Analysis | More than 1.5 billion people have faced dangerous heat this year (2024)

You only hear the horror stories. The intensive care unit at a hospital in Mali where 102 patients died of heat-related ailments in just four days in early April. The ammunition warehouse at a Cambodian military base that overheated and exploded later that month, killing 20 soldiers. Howler monkeys dying by the dozens in Mexico’s scorching spring heat, their lifeless bodies tumbling from tree limbs.

Yet anecdotes, however outrageous, cannot capture the scale and scope of the heat that descended upon the planet this spring. By the end of May, more than 1.5 billion people — almost one-fifth of the planet’s population — endured at least one day where the heat index topped 103 degrees Fahrenheit, or 39.4 degrees Celsius, the threshold the National Weather Service considers life-threatening.

On May 4, the heat index in Bangkok topped 117 degrees Fahrenheit amid a Southeast Asian heat wave that shattered temperature records across the continent. Six days later, Thailand’s Health Ministry announced that 61 people had died from the heat.

The Bangkok metro area’s 18 million inhabitants are among the 111 million people who suffered at least 60 days of dangerous heat in the first five months of this year, a Post analysis of weather station records from more than 14,000 cities showed. Taken together, the cities included in the analysis are home to slightly less than half the world’s population, so the actual number of people exposed to dangerous heat is probably much higher.

The heat index accounts for temperature and humidity to better capture the threats heat poses to human health than temperature alone. Exposed to a heat index of 103 degrees, a healthy person will probably suffer heat cramps and heat exhaustion, and prolonged exposure can cause heat stroke, according to the National Weather Service. Much lower heat indexes have proved deadly for vulnerable people, such as the elderly and those who take medications that inhibit sweating.

When quantifying heat’s threat to the human body, it is important to account for humidity because it undermines our natural defenses against the heat. With more moisture in the air, sweating fails to cool us down as effectively. That makes it harder to regulate our core temperature — and it’s our core temperature, rather than the temperature we take with a thermometer, that gets us in trouble on hot days.

“The heart really doesn’t like to get warmer,” said Kristie Ebi, an epidemiologist at the Center for Health and the Global Environment at the University of Washington, adding that about half of all heat-related deaths are caused by cardiovascular problems. Heat illness can also cause muscle spasms, dizziness, vomiting, headaches, confusion, fainting and loss of consciousness.

Even able-bodied people can fall victim to extreme heat. “People typically are not aware that they’re getting into trouble with the heat until they’re really getting into trouble with the heat,” Ebi said. “You see people out doing activities where it’s quite warm, and suddenly somebody collapses.”

As global warming has nudged temperatures higher, days with dangerous heat have grown more common. Bangkok’s 76 days of dangerous heat are a record for this point in the year, according to meteorological data beginning in 1979. It is also more than double the city’s average. Other cities follow a similar pattern. Lagos, Nigeria, for instance, has faced 13 times as many dangerous heat days this year than average.

Below, you can see how the number of days with dangerous heat between January and May has risen in a selection of large cities. These cities are part of a trend playing out across the planet. Of 150 cities with more than 1 million people that face at least one dangerous heat day on average by the end of May, 135 have had more dangerous heat days than average this year.

In the northern hemisphere, summer hasn’t even arrived yet in what scientists project could be the hottest year in recorded history, breaking the record set last year. Researchers have linked the rise in temperatures to the El Niño climate pattern and decades of emissions of greenhouse gases such as carbon dioxide and methane.

Long-term projections indicate that future warming will also lead to milder winters, sparing people in the wealthy Global North. But in hotter, less wealthy countries — the places where people are least able to buy air conditioners, where poor laborers can least afford to miss work, where water is scarcer and the power grid shakier — summer heat will grow more dangerous.

There is, however, a morbid silver lining to heat waves’ lengthening duration. When a heat wave hits, mortality starts to rise after about 24 hours, as people unable to cool themselves down at night begin to perish, Ebi told me. But after a certain point, the death rate drops. Those most susceptible to heat’s ravages have already passed away, leaving behind a population more fortified against the danger.

“A healthy adult can handle more days. It may not be pleasant, but, frankly, people have done this for millennia,” Ebi said. “It’s the people who are more susceptible to heat who we need to think about. And as the population is aging and taking more prescription medications, the pool of people who are susceptible is much larger.”

Niko Kommenda contributed to this report.