Sourced from the Living Architecture Monitor
An urban heat island (UHI) is defined as an urban area that is significantly warmer than its surrounding rural landscape due to human activities. According to the U.S. Environmental Protection Agency, urban air temperatures can be as much as 5.6 oC (10 oF) warmer than the surrounding countryside. Warmer summer day and night temperatures are a problem for several reasons. They are a health hazard due to atmospheric ozone concentrations that increase with temperature as well as the increased demand for energy to power air conditioners and refrigeration systems. This in turn increases CO2 in the atmosphere that further exacerbates the warming trend.
The primary cause for the UHI is that humans have replaced vegetated surfaces with hardscapes such as roads, parking lots, and rooftops. Building materials such as asphalt and concrete have a much higher thermal mass capacity relative to plant tissue and therefore absorb and retain more heat. Dark surfaces absorb more solar radiation because of their lower albedo (level of reflectiveness) values. The albedo of green roofs ranges from 0.7 to 0.85, a value much higher than the albedo of bitumen, tar, and gravel roofs (typically from 0.1 to 0.2). Also, evapotranspiration (ET) (evaporation from a surface plus transpiration through plants) plays a major role. Fewer plants results in less transpiration, less absorption of CO2 via photosynthesis, and reduced shading of surfaces. Furthermore, because urbanization increases runoff, the quantity of water available for evaporative cooling is reduced. A great deal of incoming solar energy that would have been used to evaporate water is instead transformed into sensible heat. Coupled with the loss of vegetation and heat absorbing properties of impervious surfaces such as conventional rooftops, this results in higher ambient air temperatures. Since the albedo of urban surfaces is generally 10% lower than the albedo of rural surfaces, urban areas can have higher ambient air temperatures.
There is much published research regarding how green roofs mitigate UHI and how plant species can influence energy savings within an individual building. There is no doubt that a green roof is beneficial relative to a conventional roof. However, there is not much published pertaining to how individual plant species on a green roof influence UHI. In theory, plant species with greater biomass and higher transpiration rates should provide a greater cooling effect. This has been shown to be true in some cases. However, in other situations this has been shown not to be true.