by Dan McCabe
Greenhouse gases have earned a bad name for their impacts on global climate, but in modern cities, the built environment itself can contribute to climate change just as much. In order to quantify and analyze the impacts of urbanization on local and regional temperature and hydroclimate, Georgescu et. al. (2014) modeled the impacts of urban expansion in the contiguous United States in a variety of scenarios. The authors considered a range of different predicted population levels in the United States for the year 2100. Using advanced atmospheric models, they found that if no urban climate change mitigation measures were put into place by then, summertime urban-induced warming of 1–3 °C can be expected in cities, with exact values varying by location. These increased temperatures are due solely to the effects of the built environment, as simulations were run using climate data from 2001-2008 without any assumptions about future warming due to increased greenhouse gas emissions.
The model also evaluated the impacts of certain urban climate adaptation strategies—specifically, the implementation of green roofs, reflective cool roofs, and a hybridization of the two roof styles. The results suggested that widespread implementation of these strategies could entirely offset the warming caused by urban development. While this result is promising, it came with some caveats. For example, the authors found that widespread adoption of cool roofs, counteracts summertime warming but exacerbates winter cooling, leading to increased heating demand and energy usage in winter. These adaptation strategies also exhibited substantial effects on hydroclimate, as cool roofs showed a tendency to decrease precipitation and green roofs to increase it. Interestingly, these impacts appeared to extend beyond local areas and have noticeable effects on regional hydroclimate, which need to be addressed in any future plans for urban climate change adaptation.
The authors’ research emphasizes a need for comprehensive responses to climate change. While the model revealed many universal warming trends for urban areas, different cities suffered distinct consequences. However, the need to address the effects of urban warming is clear. In the case of maximum predicted greenhouse gas emissions by 2100, regional contribution to urban warming is expected to range from 15-27% (up to 50% locally), and is even higher in the case of lower emissions. The results of this research demonstrate that local and regional climate change due to urbanization is likely to be a problem regardless of global trends, and the policy needed to combat it should comprehensively address climatic, hydrological, and socioeconomic concerns unique to the regions it covers.
Georgescu, M., Morefield, P. E., Bierwagen, B. G., Weaver, C. P., 2014. Urban adaptation can roll back warming of emerging megapolitan regions. Proceedings of the National Academy of Sciences 111, 2909-2914. http://www.pnas.org/content/111/8/2909.