Climatic impacts of megapolitan expansion

our results still indicate considerable local warming, up to about 2 degrees Celsius.”

An additional experiment was conducted to examine an adaptation where all of the buildings were topped by highly reflective white or “cool” roofs.

“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 cool roofs, like the maximum-growth scenario without this adaptation approach, further reduce evapotranspiration – water that evaporates from the soil and transpires from plants.

Ultimately, comparison of summertime warming resulting from Sun Corridor expansion to greenhouse-gas-induced summertime climate change shows that through mid-century the maximum urbanization scenario leads to greater warming than climate change.

The authors say, however, that pinning precise figures on the relative contribution of each effector is difficult.

“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.

The group conducted their numerical simulations using an “ensemble-based” approach. By modifying their model’s initial conditions and repeating their simulations a number of times, they were able to test the robustness of their results. In all, nearly half of a century of simulations were conducted.

“By incorporating differing Sun Corridor growth scenarios into a high-performance computing modeling framework with projections obtained from Maricopa Association of Governments, we quantified direct hydroclimatic impacts due to anticipated expansion of the built environment,” Mahalov added. Simulations were conducted at ASU’s Advanced Computing Center (A2C2).

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.

— Read more in M. Georgescu et al., “Summer-time climate impacts of projected megapolitan expansion in Arizona,” Nature Climate Change (12 August 2012) (doi:10.1038/nclimate1656)