As Earth’s climate continues to warm, the urban heat island effect raises concerns that city-dwellers will suffer more heat stress than their rural counterparts. However, new research suggests that some cities actually experience a cooling effect.
More than 60 percent of urban areas in India experience a day-time cooling effect, according to the study, which was published in Scientific Reports. The cooling effect has been observed in the past, but this paper is the first to directly identify a cause: lack of moisture and vegetation in non-urban areas surrounding the city.
“When the areas around cities are running low on water and they aren’t being irrigated, they turn into hot, dry, barren fields,” said Matthew Huber, a professor of earth, atmospheric and planetary sciences at Purdue University. “When that happens, there’s actually more water available to evaporate in the cities than the surrounding countryside. It’s like the cities are sweating.”
More commonly, cities are warmer than their rural surroundings, known as the heat island effect. As cities develop, they lose vegetation and surfaces are paved or covered with buildings. With less shade and moisture, they aren’t able to keep themselves as cool.
Huber and his collaborators obtained temperature data from 89 cities in India and then used a climate model to determine the effects of irrigation. By turning irrigation “on and off” in the model, they found that both urban heating and cooling effects are largely controlled by agriculture and moisture availability from irrigation.
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