AP Analysis: Exposure to extreme heat has tripled since 1983

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World leaders have pledged to limit the Earth’s temperature rise to 1.5 degrees Celsius (2.7 degrees Fahrenheit) compared to pre-industrial times.

But how does it feel?

It is difficult to convey, because you may not notice changes in the average temperature. But, depending on where you live, you may notice when it’s very hot.

To better understand the problem, Columbia University’s School for the Climate recently released a global dataset with estimates of both population and temperature. The Associated Press analyzed the data – ranging from 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 global average temperatures rise, so do the hottest daily temperatures. And, in some places, hotter days can be dangerous to human health, causing heat stress.

Heat stress can create a number of health problems, including rashes, cramps, and heatstroke. Hot air isn’t the only risk factor for this. Other factors include humidity, wind speed, and the amount of shade.

You may know the Heat Index, which takes temperature and humidity into consideration 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 work in full sun on an extremely hot, windless day.

More and more climate scientists and meteorologists are advocating the use of a different metric to understand extreme heat. It’s called wet bulb globe temperature and takes into account temperature, humidity, wind speed, sun angle, and cloud cover.

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

When the wet bulb globe temperature exceeds 30 degrees Celsius (86 degrees Fahrenheit), people are advised to start resting if they work outdoors.

EXPOSURE TO HEAT IS INCREASING

To match heat measurements with population estimates, the researchers calculated mean temperature data on 13,115 urban centers identified in a data set produced by the European Union.

Of these urban centers around the world, nearly half have experienced a trend of increasing 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, has been South Asia, where India alone accounts for 37% of the population living in areas with a growing tendency to extreme heat.

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

SIGNIFICANT INCREASES IN EXPOSURE TO HEAT

In Dhaka, Bangladesh, the population more than tripled from about 7.7 million in 1983 to 24 million in 2016. As the city grew by more than 16 million people, the number of days of extreme heat also increased by 1. , 5 days a year, until Dhaka experienced about 50 dangerously hotter days a year than in 1983.

This large population growth, coupled with the area’s warming trend, reveals that Dhaka has had the largest increase in heat exposure in the world.

Population growth and temperature rise both contribute to exposure trends. In some cases, these have the same effect. This was the case in Calcutta, India, where the population grew by 6 million and the number of hot days grew by 1.76 each year. Both of these increases contributed to a strong trend in exposure.

Meanwhile, New Delhi has added nearly 14 million people. While the city added 1.12 more warm days each year, the rising population is what made the Delhi exposure trend the steepest in India.

THE CAPITAL OF INDONESIA

This dataset focuses on the past, but could help world leaders make more informed decisions in the future. Indonesia is planning to move the country’s capital from Jakarta, a city that is sinking below sea level, to Kalimantan. The development site is located in a jungle area between two cities, Samarinda and Balikpapan. Both cities have increasing heat exposure trends.

People in Balikpapan, located at the mouth of Bakpapan Bay, could expect 10 more days of extreme heat in 2016 than in 1983.

Samarinda, located on the humid delta of the Mahakam River, could expect another 19 days. Jakarta, while sinking, did not experience a trend of exposure to the significant increase in the dataset.

THE GROWING CITIES OF AFRICA

Many cities that already experience extreme heat are growing rapidly.

Douala, Cameroon, grew from about 760,000 people in 1983 to nearly 3 million in 2016. The UN demographic projections suggest the tally will double by 2035. Douala’s citizens endured 76 days of extreme heat in 2016.

Douala is representative of sub-Saharan Africa as a whole. According to the United Nations World Population Outlook 2019, most of the world’s population growth will come from this region in the coming years and is growing rapidly at a time when warming trends are on the rise in cities in the area. If world leaders are able to limit the rise in global mean temperature, people in this part of the world will likely feel the difference in the resulting heat exposure more.

Dim Coumou, a climate professor at Vrije Universiteit Amsterdam, said the combination of growth in African cities and climate change poses a serious risk.

“As the population in these megacities increases, there are more buildings, more concrete and a greater heat island effect, which makes heat waves worse,” Coumou said. “I think it’s a dangerous combination.”

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.

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