According to the United Nations’ 2011 Revision of World Urbanization Prospects, global urban population is expected to gain more than 2.5 billion new inhabitants through 2050. Such sharp increases in the number of urban dwellers will require considerable conversion of natural to urban landscapes, resulting in newly developing and expanding megapolitan areas. Could climate impacts arising from built environment growth pose additional concerns for urban residents also expected to deal with impacts resulting from global climate change?
In the first study to attempt to quantify the impact of rapidly expanding megapolitan areas on regional climate, a team of researchers from Arizona State University (ASU) and the National Center for Atmospheric Research has established that local maximum summertime warming resulting from projected expansion of the urban Sun Corridor could approach 4 degrees Celsius. This finding establishes that this factor can be as important as warming due to increased levels of greenhouse gases. Their results are reported in the early online edition (Aug. 12) of the journal Nature Climate Change.
Arizona’s Sun Corridor is the most rapidly growing megapolitan area in the United States. Nestled in a semi-arid environment, it is composed of four metropolitan areas: Phoenix, Tucson, Prescott and Nogales. With a population projection expected to exceed 9 million people by 2040, the developing Sun Corridor megapolitan provides a unique 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. 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,” 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.
The authors utilized projections of Sun Corridor growth by 2050 developed by the Maricopa Association of Governments (MAG), the regional agency for metropolitan Phoenix provides long-range and sustainably oriented planning. Incorporating maximum and minimum growth scenarios into a state-of-the-art regional climate model, the researchers compared these impacts 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. 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,” Georgescu said.
An additional experiment was conducted to examine an adaptation where all of the buildings were topped by highly reflective white or “cool” roofs.