Vegetation might exacerbate urban heat island effect in very dry cities
As temperatures rise around the world, city heat becomes increasingly unbearable during the hottest seasons. The urban heat island effect causes cities to become significantly warmer than surrounding rural areas due to human activities and building materials that trap heat.
To mitigate these effects, many urban planners have looked to urban vegetation as a solution. While this has led to noticeable cooling in many cities, a new study, published in Science Advances, finds that incorporating vegetation into city landscapes can actually cause increased heating under certain conditions.
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"Amid these challenges, urban trees stand out as a nature-based solution to mitigate these impacts, offering shade to reduce direct sunlight and facilitating cooling through evapotranspiration," the study authors write.
While shade helps to block direct sunlight to reduce heating, evapotranspiration cools the air through the evaporation of water that trees and other plants pull up from their roots. This effect is stronger with trees, because their roots can reach further down into the ground. However, some local studies in Africa and Europe have observed higher surface temperatures with increased urban tree cover, raising questions about how well the incorporation of urban vegetation works to cool down cities.
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Despite the known cooling effects of shade and evapotranspiration, the new study found that cities in more arid regions were experiencing warming instead of cooling. Although trees still performed significantly better than other vegetation.
"Across global cities, urban grasslands provided a cooling effect in 78% of cases, while urban trees did so in 98%, showing consistently lower surface temperatures than built-up areas. However, in up to 22% of cities, predominantly in arid regions with annual precipitation below ~1000 mm (an empirical threshold), urban grassland (n = 164) and cropland (n = 145) exhibit higher surface temperatures than built-up areas, indicating a net warming effect. In 13 cities (2%) of arid regions, urban trees also exhibit a warming effect," the study authors explain.
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They found that trees failed to cool 25% of cities during extreme heat, and grasslands and croplands fail in 71% and 82% of cities, respectively.
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Still, vegetation-especially trees-perform very well in cooling cities in many climates. The study authors suggest that, in the future, urban planners should tailor strategies to local climate, prioritizing water-efficient species and complementary cooling measures in arid cities, as misguided greening could worsen urban warming if not adapted to local conditions.