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World temperatures are rising at an unprecedented rate.

According to the Intergovernmental Panel on Climate Change, the Earth’s average temperature is on track to increase by between 2 and 7 degrees Celsius (4 to 13 degrees Fahrenheit) this century. This dramatic change in temperature will produce a climate never before experienced by human civilization. Cities are often significantly warmer than the surrounding landscapes because urban surfaces absorb more sunlight than natural landscapes, cities lack vegetation, which cools landscapes by evaporating water, and urban areas release more heat from human activity including air conditioning, vehicles, and industry. The difference between outside air temperatures in a city and its surrounding rural areas can be 5 to 9 degrees Celsius (9 to 16 degrees Fahrenheit) or more in summer months.1 This phenomenon is called the summer “urban heat island effect.” Addressing this heating effect will only become more important because the world is rapidly urbanizing—within 50 years an estimated 80 percent of the world’s population will live in an urban area.

Higher temperatures adversely affect our health, our energy consumption, and our environment.

Rapidly increasing temperatures stress ecosystems, increase the frequency and duration of heat waves and exacerbate air pollution. Together, these factors are creating serious health risks to people around the world. In addition, increasing wealth in the developing world is spurring the rapid deployment of air conditioners that are taxing electrical grids with their energy demands.

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Studies of a city’s “urban fabric” indicate that about 60 percent of urban surfaces are covered by roofs or pavements. About 20 to 25 percent are roofs and 30 to 45 percent are pavements.3 Because these surfaces are dark and typically absorb over 80 percent of sunlight that contacts them and convert that solar energy into heat, our built environment exacerbates the warming effects of climate change. Replacing and upgrading roofs and pavements with more reflective materials could reverse this warming, turning urban surfaces into assets instead of burdens. Vegetated roofs, permeable pavements, and shade trees are other cooling strategies that are complementary with cool roofs. Cool roofs paired with appropriate levels of roof insulation help keep buildings more thermally comfortable. Cool, reflective roofs and pavements should be a priority strategy because they are cost-effective, typically pay back within one year, and help cities both mitigate and adapt to climate change while making them more desirable and comfortable places to live.

A practical guide to cool roofs and cool pavements

13 May 2019