Between climate and density, cities are increasingly threatened by extreme heat

According to a scientific study published on Monday, there is a risk that the poor population in these cities will face increasing exposure to extreme heat caused by their density and global warming.

“In Africa and South Asia, where hundreds of millions of urban poor (…) already live without adequate investment, humanitarian intervention and government support, extreme heat can severely limit people’s ability to urbanize,” write the authors of the Proceedings of the National Academy of Sciences “published study.

The “heat islands” come into question because of the density of the cities, where the concrete, asphalt and the slightest vegetation tend to trap the heat.

To arrive at their results, the researchers examined more than 13,000 cities by setting an extreme heat threshold at 30 degrees Celsius using the Wet Bulb Globe Temperature, an index that takes heat into account.

At this point, “even the healthiest people find it difficult to stay outside for long periods of time, and people in poor health can become very sick or even die,” explains the Earth Institute at the University of Columbia, New York, affiliated with the study .

– “Bass degrees of latitude” –

Of the 13,115 cities surveyed, the number of people / days (number of people affected times the days) exposed to this extreme heat rose from 40 billion in 1983 to 119 billion in 2016, researchers from the University of California Santa Barbara calculate . Twin cities of Minnesota, Arizona, and Columbia.

Most of the cities affected are concentrated “in the lower latitudes”, especially in South Asia (India, Pakistan, Bangladesh) and in sub-Saharan Africa (Nigeria), “but cover a wide range of climates,” write the authors of the study.

In 2016, 1.7 billion people were exposed to these temperatures for more than one day a year, adds the institute.

Among the hardest hit cities are Dhaka, the capital of Bangladesh, which saw an increase of 575 million person-days of extreme heat during the period studied (1983-2016), an increased exposure due to 80% of the explosion (from four to 22 million), so the Earth Institute.

“This does not mean that Dhaka did not experience significant warming, but population growth was even faster,” the institute writes.

Other cities show similar trends like Shanghai, Guangzhou (China), Rangoon (Burma), Bangkok (Thailand), Dubai (United Arab Emirates), Hanoi (Vietnam), Khartoum (Sudan) or several cities in Pakistan, from India and the Arab Peninsula.

On the other hand, in some other major cities, almost half or more of their exposure was caused by global warming alone compared to population growth. These are Baghdad (Iraq), Cairo (Egypt), Kuwait City, Lagos (Nigeria), Calcutta, Bombay (India) and other major cities in India and Bangladesh.

The authors emphasize the importance of distinguishing between demographic and climatic factors in order to better define local policies.

In the United States, about forty major cities have increased their exposure “very rapidly, mainly in Texas and on the Gulf Coast” of Mexico (Alabama, Florida, Louisiana, Mississippi, Texas) as the study s’ halted in 2016 , does not take into account the fatal heat records that occurred in the northeastern United States and southern Canada in the summer of 2021.