Expert Commentary

Reflective surfaces in urban areas: Global cooling?

2009 study by the Lawrence Berkeley National Laboratory on the potential benefits of increasing the reflectivity of urban surfaces such as pavement and roofs.

On summer days, city air can be 5 or more degrees warmer than that in surrounding areas. Known as the “urban heat island effect,” it results in part from limited shade trees and the large surface area of pavement and dark roofing materials.

A 2009 study by Lawrence Berkeley National Laboratory researchers published in Climatic Change, “Global Cooling: Increasing World-Wide Urban Albedos to Offset CO2,” examines the potential benefits of increasing the reflectivity of urban surfaces such as pavement and roofs.

The term “albedo” refers to the ability of a surface to reflect solar radiation. For example, fresh snow reflects 80% to 90% of the sunlight that falls on it, while a dark surface such as asphalt reflects just 4% and absorbs the rest. Light energy that is absorbed is later released in the environment, warming it.

On average, pavement and roofs constitute over 60% of urban surfaces. Using reflective materials, the authors calculated that roof and pavement albedos can be increased by about 0.25 and 0.15, respectively; overall, this would increase urban albedos by approximately 0.1. The study’s authors calculated that:

  • Every 100 square feet of roof area turned from a dark color to white is equivalent to offsetting the emission of one ton of heat-trapping, atmospheric CO2.
  • The 20 tons of CO2 that the average American generates annually could be offset by 2,000 square feet of white roof.
  • Permanently retrofitting roofs and pavement in tropical and temperate regions of the world would offset 44 gigatons of CO2 emissions, the equivalent of a year and a half of global CO2 production.
  • At $25 per ton of CO2,44 gigatons CO2 are worth about $1.1 trillion.
  • While winter heating needs would be increased, this would be more than offset by savings in air-conditioning costs: Americans would save $2 billion annually overall.
  • There would be additional benefits such as improved urban air quality.

Keywords: global warming, greenhouse gases, carbon dioxide, consumer affairs