Extreme heat is the deadliest natural hazard in the United States.
Between 1992 and 2021, it killed an average of 148 people every year. In the same 30-year period, floods killed about 88 annually while hurricanes killed 45 yearly.
This figure is likely an underestimate, as potential problems have been found with the way deaths due to extreme heat are counted. For instance, National Oceanic and Atmospheric Administration (NOAA) originally counted 200 deaths due to a heat wave that struck the Pacific Northwest in June 2021, but has since amended its description of the event to note the potential for hundreds of excess deaths during the period.
The consequences of extreme heat have an outsized impact on vulnerable groups. Immigrant workers have a tripled risk of heat-related death compared to U.S. citizens, with over 85 percent of non-citizen deaths occurring among Hispanics in one study by the Centers for Disease Control and Prevention (CDC). The New York City Department of Health and Mental Hygiene found that Black residents died at twice the rate from extreme heat as did white residents.
Cities can combat extreme heat and cool the climate by implementing more reflective pavements, surfaces high in albedo, a measure of reflectance that ranges from 0 (reflects no sunlight) to 1 (reflects all sunlight): Think of the deepest black and the brightest white as comparable to each end of the spectrum.
Surfaces with high albedo stay cooler in the sun. As a practical example, imagine you are at the beach on a hot day and are walking on the light-colored sand barefoot. Would you dare to step onto the asphalt parking lot without sandals? If not, you intuitively understand that the higher-albedo sand feels cooler than the lower-albedo asphalt.
A cooler, high-albedo surface warms the air less than a hotter low-albedo surface, helping reduce local air temperatures. The MIT Concrete Sustainability Hub found that if Houston increased the albedo of its pavement by 0.20, maximum summer air temperatures would decrease by 2.7 degrees Fahrenheit (1.5 degrees Celsius). While this may not sound impressive, climate simulation studies suggest that the lowered air temperatures would result in 59 percent fewer heat waves affecting the city.
A study conducted by the MIT Joint Program on he Science and Policy of Global Change found that increasing the average albedo of roofs, building walls, and hardscapes (including pavements) by 0.20 across the United States would result in a 22 to 50 percent decrease in the number of days exceeding heat index thresholds for “extreme caution” and “danger.”
Climate change may be viewed as a “symptom” of Earth’s energy imbalance in which the planet absorbs more radiation than it emits to outer space. This process of positive “radiative forcing” is raising surface temperatures and melting highly reflective sea ice at the poles. This, in turn, creates a cycle of worsening energy imbalance.
Raising pavement albedo is one way that cities can begin to break this cycle and reduce energy accumulated on the surface by reflecting more of it back to outer space. If the United States increased its average albedo of all pavements by 0.20, the country could attain a global warming impact savings equivalent to taking about 3.75 million cars off the road for one year due to the lowered atmospheric temperature. This one-time benefit would remain as long as the increased average albedo is maintained.
Implementing reflective pavements is one of many tools available to adapt to climate change and extreme heat, but one that could be especially helpful in cities because roughly 30 to 45 percent of the U.S. urban area is paved and 94 percent of U.S. roads are surfaced with asphalt.
Cities must consider the many options at their disposal to boost pavement reflectance. For instance, cities could add reflective coatings to asphalt streets to increase their albedo to 0.48 from 0.11, or install concrete pavements, which naturally have an albedo of 0.30 and do not require frequent maintenance to maintain high reflectance. Alternatively, they could pursue one of many new approaches to increase pavement reflectance.
There are limits to how high of albedo we should strive for — surfaces with values above 0.50 may cause thermal discomfort for pedestrians due to the amount of sunlight reflected. Careful attention must be paid to how local climate and the layout of neighborhoods influence the efficacy of reflective pavements. Materials research is also needed to reduce the energy and carbon footprints of reflective pavement technologies.
While there is an upfront cost involved in implementing high-albedo pavements, it is worth it in the long run to reduce the frequency of deadly heat waves and mitigate the impact of global warming.
Extreme heat kills, and it kills inequitably. Inaction threatens the lives of city residents, and this danger falls disproportionately across the city.
To protect vulnerable communities, federal and local officials must cool cities with high-reflectance pavements before the opportunity to limit the impacts of global warming vanishes.
Hessam Azarijafari, Ph.D., is the incoming deputy director of the MIT Concrete Sustainability Hub (CSHub). He studies the economic and environmental implications of infrastructure systems.
Ronnen Levinson, Ph.D., is a staff scientist and leader of the Heat Island Group at Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California. He develops cool roof, wall and pavement materials and quantifies the energy and environmental benefits of cool surfaces.
Andrew Laurent is the communications administrator of the MIT Concrete Sustainability Hub (CSHub).
Research from the MIT CSHub is sponsored by the Portland Cement Association and the RMC Research and Education Foundation. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.