As urban planners look to expand green spaces to help cool cities, a new study finds that, in arid regions, grassy areas can actually have a warming effect.
Analyzing satellite data from more than 700 large cities around the world, researchers found that green areas are generally cooler, but not always. In 22 percent of cities, grassy areas are actually warmer than built-up areas. These cities tend to be in regions that see less than 40 inches of rain yearly.
Plants help cool cities by soaking up water from the ground and releasing it through their pores. Just as when humans sweat, water evaporates, with a cooling effect.
In dry regions, however, plants recycle only small amounts of water, so the effect is minimal. At the same time, their dark green leaves absorb more heat from the sun than other, lighter surfaces, such as concrete. In the final tally, grasses in many arid cities have a net warming effect. The findings were published in Science Advances.
Authors also note that trees are much better than grasses at cooling off cities. Along with providing shade, trees grow long roots that can tap moisture deeper underground, allowing them to dispense large volumes of water, even during hot spells that dry out topsoil. In only 2 percent of cities did trees have a net warming effect, the study found.
Authors say that planners in arid regions should look to plant hardy trees, rather than grasses, and to build roads and rooftops that are light in color, and thus reflect sunlight.
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