April 12, 2012

Lighter roofs and roads would offset 130 to 150 billion tons of CO2

The combined size of global urban areas is around 2 million square kilometers.
-- Over 50% of the world's population currently lives in urban areas. This is expected to increase to 70% by 2040.
-- Pavements and roofs comprise over 60% of urban surfaces (25% roof and 35% pavement).

"Typically roofs are resurfaced (or changed) about every 20-30 years; paved surfaces are resurfaced about every ten years. When roofs or paved surfaces are installed, they can be changed to materials with high solar reflectance, typically at no incremental cost," the researchers write.

Lead author Professor Hashem Akbari said: "It is all based on planning, codes and policies. If we really put the nuts and bolts in place, we can get close to 100 per cent of urban areas increasing the albedo of surfaces."

Increasing the reflectance – commonly known as albedo – of every urban area by 0.1 will give a CO2 offset between 130 and 150 billion tonnes. This is equivalent to taking every car in the world off the road for 50 years, assuming a single car gives off around 4 tonnes of carbon dioxide a year.

Solar reflective urban surfaces (white rooftops and light-colored pavements) can increase the albedo of an urban area by about 0.1. Increasing the albedo of urban and human settlement areas can in turn decrease atmospheric temperature and could potentially offset some of the anticipated temperature increase caused by global warming. We have simulated the long-term (decadal to centennial) effect of increasing urban surface albedos using a spatially explicit global climate model of intermediate complexity. We first carried out two sets of simulations in which we increased the albedo of all land areas between ±20° and ±45° latitude respectively. The results of these simulations indicate a long-term global cooling effect of 3 × 10−15 K for each 1 m2 of a surface with an albedo increase of 0.01. This temperature reduction corresponds to an equivalent CO2 emission reduction of about 7 kg, based on recent estimates of the amount of global warming per unit CO2 emission. In a series of additional simulations, we increased the albedo of urban locations only, on the basis of two independent estimates of the spatial extent of urban areas. In these simulations, global cooling ranged from 0.01 to 0.07 K, which corresponds to a CO2 equivalent emission reduction of 25–150 billion tonnes of CO2.

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