GCCA gathered a team of top climate researchers, including Larry Kalkstein (University of Miami), Jennifer Vanos (Texas Tech University), David Sailor (Portland State University), and Scott Sheridan (Kent State University) to quantify the impact that typical urban heat island mitigation strategies, such as reflective roofs and vegetation, have on weather conditions and estimated mortality during extreme heat events. Many residents within American cities are vulnerable to health and even mortality risks caused by extreme heat events. In numerous cities, the health impacts of these heat events are exacerbated by the fact that the city is significantly warmer than the surrounding rural areas during the summer.

Research on urban environments has found that a number of strategies can reduce excess urban heat by generating perceptible ambient changes in temperature, thus making urban populations more resilient to extreme heat events. Examples of such strategies include green roofs, shade trees, and vegetation, as well as surfaces that reflect sunlight rather than absorb it as heat (e.g., cool roofs and cool pavements). Although these UHI mitigation measures often save energy and make economic sense for building owners, policymakers are increasingly drawn to UHI mitigation to gain the health benefits of cooler cities. The findings of this paper will help urban planners and city officials looking to further quantify the health and life-saving benefits of reducing summer urban heat islands with cool surfaces and increased vegetation.

The team estimated reductions in heat-related mortality in three cities: Baltimore, MD, Los Angeles, CA, and New York, NY. The team identified four actual multi-day extreme heat events in each city, modeled the impact of increased surface reflectance and increased vegetative cover on meteorological conditions using three scenarios:

1. Increase urban surface reflectance by 0.10 (0.15 to 0.25);
2. Increase surface vegetation by 10% and reflectance by 0.10;
3. Increase surface reflectance by 0.20 (0.15 to 0.35).

Changes in air temperature and humidity during the heat events (as measured by dew point temperature) in all cities were small, commonly less than a 1^oF decrease. The study found that reflectivity and vegetated cover were equally effective urban cooling strategies. During the period between 1948 and 2011, Baltimore, Los Angeles, and New York lost an average of 87 people each summer (June-July-August) due to heat-related mortality (Kalkstein et al., 2011). This study finds that deploying UHI mitigation strategies would save up to 32 lives in Baltimore, 22 lives in Los Angeles, and 219 lives in New York over a 10-year period.

Read the full study on our Cool Roofs and Cool Pavements Toolkit.