Such green roofs as the one atop Chicago’s city hall could be used on a larger scale to slow or halt temperature increases in cities as urban populations increase and the global climate warms. Wikimedia Commons/TonyTheTiger
A research team finds that a wide-scale deployment of green and cool roofs could combat the heat island effect in urban areas and might even alter precipitation levels.
March 11, 2014—Green roof and cool roof technologies, properly deployed, have the potential to dramatically cool the rising temperatures in growing metropolitan areas attributed to the “heat island” effect, according to new research published in the Proceedings of the National Academy of Sciences (PNAS).
The issue is likely to become a pressing concern as the population of the United States grows from 315 million in 2012 to the more than 420 million by 2060 that is projected by the U.S. Census Bureau. The most aggressive estimates of the U.S. population in 2100 reach nearly 700 million. This population growth is expected to take place largely in rapidly expanding metropolitan areas, the boundaries of which have begun to merge in some regions to form megapolitan areas.
A team of researchers led by Matei Georgescu, Ph.D., an assistant professor at Arizona State University, examined the effects of these projected population and urban expansion trends utilizing the advanced research version of the Weather Research and Forecasting (WRF) model, an open-source climate and weather prediction model system designed for both research and forecasting that is maintained and supported by the National Center for Atmospheric Research in Boulder, Colorado. The urban expansion projections that were used in the analysis were developed by the U.S. Environmental Protection Agency.
The research findings, “Urban Adaptation Can Roll Back Warming of Emerging Megapolitan Regions,” were published February 10, 2014, in the PNAS Early Edition. The results indicate that by 2100, megapolitan regions will experience a near-surface temperature warming in the range of 1° to 2° C attributable solely to the heat island effect. Some areas, such as Chicago, will top 3° C. This warming is separate from and additional to that projected from climate change.
The team then examined the implications of high-albedo cool roofs, green roofs (those covered with living plants), and a hypothetical combination of the two that includes the reflectivity of cool roofs and the evapotranspiration of green roofs. The findings present a complex picture, but indicate that each of the strategies significantly mitigates the heat island effect expected by urban expansion.
“Our results indicate that adaptation to urban-induced climate change depends on specific geographic factors,” said Georgescu, in written responses to questions posed by Civil Engineering online. “What works over one geographical area may not be optimal for another. This is in contrast to work that has touted one-size-fits-all solutions to urban heat island amelioration.”
In general, the cool roofs were more effective at reducing heat in megapolitan areas than green roofs, but the results varied greatly. The difference is just 0.2° C in Florida, for example, but 1.2° C in California in spring and summer. And cool roofs lower temperatures in cold months as well as warmer ones, so in some locations this would increase energy demands for heating.
The research also indicates that large-scale cool-roof or green-roof deployment can significantly reduce precipitation in some megapolitan areas and increase it in others. This creates a need for a careful assessment of the trade-offs.
“From an engineering perspective, it was interesting that cool roofs were more effective at lowering near-surface temperatures than green roofs, for all regions investigated. Although this is an interesting result, focusing exclusively on near-surface temperature is not a sufficient way to characterize climate system effects, since it ignores the consequences for moisture availability, clouds, and rainfall,” Georgescu said.
“By more comprehensively addressing impacts from each of the urban adaptation strategies, on climate rather than merely near-surface temperature, we can begin to peel the layers of trade-offs associated with each approach,” Georgescu said. “From a sustainability perspective, better solutions to urban adaptation will provide multiple benefits for people and environment, rather than a narrowly focused view on near-surface temperature only.”
The results reinforce the value of locally centered, “bottom-up” decision making to determine the best adaptation solutions for a given region, Georgescu noted.
“It was interesting to find that each of the strategies investigated completely offset warming due to urban expansion and even offset a percentage of future greenhouse [gas-induced] warming over large regional scales,” Georgescu said.
Georgescu said the team is now extending this research by focusing more closely on specific megapolitan areas “at very high resolution.” This next round of research will consider air quality consequences as well as temperature and climate.