|Fig. 1: Typical European city street, Valencia, Spain. (Source: (Wikimedia Commons).|
Climate change is a pressing issue that impacts various ecosystems across the globe and manifests itself in many ways: decreasing snowpack, shrinking glaciers, declining summer flows, rising sea level and temperature, more intense heat waves, stronger hurricanes, and continued temperature increase to name a few. Climate change has been induced in large part due to the greenhouse effect heat being trapped in the atmosphere by greenhouse gases including water vapor (H2O), nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). Human activity i.e. growing consumption of goods and services is a leading factor in the increase of greenhouse gases.  Increased urbanization is another contributor to climate change and rising temperatures. The phenomenon of cities and urban environments being warmer than the surrounding landscape is known as an urban heat island. City planners and environmental scientists have been analyzing methods for mitigating the warming effect of metropolitan areas.
Urban heat islands are not only a problem due to the fact that they are contributing to the warming of the Earth, but also because they are linked to higher cooling energy and power consumption.  In addition to adverse environmental effects, urban heat islands also negatively impact physical health.  The main cause of urban heat islands is the reduction of green vegetation.  Urban areas made up mainly of buildings, asphalt, bare-soil and short grasses that do not have the same cooling advantages as vegetation.  Three considerations to help mitigate the effects of urban heat islands are urban geometry, surface color, and vegetation. 
Urban geometry consists of city planning factors such as aspect ratio, building density, and street orientation. Studies found that aspect ratios and building densities can be manipulated to provide optimal shelter from wind, dispersion of air pollutants, warmth, and solar access.  European cities with high building densities and deep canyons foster more optimal environments than North American cities whose layouts are far more spread out (see Fig. 1).  Deeper canyons are also linked to cooler temperatures due to increased shade.  Urban geometry is rather difficult to implement however, because most cities are already established and implementing these types of changes would be impossible.
Surface color is another area that city planners can take into account to help combat urban heat islands. Increasing the albedo a measure of reflectiveness of buildings and large areas in cities can be achieved by whitewashing surfaces which in turn reduces the absorption of shortwave radiation.  Increases in albedo have been found to reduce annual cooling energy by 19 percent and peak cooling demand by 14 percent.  Implementing changes in surface colors is a relatively easy process and is ripe for large-scale implementation.
Urbanization and the ensuing decrease in vegetation densities in cities have greatly contributed to rising temperatures. Vegetative areas provide shade to heat-absorbing surfaces and therefore can greatly reduce local air temperatures.  Studies found that appropriately dispersed smaller green areas were more effective at cooling surroundings than concentrated larger green areas.  Simulations also found that shade reduced annual cooling energy use by 16 percent.  In using vegetation as a provider of shade to buildings it is important to consider the location of the trees. Shading during peak electricity usage hours will give the most break to the air-conditioning load and therefore largest decrease in energy use.  Although vegetation and green areas are easily implemented, they are currently losing potential space to projects like parking lots. Cities need to prioritize the importance of implementing various techniques to decrease urban heat islands otherwise the effects will continue to worsen.
© Quinn Brodey. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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