The solution to cooler cities hides under our feet

Lighter-colored and more reflective streets could go so far as to lower the temperature of cities by about 1.5°C. Researchers at MIT have identified how strategic construction of roads and sidewalks can have environmental benefits.

Ahead of the arrival of heat waves, one solution to lowering the temperature of cities is literally "under the feet" of citizens: it's roads and sidewalks. Research by MIT has revealed that by building streets and pavements with a lighter, more reflective color, it could be possible to lower the temperature by about 1.5°C and reduce the frequency of heat waves by 41%. However, surfaces need to be strategically structured because otherwise the opposite effect of warming nearby buildings instead of cooling them could be achieved.

How to make cities cooler

A dark asphalt or sidewalk attracts heat by raising the temperature of the surrounding air. Researchers at MIT's Concrete Sustainability Hub tried modeling these surfaces to determine the right balance to reduce heat and help cities also decrease greenhouse gas emissions. The analysis assumes that all surfaces, depending on the amount of radiation they absorb or reflect, can affect the air temperature in the surrounding environment.

In urban areas, about 40 percent of the ground is paved, and that pavement absorbs solar radiation. The heat absorbed into the pavement mass is given up gradually, warming the atmosphere of the area. This worsens the effects of heat waves, making cities regularly a few degrees warmer than nearby rural areas and leafy suburbs. Using reflective materials on pavement could prevent heat buildup and help combat climate change. An example of such an effect is white roofs.

The MIT researchers explained that to estimate the reflectivity of a pavement, they used a measure called albedo, which refers to the proportion of light reflected from a surface. The lower the albedo of a surface, the more light it absorbs and, consequently, the more heat it traps. Usually those with a lower measure of albedo are darker surfaces. Pavements such as asphalt, for example, have a low albedo of about 0.05-0.1, meaning they reflect only 5% to 10% of the light they receive and absorb up to 95%.
Sidewalks that instead use brighter additives, reflective surface coatings, or lighter materials such as concrete can triple their albedo, sending more radiation back into the environment. An MIT CSHub model estimated that an increase in pavement albedo on all U.S. roads could reduce energy consumption for cooling and reduce greenhouse gas emissions equivalent to 4 million cars driven in a year.

An opposite effect, however, occurs if brighter sidewalks reflect radiation back onto buildings. In this case, so-called incident radiation can heat nearby structures in the summer. For example, in downtown Boston, with narrow streets and tall buildings that prevent light from hitting the sidewalk directly, reflective pavement could raise the temperature of homes. While in the super streets of the city and its suburbs would be able to reflect much of the sunlight going to cool the air by a few degrees.

Studies to make cities more livable are numerous. For example, some Australian researchers are hypothesizing the use of solar panels on the roofs of airports that could change the concept of energy.

Stefania Bernardini