Having lighter, more reflective roads and walkways is often used as a strategy to reduce the urban heat island effect and cut energy costs, but research from the US’s Lawrence Berkeley National Laboratory has found they could lead to increased carbon emissions.

Berkeley researchers created a lifecycle tool to assess energy, greenhouse gas emissions, air temperature and air quality consequences of urban pavement choice over a 50-year lifecycle, taking into consideration manufacture, installation, use and disposal/recycling.

They looked at traditional asphalt concrete, and compared it to lighter cement concrete as well as reflective coatings and the use of industrial waste products like slag and fly ash to offset cement emissions.

The results found that extra energy and emissions that went into cool pavement materials usually exceeded the energy and emissions savings resulting from reduced cooling needs of surrounding buildings.

“Over the life cycle of the pavement, the pavement material matters substantially more than the pavement reflectance,” Berkeley Lab’s Ronnen Levinson said.

“I was surprised to find that over 50 years, maintaining a reflective coating would require over six times as much energy as a slurry seal. The slurry seal is only rock and asphalt, which requires little energy to produce, while the reflective coating contains energy-intensive polymer.”

Researcher Haley Gilbert said the study provided an important perspective on the trade-offs of cool pavements.

“I cannot go to a city and say, ‘Cool pavements are good,’ without letting them know that there could be negative environmental consequences from deployment,” she said.

However, the research did not include other important benefits of reducing the urban heat island effect, namely health and comfort effects, which will become increasingly important in a warming world.

Dr Levinson also said there was a global cooling effect that wasn’t factored in, as making roads more reflective sent more sunlight back to space and helped to offset some of the atmospheric warming caused by greenhouse gas emissions.

“The one-time global cooling benefit of cool pavements is substantially larger than the 50-year life cycle carbon penalty or savings,” he said.

He said the report showed that technical innovation in climate-friendly cool road materials was needed to reduce lifecycle carbon impacts.

“The development of lower energy, lower carbon cool pavement solutions is still necessary.”

The research is published in Energy and Buildings.

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