The United Nations’ 2011 Revision of World Urbanization Prospects report outlined the expected growth of urban population to gain more than 2.5 billion inhabitants by 2050, which will necessarily require a massive expansion of urban landscapes, most likely at the detriment of natural landscapes.
In a first of its kind study which attempts to quantify the impact of such a massively expanding megapolitan region, researchers from Arizona State University and the National Center for Atmospheric Research have shown that local maximum summertime warming resulting from this expansion could approach 4 degrees Celsius.
The researchers focused their attention on Arizona’s Sun Corridor, the most rapidly-growing megapolitan area in the United States. Population projections for the four metropolitan areas making up the Sun Corridor — Phoenix, Tucson, Prescott and Nogales — expect the population to exceed 8 million people by 2040.
As a result, the Sun Corridor megapolitan region provided researchers from Arizona State University and the National Center for Atmospheric Research the perfect opportunity to diagnose the influence of large-scale urbanization on climate, and its relation to global climate change.
“We posed a fundamental set of questions in our study, examining the different scenarios of Sun Corridor expansion through mid-century,” says Matei Georgescu, lead author and assistant professor in the School of Geographical Sciences and Urban Planning in ASU’s College of Liberal Arts and Sciences. “We asked, ‘what are the summertime regional climate implications, and how do these impacts compare to climate change resulting from increased emissions of greenhouse gases?'”
A Warming Sun Corridor
The authors of the study used projections of Sun Corridor growth by 2050 developed by the Maricopa Association of Governments, the regional agency for metropolitan Phoenix which has been tasked with providing long-range and sustainably-oriented planning. Incorporating maximum- and minimum-growth scenarios from the population projections into a state of the art regional climate model, the researchers compared the impact of an expanding Sun Corridor region with experiments using an urban representation of modern-day central Arizona.
Their conclusions indicate substantial summertime warming.
“The worst-case expansion scenario we utilized led to local maximum summer warming of nearly 4 degrees Celsius,” said Georgescu. “In the best-case scenario, where Sun Corridor expansion is both more constrained and urban land use density is lower, our results still indicate considerable local warming, up to about 2 degrees Celsius.”
An additional experiment was conducted with the data, adding cool roofs — reflective white roofs — on top of all the buildings in the region.
“Incorporating cool roofs alleviated summertime warming substantially,” says Georgescu, “reducing the maximum local warming by about half. But another consequence of such large-scale urbanization and this adaptation approach also include effects on the region’s hydroclimate.”
The report noted that “although cool-roof implementation does offset urban-induced warming by about half, the effects on evapotranspiration from this mode of adaptation remain at least as large as those” from the maximum-growth scenario.
Maximum Urbanization Leads to Greater Warming than Climate Change
The ultimate comparison was whether Sun Corridor expansion would result in greater warming than climate change. The authors in fact found that this was the case, finding that “urbanization-induced warming … is several times greater over locales experiencing urbanization” than climate change.
That being said, the authors were quick to note that pinning precise figures on the relative contribution of each effector is a difficult task at best.
“The actual contribution of urban warming relative to summertime climate change warming depends critically on the path of urbanization, the conversion of natural to urban landscapes, and the degree to which we continue to emit greenhouse gases,” says Alex Mahalov, a co-author of the Nature Climate Change article and principal investigator of the National Science Foundation grant, “Multiscale Modeling of Urban Atmospheres in a Changing Climate,” which supported the research.
“As well as providing insights for sustainable growth of the Sun Corridor and other rapidly expanding megapolitan areas, this research offers one way to quantify and understand the relative impacts of urbanization and global warming,” says Mahalov, who is the Wilhoit Foundation Dean’s Distinguished Professor in the School of Mathematical and Statistical Sciences at ASU.
Georgescu says that one take-home message from this study is that the incorporation of sustainable policies need to extend beyond just greenhouse gas emissions. He also stresses the importance of extending adaptation strategies beyond the focus on mere average temperature.
“Truly sustainable adaptation, from an environmental standpoint, must extend to the entire climate system, including impacts on temperature and hydrology,” he says.