What makes extreme heat so dangerous?

There are many reasons. People are more likely to dehydrate in high temperatures, increasing the risk of heart attack and stroke. Heat can worsen breathing problems, especially in places with elevated pollution levels. Heat stress makes it harder for people to work and increases the likelihood of injuries. It’s hard to know exactly how many people die from heat each year; most go uncounted. Europe likely experienced 61,672 deaths attributable to heat in 2022, though that number potentially underestimates the actual total, according to a study led by the Barcelona Institute for Global Health. Emerging economies suffer more than developed ones, as there tends to be little respite from the sun; most people work outside and few have effective cooling at home. Concrete and asphalt in urban settings can trap the heat, increasing overnight temperatures and contributing to heat stress. Women and seniors have been found in studies to be the populations most affected by extremely hot weather. 

How is extreme heat measured? 

Forecasters are increasingly using measures of heat stress and discomfort — like humidex, heat index or apparent temperature — to understand the health risks posed by high temperatures. “Wet-bulb” is one of these measures. It accounts for the effects of humidity, which makes it harder for the human body to cool itself by sweating. For example, 42C with 40% humidity — think Phoenix, Ariz., in July — has a wet-bulb temperature around 30C. A lower temperature of, say, 38C, but with higher humidity of 80%, will give a wet-bulb reading around 35C. That’s high enough to trigger heatstroke even for healthy people with unlimited shade and water, and has already started to appear in coastal subtropical regions. In reality, shade and water are often limited, and heat can kill at much lower wet-bulb temperatures. A 2020 study published in the journal Science found regions affected by the 2003 European and 2010 Russian heat waves, which proved deadly for thousands, experienced wet-bulb values no greater than 28C. 

How are wet-bulb temperatures measured?

Originally by wrapping a wet cloth around the bulb of a thermometer. Scientists would record the level after moisture’s vaporization cools it down, the way the body cools down by sweating. Now, wet-bulb temperatures are measured using electronic instruments at weather stations, with further studies of hot spots assisted by satellite data from sources including NASA and the International Space Station. The National Oceanic and Atmospheric Administration in the US also developed a tool to forecast a more advanced metric of heat stress, the wet-bulb globe temperature, which factors in wind speed, sun angle and cloud cover. 

Where is this a problem? 

Traditionally, heat and humidity have been highest in South Asia and subtropical climates. Some places in India have notched wet-bulb temperatures higher than 32C; the UN predicts it will be one of the first countries to surpass a wet-bulb temperature of 35C. Planetary warming and the impact of the El Nino weather pattern mean there’s a high chance temperature and humidity records will be set this year across a sweep of regions straddling the equator, including Florida, Texas, much of Africa, India, Australia, and Central and South America, according to the University of California, Berkeley. Increasingly, typically temperate places are also seeing incredibly hot days. The UK registered a record of 40.3C in July 2022, though relatively low humidity kept the wet-bulb temperature around 25C. Barcelona experienced its hottest ever day in late July. In the US, heat alerts covered about half the population on Aug. 1, the World Meteorologial Organization said. Japan, Greece, Hungary and Croatia were among nations to experience their warmest July on record. 

What is the economic impact of heat?

In places with extreme heat, every aspect of life becomes more challenging, and inequalities become more acute, especially in cities. But even cooler places feel the effects, typically through higher food and energy prices. Coffee prices this year surged to a 45-year high as persistent heat and drought exacerbated a supply crunch in Vietnam, the world’s largest robusta producer. Dry conditions in Russia this year prompted analysts to cut wheat production forecasts, and supply of crops including rapeseed and chickpeas also remains susceptible to the impact of heat. Previous El Niños resulted in a marked impact on global inflation, adding 3.9 percentage points to non-energy commodity prices and 3.5 points to oil, according to Bloomberg Economics modeling. Power consumption also rises during peak heat, straining the grid and consumers’ pockets as prices jump. Natural gas prices advanced this summer as consumers grappled with blackout risks; Egypt, typically an exporter of the fuel, resorted to buying LNG amid higher demand. Heat also exacerbates drought, adding further stress to hydropower and nuclear power production. Extreme temperatures pose an increasing threat to outdoor activities, disrupting events including concerts, religious gatherings, and sporting events.

How is extreme heat related to climate change? 

A new branch of science, extreme event attribution, connects global warming to severe episodes of weather with a degree of specificity. Heat waves are most directly linked to humanity’s greenhouse gas pollution. And heat, along with dryness and wind, fuels forest fires, which is why scientists are now confident that climate change is exacerbating wildfires in the western US, Australia and elsewhere. (The US fire season is two months longer than it was in the 1970s and 1980s.) Global warming is making tropical cyclones — also called hurricanes or typhoons — more intense. Warmer water and moister air — two results of global warming — provide added fuel to such storms, such as the record-breaking Hurricane Beryl in July that raged through parts of the Caribbean and US. In India and Pakistan, extreme heat is 30 times more likely due to a changing climate.