Chill Out: Vegetation Reduces the Urban Heat Island Effect

March 3, 2014

By Madeline Leslie, Graduate Research Assistant

apartment building
Photo: Sam Bauer

In many places around the country heat waves during the summer are commonplace; a time when most people thankfully take advantage of indoor air-conditioning and access to nearby lakes or pools. However, as those of us in urban areas know, we frequently have to deal with average temperatures that are higher than surrounding areas, a condition commonly referred to as the urban heat island (UHI) effect. This problem is likely to only become worse in the future, as climate change raises average global temperatures and urban populations increase. Unfortunately heat waves are not just uncomfortable events; they can have serious consequences. Excessive heat can have negative effects on human health in multiple different ways, resulting in illness and even death. In addition, there are other social, environmental, and economic costs associated with high temperatures, including increased violent crime rates and global warming emissions related to higher energy usage from air conditioning systems. A recent study, Ecosystem services and urban heat riskscape moderation: water, green spaces, and social inequality in Phoenix, USA [1], sought to find out what groups of people are most affected by high temperatures and also looked at a way to reduce the risk of heat exposure in urban areas.

The authors of this study specifically wanted to find out if greater vegetation cover in the Phoenix metropolitan areas has been correlated over the past 30 years with a reduction in temperatures in specific areas. In addition the authors also investigated the potential cost of using water to reduce temperatures in urban areas, and the relationship between income levels, elevated temperatures, and vegetation cover. They did this by looking at local weather station records, satellite images collected as part of the Landsat program, and median household income from US census data.

The results showed that vegetation cover provided significant cooling ecosystem services, which were most pronounced in the summer and less pronounced in the winter. Additionally, strong relationships were found between income levels, cooling ecosystem services, and the amount of water needed to provide these services. Lower income areas tended to have less vegetation cover, a pattern which did not exist in 1970 but has been increasing over the past three decades. This points towards a growing disparity between wealthy and poor neighborhoods in terms of vulnerability to heat-related health risks.

The findings of this study are very relevant to those in the turfgrass industry and related horticultural fields. Clearly there is a need around the country to mitigate the risks of high temperatures in urban spaces. This becomes a social justice issue in places like Phoenix, where low-income areas do not have the same access to the cooling effects of vegetated landscapes as higher-income areas. Turfgrass varieties that can survive in hot environments with low amounts of water could be very beneficial in parks and on private property, especially in areas where individuals do not have the resources to maintain landscapes that require high amounts of inputs. In this way, access to the cooling benefits of vegetative cover can be made accessible to people in all income levels , rather than only some.

[1] Jenerette, G. D., S. L. Harlan, W. L. Stefanov, and C. A. Martin. 2011. Ecosystem services and urban heat riskscape moderation: water, green spaces, and social inequality in Phoenix, USA. Ecological Applications 21(7): 2637–2651.