Many of our articles discuss the benefits of a green roof landscape, especially in urban environments. You’ve probably also heard that green roofs are a great way to curb the Urban Heat Island (UHI) effect. But what is an Urban Heat Island, and why are they an issue?
What is the Urban Heat Island Effect?
The Urban Heat Island effect is when urban areas become hotter than the surrounding rural areas due to the large amount of impermeable surfaces, such as concrete parking lots and black rooftops. According to the EPA, during the summer months pavement and roofs can be 50–90°F warmer than shaded or moist surfaces (1). It is estimated that anywhere from 5%-10% of energy used by a community in the summertime is compensation for the Urban Heat Island effect (2). This increase in energy consumption has far reaching impacts, such as increased energy costs, rolling brownouts to manage consumption, pollution, and public health concerns.
Greenhouse Gases and Pollution
With an increase in energy consumption to keep homes and businesses cool, the power plants that provide this energy will release more pollutants and greenhouse gases into the atmosphere. Pollutants such as sulfur dioxide, nitrogen oxides, carbon monoxide, and mercury are just some of the more common dangers released by power plants (1). These pollutants also contribute to rising seas temperatures and climate change.
Public Health Impact
Increased daytime temperatures can impact the health of members of your local community. High heat can cause health problems in many individuals such as heat stroke, respiratory problems, cramps, and fatigue (1). Vulnerable or sensitive members of society such as children, the elderly, or those with preexisting conditions are at an increases risk for heat related illness. The EPA has reported an increase in heat related deaths throughout the country since the year 2000 as an indicator of our rapidly changing climate (5).
Denver has the 3rd hottest Urban Heat Island in the United States. In the summer, the Mile High City is on average over 23°F hotter than the surrounding rural areas (4). With 80% of Americans living in cities, the impacts of hotter temperatures and climate change are an increasing risk.
Green roofs help mitigate the impacts and reduce the Urban Heat Island effect in the community. Because they reduce the amount of nonpermiable surfaces on a roof they reduce the impact of that roof on the Urban Heat Island in your neighborhood. Green roofs also cool the area surrounding the roof through a natural process called evapotranspiration (3). This is why many cities, such as Denver, have sparked initiatives to increase the number of green roofs. Increasing the amount of trees in an urban environment, as well as the construction of nonpermiable parking lots, can also reduce the ambient temperatures during the summer months. It is up to citizens and developers to work together to decrease the risks of the Urban Heat Island effect in our city.
Akbari, H. 2005. Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation (PDF) (19 pp, 251K). Lawrence Berkeley National Laboratory.