A new study in more than 13,000 cities around the world has found that the number of days per person in which residents are exposed to extreme combinations of heat and humidity has tripled since the 1980s. The authors say that the The trend, which now affects nearly a quarter of the world’s population, is the combined result of rising temperatures and booming urban population growth. The study has been published in the journal PNAS.
Why is it hotter in cities?
The lack of vegetation and the concentration of asphalt and other impermeable surfaces that tend to trap and concentrate heat form what is called the heat island effect in cities, where temperatures are usually somewhat higher than in adjacent rural areas. “And this has its effects,” says Cascade Tuholske, a postdoctoral researcher at the Earth Institute at Columbia University and lead author of the study. “It increases morbidity and mortality. It affects people’s ability to work and results in lower economic output. It aggravates pre-existing health conditions ”.
In recent decades, hundreds of millions have moved from rural areas to cities, which are now home to more than half of the world’s population, and thus more people are exposed to these effects.
The researchers combined infrared satellite imagery and readings from thousands of ground-based instruments to determine maximum daily heat and humidity readings in 13,115 cities, from 1983 to 2016. They defined extreme heat as 30 degrees Celsius on the so-called “globe temperature scale. “wet bulb”, a measure that takes into account the multiplier effect of high humidity on human physiology. This is a point where even most healthy people find it difficult to function outdoors for long periods of time, and sick people can get worse or even die.
To arrive at a measure of person-days spent in such conditions, the researchers compared meteorological data with statistics on city populations over the same time period. The analysis revealed that the number of person-days in which city dwellers were exposed tripled, from 40 billion per year in 1983 to 119 billion in 2016. By 2016, 1.7 billion people were exposed to such conditions for several days.
Cities most affected
The most affected city in terms of person-days was Dhaka, the capital of Bangladesh, mainly due to its rapid growth, with an increase of 575 million people / days of extreme heat during the study period. Its growing population alone – 4 million in 1983, to 22 million today – caused 80% of the increased exposure. This does not mean that Dhaka has not experienced substantial warming, but rather that population growth was even faster. Other large cities showing similar population trends include Shanghai and Guangzhou, China; Yangon, Myanmar; Bangkok; Dubai; Hanoi; Khartoum; and several cities in Pakistan, India, and the Arabian Peninsula.
On the other hand, in some cities it has been seen that close to half or more of their exposure has been caused only by the warming of the climate versus the growth of the population. These include Baghdad, Cairo, Kuwait City, Lagos, Kolkata, Mumbai, and other large cities in India and Bangladesh. Populations in European cities have remained relatively static, so increases in exposure were driven almost exclusively by increased warmth . The researchers found that 17% of the cities studied added a full month of extreme hot days during the 34-year study period.
Kristina Dahl, a climate researcher at the Union of Concerned Scientists, explains that the study “could serve as a starting point for identifying ways to address local heat problems,” such as planting trees and modifying roofs with lighter colors or greenery to that do not trap so much heat . “This study shows that considerable and thoughtful investments will be needed to ensure cities remain livable in the face of an increasingly warmer climate.”
