A can of white paint should be part of the planet's arsenal against global warming, say California researchers, who have calculated that installing white roofs in the world's cities could offset 1.5 years of man-made carbon emissions.
Light-colored roofs cool the planet in two ways. First, they reflect a certain fraction of radiation back into space, which means that the earth receives less of the sun's energy. Second, a cooler house requires less air conditioning--and hence results in fewer greenhouse gas emissions from power plants.
The new study focused on the first effect. To analyze the impact of a change in the reflectivity of city roofs, scientists with Lawrence Berkeley National Laboratory (LBNL) and the state of California estimated the global number of roofs and asphalt surfaces in cities. They made a conservative estimate that those surfaces make-up 1% of the Earth's surface. Modifying the roofs with light-colored tiles, the team found that using light-colored concrete, or applying white glazes to buildings, could increase the reflectivity of urban surfaces by 10%. That adjustment would negate the warming effects of 44 gigatons of carbon dioxide, the researchers report in an upcoming issue of Climatic Change. (The world currently emits roughly 28 gigatons per year from fossil fuels.) Halting all deforestation of tropical forests could eliminate about seven gigatons of emissions, by comparison.
"We were really surprised. We didn't expect such a big [cooling] effect," says LBNL climate scientist Surabi Menon.
Assuming a global system that would put a price on carbon emissions, the scientists then calculated the value of carbon credits awarded to homeowners and businesses for making their roofs and streets lighter. At $25 per ton, the authors found the market for light-colored roofs would be worth more than $1 billion. It will be up to governments whether to offer such an incentive, says Menon, whose study didn't calculate how much it would cost to make the surfaces more reflective through the various means.
Climate modeler Ken Caldeira of the Carnegie Institution of Washington Stanford, California, calls the paper a "decent set of calculations." But he thinks that tying white roofs to carbon credits would be a mistake. That's because those who buy carbon credits often use them to produce more carbon dioxide (CO2). "The CO2 will stay in the atmosphere for 1000 years. There's no way that white roof will be around that long," he warns.