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Cool Roofs May Have Side Effects on Regional Rainfall

Rooftops with reflective paint reduce temperatures in urban areas while cutting building energy costs, but in some development scenarios they might lead to a 4 percent decline in rainfall
Roof Painting



Flickr/Michael R. Allen

As desert sands yield to asphalt and concrete, the climate is shifting in Arizona's "Sun Corridor," an expanding urban region that includes Phoenix, Tucson, Prescott and Nogales. Researchers are now finding that efforts to offset the climate shift may carry side effects of their own.

Towering buildings, dark roads and sparse vegetation combine to trap heat, making cities warmer than surrounding areas. Previous studies showed that these effects are profound. "What we saw was that urbanization-induced warming is just as important as greenhouse gas-induced climate change," said Matei Georgescu, an assistant professor in the School of Geographical Sciences and Urban Planning at Arizona State University.

In a study published last month in the journal Environmental Research Letters, Georgescu demonstrated that these effects change with the seasons and have consequences for regional hydrology, as well. "There's more to it than just average temperature."

The Sun Corridor is a good test case, according to Georgescu; it is the fastest-growing "megapolitan" region in the United States. How much population and development growth there will be is uncertain, so Georgescu and his team set a floor and a ceiling for urbanization projections up to the year 2050 based on available data from the Maricopa Association of Governments, the regional agency in charge of long-term planning.

The researchers found that cities would generate the most warming during the summers under the maximum development scenario, with warming exceeding 1 degree Celsius. Under the minimum development projections, warming ranged from 0.1 to 0.3 degrees Celsius for most of the year outside winter.

The models also showed another curious development: Cool roofs -- created when developers use reflective paint on rooftops -- do perform their intended task of reducing temperatures in urban areas while cutting building energy costs. However, they shift rainfall patterns by reducing evapotranspiration, the process by which water evaporates from the ground and enters the atmosphere. In the maximum expansion scenario, cool roofs led to a 4 percent decline in rainfall.

Modifying CO2 footprint can modify the weather
"Does that suggest that cool roofs are a negative? I think what this leads to is future research to see how they should place cool roofs to minimize impacts," Georgescu said. "Certain regions might be more appropriate for cool roofs than others."

Some changes in rain patterns also stem from development itself. "When you put this carpet of urban land use, you're forbidding the land from capturing and storing the water," Georgescu said. "We've shown in some of our previous work that locally recycled water is very important for regional rainfall."

Roger Pielke, a senior research scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder, noted that offsetting or mitigating humanity's impacts on the world often carries unintended consequences. "Any geoengineering approach will have other effects as well as for the one it is designed to respond to," he said in an email.

He pointed to research that showed how wind turbines alter regional temperatures even as they reduce carbon emissions that contribute to global climate change. Such trends mean scientists and policymakers will have to factor in how synthetic climate forcers other than greenhouse gases will change temperature, rainfall and weather extremes.

To solve this problem, Pielke suggested measuring environmental variables from a regional scale up to a global scale as a more inclusive way to assess environmental risks than the top-down approach used by the Intergovernmental Panel on Climate Change.

"This vulnerability concept requires the determination of the major threats to local and regional water, food, energy, human health, and ecosystem function resources from extreme events including climate, but also from other social and environmental issues," he said in a book chapter he co-authored in "Extreme Events and Natural Hazards: The Complexity Perspective" earlier this year.

For now, Georgescu said, he will concentrate on regional modeling because global climate models do not yet offer enough resolution to illuminate climate trends in areas like the Sun Corridor. Conducting similar studies in multiple regions around the world could help climate modelers improve their global projections and help planners anticipate local climate shifts.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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