Solutions to extreme heat can be found in our streets

Cool pavements transform a surface that would otherwise intensify heat into one that addresses both the causes and consequences of climate change.

This Arizona State University thermal image photo shows a Phoenix street coated half with new, lighter-colored pavement to the left of the crosshairs, showing as much cooler in yellow than the original blacktop at right in red, on Sept. 10, 2020. A team from Arizona State University is working with the city of Phoenix on a pilot program studying the use of "cool pavement" to reduce the heat island effect, a phenomenon that raises temperatures in urban areas covered by asphalt and concrete.Peter Crank/Associated Press

On June 29, air temperatures in Boston reached a scorching 100 degrees — 20 degrees above the average temperature for that day. That’s quite an anomaly. Yet such high temperatures are becoming less and less anomalous. Over the next decade, we can expect the days above 90 degrees in Boston to become nine times more frequent than they were in 1990. Much of this heat increase can be attributed to climate change, but it’s also the result of the urban heat island effect, which can warm cities by as much as 7 degrees.

In light of this threat, Boston needs to do more to address urban heat. Currently, the city has taken two main approaches: planting more vegetation and implementing public cooling centers with abundant air conditioning.

Both are imperfect solutions. Air conditioning is carbon-intensive and subject to grid failures (which often occur during extreme heat events) while the planting of additional vegetation is strongly limited by the extent of manmade surfaces.

So as Boston undertakes its second heat resilience study, it should consider an additional strategy that cities like Tokyo and Los Angeles have already employed: cool pavements.

Pavements make up around 40 percent of urban space in the United States but tend to absorb 90 percent of the solar radiation they receive. This radiation is then reemitted as heat, resulting in more intense heat islands and higher air temperatures across the region.

Cool pavements have the opposite effect. Since they are roughly three times more reflective than conventional pavements, they can redirect a greater proportion of the radiation they receive. This means that they not only cool their surroundings but that they also send radiation back into the atmosphere, directly offsetting global warming.

Essentially, cool pavements transform a surface that would otherwise intensify heat into one that addresses both the causes and consequences of climate change.

Cool pavements come in many varieties. In some instances, a brighter coating can be painted on top of an existing pavement to increase its reflectivity. It’s also possible to use naturally reflective materials such as concrete or lighter colored aggregates or binders. Regardless of which method is used, it’s cheapest to implement cool pavements during a new construction project rather than through retrofits.

A City of Phoenix Street Transportation Department crew sprays light-colored pavement over blacktop on a Phoenix street on Aug. 11, 2020. A team from Arizona State University is working with the city of Phoenix on a pilot program studying the use of "cool pavement" to reduce the heat island effect, a phenomenon that raises temperatures in urban areas covered by asphalt and concrete.Associated Press

Research at the MIT Concrete Sustainability Hub has shown that if cool pavements were applied around the country, they could reduce the frequency of heatwaves by 41 percent across all urban areas. Cool pavements have another benefit: They would also change the earth’s energy balance by trapping less energy in the atmosphere and sending more energy out of the atmosphere. This would counteract the effects of greenhouse gases, offsetting enough CO2 to remove the equivalent of about four million cars from the road each year.

But cool pavements are so potent that they need to be implemented with care. While they will always lower air temperatures, they can sometimes reflect energy into buildings instead of the atmosphere. This can warm structures, increasing emissions and energy consumption from AC in the summer.

So, with all this in mind, would cool pavements benefit Boston? According to our research, the answer is yes.

An analysis we conducted found that every neighborhood in Boston would emit less greenhouse gases with cool pavements. Some areas stood out. Chinatown and the Leather District — neighborhoods already highly vulnerable to heat — saw some of the greatest decreases in energy consumption from cool pavements due to the plethora of unobstructed roadways that could reflect light and keep air cool. Meanwhile, the Financial District saw significant benefits as well, with lower air temperatures reducing emissions from AC usage.

Overall, if Boston were to properly implement cool pavements, it would reduce its CO2 emissions by 1.5 million tons over 50 years — between 1 and 3 percent of the reductions needed to meet its 2050 carbon neutrality pledge. These reductions would come from not just reflectivity, but also from better road quality over time. That’s because cool paving options, like concrete, can improve the surface quality and stiffness of roads, leading to savings in vehicle fuel consumption.

These greenhouse gas savings would match those of even the most ambitious of tree planting initiatives. Since one tree absorbs about one ton of CO2 over its life, the CO2 reductions from cool pavements in Boston would equal the carbon uptake of 1.5 million trees — making cool pavements an excellent alternative to vegetation in already built-up areas.

It’s important to remember too that these benefits are entirely separate from the lower air temperatures and reduced health risks the city would experience: We found that a switch to “cooler’ paving materials would also decrease peak summer air temperatures by up to around three degrees.

Massachusetts has some of the worst roads in the nation, and with infrastructure spending hot on Capitol Hill, there’s an opportunity to steer the Commonwealth’s infrastructure in a more innovative, sustainable direction.

With an investment in cool pavements, we could deliver energy savings and lower air temperatures to the state’s most vulnerable neighborhoods. And as the city of Boston builds bus and bike lanes and redesigns busy corridors, cool pavements could also ensure that these projects address both transportation and climate equity.

Urban heat is an inevitable problem. Yet it is not intractable. There is an array of solutions that can help moderate rising air temperatures in Greater Boston. So far, the city has narrowly focused on tree planting and cooling centers. But if the city would reflect on how to pave, it’s possible to build a cooler (and brighter) future.

Randolph Kirchain is codirector of the MIT Concrete Sustainability Hub, which was established in collaboration with the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation. Hessam Azarijafari is a postdoctoral associate at the Hub.