Could ‘cool pavements’ help in the battle against climate change?

Pavements account for up to 40% of the urban surfaces in cities in America and Europe – and can lead to cities warming up.

Pavement surfaces can absorb solar radiation and warm the area around them by re-emitting the radiation as heat.

Researchers at MIT’s Concrete Sustainability Hub in the US are investigating “cool pavements”, which reflect more solar radiation and emit less heat than conventional paving surfaces.

In Europe, the EU’s UrbanGreenUP project is also investigating them.

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The MIT study found that cool pavements could reduce air temperatures in Boston by 3C, and reduce greenhouse gas emissions by 3%.

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It’s well known that darker surfaces get hotter in sunlight than lighter ones. Climate scientists use “albedo” to help describe this phenomenon.

The paper’s lead author, postdoctorate student Hessam AzariJafari, said: “Albedo is a measure of surface reflectivity. Surfaces with low albedo absorb more light and tend to be darker, while high-albedo surfaces are brighter and reflect more light.”

Normal paving surfaces, like tarmac, possess a low albedo and absorb more radiation and emit more heat.

Cool pavements use brighter materials that reflect more than three times as much radiation and, consequently, re-emit far less heat.

Randolph Kirchain, a researcher in the Materials Science Laboratory and co-director of the Concrete Sustainability Hub, said: “We can build cool pavements in many different ways.

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“Brighter materials like concrete and lighter-coloured aggregates offer higher albedo, while existing asphalt pavements can be made ‘cool’ through reflective coatings.”

Cool pavements exert direct and indirect impacts on climate change.

AzariJafari said: “The one direct impact is radiative forcing. By reflecting radiation back into the atmosphere, cool pavements exert a radiative forcing, meaning that they change the Earth’s energy balance by sending more energy out of the atmosphere – similar to the polar ice caps.”

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The researchers found that cool pavements also changed the way that people nearby used energy – sometimes in complex ways.

But overall, areas with cool pavements saw a drop in emissions, the researchers said.

AzariJafari said: “On the one hand, by lowering temperatures, cool pavements can reduce some need for air conditioning [AC] in the summer while increasing heating demand in the winter.

“Conversely, by reflecting light – called incident radiation – on to nearby buildings, cool pavements can warm structures up, which can increase AC usage in the summer and lower heating demand in the winter.”

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