A bit of reflection may curb warming
WASHINGTON – Could climate change be staved off by making the United States look like the set of “Mamma Mia!”?
That was suggested in a recent talk by Energy Secretary Steven Chu – although, because he was speaking to Nobel laureates, he did not mention the ABBA musical set in the Greek Islands. He said that global warming could be slowed by a low-tech idea that has nothing to do with coal plants or solar panels: white roofs.
Making roofs white “changes the reflectivity … of the Earth, so the sunlight comes in, it’s reflected back into space,” Chu said. “This is something very simple that we can do immediately,” he said later.
Chu has brought increased attention to an idea that – depending on your perspective – is either fairly new or as old as Mediterranean villages, desert robes and Colonel Sanders’ summer suit. Climate scientists say that the reflective properties of the color white, if applied on enough of the world’s rooftops, might actually be a brake on global warming.
But if anybody is seriously considering a global whitewash, “simple” and “immediate” are probably not words that come to mind.
“I don’t think that it could ever be done at a sufficient scale,” said Ken Caldeira, a climate scientist at the Carnegie Institution in Stanford, Calif. He added: “It’s hard enough, in many of the cities of the world, to keep the streets swept, much less to keep the city reflective.”
White roofs work because of the physics of sunlight. Dark roofs absorb and hold more than 80 percent of solar energy, while white ones can reflect 75 percent of it away. That makes a white-roofed building cooler and cheaper to air-condition.
Because of that energy savings, California has since 2005 required most flat-roofed buildings to have white tops, and Wal-Mart has installed them on about 75 percent of its U.S. stores. In January, Washington, D.C., will require new flat roofs on commercial buildings to be covered in vegetation or a reflective material.
Now scientists are wondering whether white roofs might keep the world cooler, too.
The idea does not treat the root cause of climate change, which is heat-trapping pollution such as carbon dioxide and methane. But white roofs do help with the primary symptom: heat. The light they reflect escapes through the polluted atmosphere like a BB through a greenhouse.
“We may have to figure out a way to artificially cool the planet while the atmosphere is still super-saturated with greenhouse gases,” said Mike Tidwell of the Chesapeake Climate Action Network. This could be it, he said, “because the planet, it’s a closed system, it’s an absolutely closed system, except for one thing: sunlight.”
How well it would do, scientists say, depends on the number of roofs.
In his talk, Chu cited new research from his former laboratory, the Lawrence Berkeley National Laboratory in California, which imagined the result of painting white about 63 percent of the roofs in 100 large cities in tropical and temperate areas worldwide.
It estimated that would provide about the same climate benefits as taking all the world’s cars off the road for 10 years.
Hashem Akbari, a Lawrence Berkeley scientist who co-wrote the study, said: “It buys us precious time” to figure out ways to limit greenhouse-gas emissions or remove the gases from the atmosphere. “It basically buys us time until we come up to our senses.”
With that kind of potential, Chu told his London audience, “I would love to appeal to all people. We should convert to white limousines” – here the laureates laughed – “and white roofs.”
But, as with any proposal to paint large portions of the world the same color, there are skeptics.
Making roofs white “is one of many things that we need to do simultaneously” to combat climate change, said Daniel Lashof of the Natural Resources Defense Council. But, he added, the amount of space that might get painted is “just not enough area to significantly affect the reflectivity of the Earth.”
A spokeswoman for Chu said the Energy Department is exploring ways to encourage more white roofs on private and public buildings. (For now, Google Maps shows that Chu’s own headquarters is a light beige on top.) She also noted that some homeowners who purchased a “cool” roof would be eligible for an expanded tax credit intended for “weatherizing” homes.
There is also the winter problem: In a cold climate, a dark roof can lower heating costs by soaking up the winter sun. White-roof advocates counter that, in the continental United States, the “winter penalty” is just 10 percent of the overall savings.
“As far north as Toronto, it pays,” said Arthur Rosenfeld, a member of the California Energy Commission.