Exposure to extreme heat has tripled since 1983

World leaders have committed to limiting Earth’s rising temperature to 2.7 degrees Fahrenheit above pre-industrial times.

But what does that feel like? It’s difficult to convey, because you may not notice changes in average temperature. But, depending on where you live, you might notice when it’s extremely hot.

To better understand the issue, Columbia University’s climate school recently published a global dataset with estimates of both population and temperature. The Associated Press analyzed the data — spanning 1983 to 2016 — and found that exposure to extreme heat has tripled and now affects about a quarter of the world’s

population.

HOW HEAT IS MEASURED

As the global average temperature rises, so do the hottest daily temperatures. And, in some places, the hottest days can be dangerous to human health, causing heat stress.

Heat stress can create a host of health problems, including rashes, cramps and heat stroke. Hot air is not the only risk factor for it. Other factors include humidity, wind speed and the amount of shade.

You may be familiar with the heat index, which takes into consideration temperature and humidity to suggest what it feels like in the shade on a hot day. But even the heat index doesn’t tell you what it’s like to be working in the full sun on an extremely hot windless day.

Increasingly, climate scientists and meteorologists are advocating for the use of a different metric for understanding extreme heat. It’s called wet-bulb globe temperature and it takes into account temperature, humidity, wind speed, sun angle and cloud cover.

The new dataset uses estimates of both population and wet-bulb globe temperature to better understand how many people are affected by dangerously hot temperatures and where they live.

When the wet-bulb globe temperature exceeds 86 degrees Fahrenheit, people are advised to start taking rests if they’re working outdoors.

HEAT EXPOSURE IS RISING

To match heat measures with population estimates, the researchers averaged temperature data over 13,115 urban centers identified in a dataset produced by the European Union.

Out of those urban centers around the world, nearly half experienced an increasing trend in heat exposure.

In 2016, just under 1.7 billion people lived in those areas, with the majority in Asia and sub-Saharan Africa.

The most affected region, by far, was southern Asia, where India alone accounted for 37 percent of the population living in areas with an increasing extreme heat trend.

With population growth from 1983 to 2016 for each city and estimates for the year-to-year increase in annual counts of dangerously hot days, the AP was able to identify the cities in the world where exposure to extreme heat is increasing most.

NOTABLE HEAT EXPOSURE INCREASES

In Dhaka, Bangladesh, the population more than tripled from about 7.7 million in 1983 to 24 million in 2016. While the city grew by more than 16 million people, the number of extreme heat days also increased by 1.5 days a year, until Dhaka experienced about 50 more dangerously hot days a year than it did in 1983.