Cool roofs may effect regional rainfall
Image: 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.
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6 Comments
Add CommentInstall green roofs and use rainfall and the building's grey water to irrigate them. Use a greenhouse and you can pretreat raw sewage.
Reply | Report Abuse | Link to thisWhite roofs -> Lower rainfall in urban 'regions' -> then more rainfall for farming 'regions' ?
Reply | Report Abuse | Link to thisFirst, 4% is not much of a decrease. Second, does that decrease add to or reduce the change to rainfall caused by the initial (and continuing) urbanization? But of course the real take home lesson is the one that is the focus of the report, anything we do will have effects that cascade through the ecosystem, meaning ecologists need to be involved from the beginning to identify these unintended consequences. Would it not have been nice to identify in advance the unintended consequences of the initial urbanization?!
Reply | Report Abuse | Link to thisGood news, this article does a great job of pointing out a variety of "unintended consequences", i.e. there is no easy fix, or ecology is complicated... bad news (at least for me) I'm losing all sense of scale. 4% decrease in rainfall, where exactly? Less heat absorption, how much less? which helps how much? And finally, relative to things like dams on the Colorado, Lake Mead, no water reaching the Gulf of Mexico, watering the golf courses in the deserts... played out against 400 ppm CO2 and rising... isn't the Sun Corridor a rather high risk 'megapolitan' area in terms of consequences of said global climate changes? Might those "unintended consequences" include people migrating to areas less impacted? What is there now is not sustainable, surely the models don't just assume more and more of the same.
Reply | Report Abuse | Link to thisI would like to point everyone to the published journal article (it is free): http://iopscience.iop.org/1748-9326/7/3/034026
Reply | Report Abuse | Link to thisI will address some of the excellent questions posed in the comments:
1. The 4% decrease in rainfall is relative to the non-adaptive maximum expansion scenario. The maximum expansion scenario (relative to a baseline) lowers precipitation by 12%.
2. All temperature and precipitation impacts are averaged over the entire state. Local impacts are greater. Figure 6, for example (viewable from the link above, to the article), displays locations where relative precipitation change is greatest. From that figure, one can see significant precipitation changes, due to Sun Corridor expansion, over higher terrain (for example, north of current Metro Phoenix).
According to prophesy, there is a chance of a super solar storm in this time period. Dec 21 2012 seems reasonable. Such an occurrence would destroy asphalt roof, roads. 700 % for 7 days. No wonder coke is said to add flame retardants to our drinks. They are worried about our surviving a solar storm.....;)
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