Benefits of green roofs: A systematic review of the evidence for three ecosystem services

Green roofs are often claimed to provide a range of environmental, economic and social benefits, or 'ecosystem services'. These reported benefits, suggests that green roofs could play a significant role in sustainable urban development, and consequently green roofs are now widely used as tools in urban planning strategies. Accordingly, it is relevant to assess whether the benefits of green roofs and comparative advantages over conventional roofs rest on a robust evidence base. A considerable number of studies of the ecosystem services delivered by green roofs have appeared over the last few decades, but a rigorous assessment of the overall level of evidence is lacking. Using a systematic review approach, this study seeks to evaluate the documentation relating to three selected green roof ecosystem services: reduction of the urban heat island effect, reduction of urban air pollution, and reduction of building energy consumption. The number of studies quantifying effectiveness with original data was found to vary significantly from service to service: 17 studies reported cooling at street level ranging between 0.03-3 C degrees, four reported pollution removal at roof level e.g. removal of small particles, PM10, ranging between 0.42-9.1 g/m(2) per year, and 41 reported on building energy consumption, of which 20 were comparable and claimed changes in annual consumption ranging between an increase of 7% to a 90% decrease. The large spans in documented effectiveness are ascribed to heterogeneity in context and design parameters of the identified studies. Analysis of the identified studies suggests that some parameters are of key importance for the effectiveness but further research is needed to clarify the complex relation between ecosystem service effectiveness and the parameters influencing it.