Planning for future heat waves. Invoking the “Oasis” effect.

August 18, 2015

Link to article: “Challenges associated with projecting urbanization-induced heat-related mortality,” Hondula, et al.

Recent and substantial increases in heat and heat wave mortality and morbidity impel society to brace for heat events of even greater severity and duration. Unfortunately, research on the relationship between future heat increases and its health impacts generally overlooks how choice of exposure metric (mean, minimum & maximum temperature) effects downstream projections. Additionally, regional growth management plans are generally not incorporated into climate impact models. Both considerations are important for climate change modeling to provide an accurate big picture framework for mitigation planning.

David Hondula et al., compared projections of heat-related deaths for Maricopa County, Arizona under three different urbanization scenarios; high growth, low growth, and adaptation-oriented growth, using actual Maricopa Association of Governments regional planning projections out to the year 2050. The scenarios considered a maximum and minimum expansion for both urban extent and density. The adaptation-oriented growth scenario incorporated highly reflective cool roofs as an adaptation strategy.  Heat impacts under the various growth scenarios were compared using mean, minimum and maximum temperatures forecasted with the Weather Research and Forecasting Model. Interestingly, researchers observed highest mortality associated with the minimum temperature metric, while maximum temperature, the most widely used metric, was associated with lower mortality. Hondula et al., ascribed this counter-intuitive result to an “oasis” effect common to arid environments: arid soil and the absence of vegetation cause daytime desert temperatures to rise faster than those of adjacent urban centers, effectively cooling them precisely when mortality and morbidity would otherwise be greatest (Georgescu et al., 2011). In the case of Phoenix using maximum temperature assigned at the county scale to predict health impacts that may be dominated by the urban core might introduce exposure misclassification.

With two billion people living in arid zones, many of them already seeing the effects of climate change, these finding are critically important. Examining the influence of regional planning decisions on heat-island impacts is relevant for decision makers to consider since strategies for adaptation are geographically dependent and may need correct local data to support policy formation.

Hondula DM, Georgescu M, Balling RC. Challenges associated with projecting urbanization-induced heat-related mortality. Science of the Total Environment, 2014. 490:538-544. doi: 10.1016/j.scitotenv.2014.04.130

Georgescu M, Moustaoui M, Mahalov A, Dudhia J. An alternative explanation of the semiarid urban area oasis effect. Journal of Geophyics Research 2011;116:D24113.