The last eight years have been the hottest on record. In early September, scorching temperatures reached all-time highs in much of California, with many cities clocking in at over 110 degrees Fahrenheit. Last year, the Pacific Northwest experienced its own unprecedented heat wave, with regions typically known for temperate climates enduring their own sweltering conditions (121 F in British Columbia, for example). These developments are concerning. Heat is not only the leading weather-related cause of death — more than 1,400 people died during the 2021 heat wave — but also one of the greatest climate injustices facing our nation: Residents of low-income areas and communities of color are more likely to die or become ill from heat exposure.
Climate scientists have known for decades that urban regions can be hotter than outlying areas because cities have large concentrations of impervious, dark-colored (or “low-albedo”) surfaces like buildings, parking lots, and roads. These surfaces absorb incoming sunlight and slowly radiate it back into the atmosphere throughout the late afternoon as heat, creating what are known as urban heat islands.
Climate scientists have also known for decades that replacing low-albedo materials with lighter-colored (high-albedo) materials will reflect sunlight rather than absorb it. Theoretically, then, an urban region that replaced enough low-albedo material with high-albedo surfaces could offset the urban heat island effect.
Asphalt streets, at least superficially, are a logical target for such intervention. Streets are an abundant type of low-albedo surface, accounting for 18% of urban land area in large counties nationwide. They are also, unlike many other low-albedo surfaces (like parking lots and roofs) publicly owned, so cities can move quickly to change them. Cities around the world have begun resurfacing streets with high-albedo “cool” pavements that immediately reduce surface temperatures by more than 18 degrees F in areas without shade. Los Angeles, for example, has been pursuing a cool pavement pilot program to resurface some of the 7,500 miles of streets in its road network in neighborhoods that typically see triple-digit summer temperatures. The idea is simple: As more cool pavement gets laid down, the city itself cools off.
The reality, however, is not so straightforward.
The urban heat island is, in one respect, a climate science success story. It is a highly technical concept that has entered mainstream planning lexicon, and guides climate action in major cities. But it can also be a misleading concept. It’s easy to think of the heat island as a linear relationship between low-albedo urban surfaces and warming (e.g., more dark pavement, more urban heat). But this only holds true if specific conditions are met, and reducing the urban heat island is not always the same as reducing urban heat.