heat waves have major direct and indirect health effects: Extreme heat caused an average of 138 deaths per year in the US between 1991 and 2020, according to the National Weather Service. High temperatures increase the likelihood of heat exhaustion and heat stroke. They can raise blood pressure, make certain medications less effective, and worsen neurological conditions like multiple sclerosis.

Air pollution also gets worse as rising temperatures increase the rate of formation of hazards like ozone. Such pollutants in turn exacerbate heart and lung problems.

The rise in nighttime temperatures is particularly worrisome for public health. Without much overnight cooling, people living through a heat wave experience higher cumulative heat stress, increasing risks of problems like dehydration and disrupting sleep, which can further worsen exhaustion and stress from high temperatures.

Alongside the heat, another important factor to consider for human health is humidity. The amount of moisture in the air affects how well sweat can evaporate off the body and cool it off. In some parts of the world, like the southwestern US, heat waves have become drier. But in other regions like the Persian Gulf and South Asia, higher temperatures are instead increasing humidity.

The key metric here is known as the wet-bulb temperature, where a thermometer is wrapped in a damp cloth, revealing the lowest temperature achievable by evaporative cooling (i.e., sweating) under a given set of heat and humidity conditions. The upper limit wet-bulb temperature for human survival is 95 degrees Fahrenheit (35 degrees Celsius), during which even standing in the shade with unlimited water can be life-threatening.

Since 1979, these dangerous conditions have become twice as common in several regions of the world, including South Asia and the Persian Gulf, researchers found in 2020. They warned that further warming this century could render many of the most densely populated parts of the world uninhabitable during the hottest times of year.

The timing of heat waves is changing: Periods of extreme heat that occur early in the season tend to have greater public health impacts. That’s because people are less acclimated to heat in the spring and early summer. Cooling infrastructure may not be in place, and people may not be taking heat precautions like staying hydrated and avoiding the sun. That’s why early-season heat waves in the US, as we have seen across the country this year, are so troubling. As climate change makes heat waves more common, it also increases the frequency of early- and late-season extreme temperatures, lengthening the hot season.

The worst effects of heat aren’t always in the hottest places: While absolute temperatures may rise higher in already warm areas like the southwestern US, heat waves can have their deadliest impacts in cooler regions, where high temperatures are less common. Warmer areas often already have air conditioning in homes and offices, while regions that usually don’t get as warm have less cooling infrastructure and fewer places to find relief. The people in these regions are also less acclimated to high temperatures and may not recognize warning signs of heat-induced health problems.

Some people are far more vulnerable to extreme heat: Elderly people and very young children face some of the highest risks from extreme heat. People with certain health conditions, like high blood pressure and breathing difficulties, also face greater harm. But even otherwise healthy people can suffer from heat waves if they are exposed for long durations, such as those working outdoors in agriculture and construction.

Heat waves exacerbate structural inequalities: While cities can warm up faster than their surroundings, poorer neighborhoods — which are disproportionately home to people of color — tend to get hotter. These neighborhoods often have less tree cover and green spaces, and more paved surfaces that soak up heat. At the same time, lower-income residents may have a harder time affording crucial cooling. The pattern of heat inequality plays out on an international scale, too, with lower-income countries already facing higher health and economic costs from heat waves.

The tools used to cope with heat are also stressed by it: Power plants, which provide electricity for everything from fridges to air conditioners, themselves need to be cooled, and they become less efficient as the weather warms. Power lines have lower capacities under extreme heat, and hardware like transformers experience more failures. If enough stress builds up, the power grid can collapse just when people need cooling the most. Power disruptions then ripple through other infrastructure, like water sanitation, fuel pumps, and public transit.

We’re running out of time to act: All this means that heat waves are going to become an increasingly impactful and costly fact of life across the world — from the direct impacts on health to stresses on infrastructure.

But since humans share a significant portion of the blame for extreme heat waves, there are also actions people can take to mitigate them. Increasing energy efficiency can relieve stress on the power grid, and adding power sources that don’t require active cooling like wind and solar can boost capacity without adding greenhouse gas emissions.

Improving public health outreach and providing more cooling resources and education, particularly in the most vulnerable neighborhoods, could reduce some of the worst human effects. “Basically all of the damages from heat waves, at least on the health side, are preventable if we warn people effectively and just help our neighbors during an event,” Baldwin said.

Humanity must curb its output of heat-trapping gases to limit just how hot the planet will get. It may take years or decades for these reductions to show up in the climate system, but they have to begin now.

Update, June 5, 2024, 4:30 pm ET: This story, originally published in June 2021, has been updated several times, most recently with new heat waves across the US.

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