Urban heat islands form when urban centres are relatively warmer than the rural areas nearby. These higher temperatures can have severe negative impacts on the health of the residents, even resulting in health-related deaths, and translate into higher electricity demands to power fans and air conditioners. According to a 2001 paper, alleviating the heat island effect could lower the energy used to power air conditioners in the United States by around 20% and save billions of dollars. Heat islands also mean warmer air temperatures, which increases smog production, potentially reducing visibility in cities. Urban trees have frequently been proposed as a way to reduce urban heat. They have been found to significantly reduce land surface temperatures and air temperatures in urban areas.
Trees have a known cooling effect on their surroundings, primarily as a result of shading and transpiration. Tree canopies stop radiation from the sun from reaching the ground, which has a high heat absorption capacity. Without trees, when short wavelength radiation from the sun is absorbed by the ground, it is re-emitted as long wavelength radiation, resulting in higher temperatures in the vicinity. There can even be more than a 20°C surface temperature difference between shaded locations and asphalt locations directly out in the sun. Cooling by transpiration varies based on tree species and local environmental conditions. Heat energy is consumed for the evaporation of water transpired by trees, resulting in a reduction in the temperature of leaf surfaces and nearby air temperature.
A study examined the land surface temperature differences in locations covered completely by urban trees and locations covered completely by urban fabric during periods of hot temperature extremes in Europe. It found that the temperature was as much as 8-12°C lower for urban trees than for urban fabric in Central Europe and up to 4°C lower in Southern Europe.
The planting of urban trees could be a relatively inexpensive method of reducing urban heat in cities, with additional benefits like neighbourhood beautification. Trees also reduce CO2 levels- both directly, through carbon sequestration, and indirectly, since lower temperatures mean lower energy demands.
Works Cited
Akbari, H, et al. “Cool Surfaces and Shade Trees to Reduce Energy Use and Improve Air Quality in Urban Areas.” Solar Energy, vol. 70, no. 3, 2001, pp. 295–310, www.sciencedirect.com/science/article/pii/S0038092X0000089X?via%3Dihub, https://doi.org/10.1016/s0038-092x(00)00089-x.
Schwaab, Jonas, et al. “The Role of Urban Trees in Reducing Land Surface Temperatures in European Cities.” Nature Communications, vol. 12, no. 1, 23 Nov. 2021, https://doi.org/10.1038/s41467-021-26768-w.
Winbourne, Joy B, et al. “Tree Transpiration and Urban Temperatures: Current Understanding, Implications, and Future Research Directions.” BioScience, vol. 70, no. 7, 24 June 2020, pp. 576–588, https://doi.org/10.1093/biosci/biaa055.
Yang, Yixuan, et al. “How Can Trees Protect Us from Air Pollution and Urban Heat? Associations and Pathways at the Neighborhood Scale.” Landscape and Urban Planning, vol. 236, Aug. 2023, p. 104779, www.sciencedirect.com/science/article/pii/S0169204623000981, https://doi.org/10.1016/j.landurbplan.2023.104779.
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