Urban Heat Island Effect

Cities and regions facing increasing temperatures related to climate change are looking to green infrastructure as a strategy for offsetting the urban heat island effect and mitigating some of the impacts of changing climate for public health reasons.

According to the American Rivers report “The Value of Green Infrastructure,” as urban areas develop and buildings, roads, and other infrastructure replace open land and vegetation, surfaces that were once permeable generally become impermeable and dry. This development causes urban regions to experience warmer temperatures than their surroundings, a phenomenon known as the urban heat island (UHI) effect. UHI can affect public health by causing respiratory difficulties, exhaustion, heat stroke and heat-related mortality. UHI also contributes to elevated emission levels of air pollutants and greenhouse gases through the increased energy demand (via greater air conditioning needs) that higher air temperatures cause. Additionally, UHI puts a greater demand on outdoor irrigation needs thus increasing water demand and its associated energy uses.

Green infrastructure techniques can be used to help mitigate UHI and improve air quality through increased vegetation, reduced ground conductivity and decreased ground level ozone formation. As reported by American Rivers, various studies have estimated that trees and other vegetation within building sites reduce temperatures by about 5oF when compared to outside non-green space. At larger scales, variation between non-green city centers and vegetated areas has been shown to be as high as 9F. Likewise, recent studies done on permeable pavement have found that it reduces or lowers the negative impacts of UHI through its porosity, which serves to insulate the ground better and allow more water evaporation. Both of these effects aid in cooling temperatures and mitigating the UHI effect.

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