Temperatures in cities are rising around the world and urban heat stress is already a major problem. Extensive surfaces of man-made materials absorb the sun’s energy, and lead to temperatures well above those in the surrounding countryside. This is known as the urban heat island effect, and it can lead to greater energy use, higher air pollution levels, and a greater risk of heat-related illnesses, as well as death.
Some cities have already started implementing mitigation strategies, with tree planting prominent among them. Planting trees can cool the climate by absorbing carbon dioxide, providing shade, and releasing water vapor, which lowers air and surface temperatures.
However, while trees can cool cities significantly during the day, new research from the University of Cambridge in the U.K. shows that tree canopies can also trap heat and raise temperatures at night.
According to the study, which was recently published in the journal Communications Earth & Environment, planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.
The researchers found that in temperate climates, trees can cool cities by up to 6°C during the day but can increase nighttime temperatures by 1.5°C. Cities with open layouts in temperate and tropical climates benefit from a mix of evergreen and deciduous trees, enhancing cooling by 0.5°C more than in cities with only deciduous or evergreen trees.
The researchers hope their findings will help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.
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Wrong trees in the wrong place can make cities hotter at night, study reveals
Photo, posted October 29, 2017, courtesy of Lars Plougmann via Flickr.
Earth Wise is a production of WAMC Northeast Public Radio
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