The idea that white roofs can both reduce the average temperature of cities and reduce the amount of energy required for cooling buildings received both scientific verification and practical application yesterday.
A report from Lawrence Berkeley National Labs said that if the world’s 100 largest and hottest cities converted to white roofs and lighter colored pavement, they would achieve a one-time offset of 57 gigatons of carbon emissions. That’s equivalent to taking all the autos of the world off the road for 11 years, according to Energy Secretary Chu.
The report emerged from the national lab that Chu used to run, and simultaneous with its release, the Department of Energy announced a plan to install “cool roof” technologies on its facilities and buildings. As new buildings are built or as roofs need to be replaced on DOE buildings, cool roofs will be installed. Chu also issued a letter to the heads of other federal agencies encouraging them to follow suit.
“Cool roofs are one of the quickest and lowest cost ways we can reduce our global carbon emissions and begin the hard work of slowing climate change,” Chu said in a statement about the initiative. “By demonstrating the benefits of cool roofs on our facilities, the federal government can lead the nation toward more sustainable building practices, while reducing the federal carbon footprint and saving money for taxpayers.”
Previous studies had shown the method to be effective as well, but the model used in the most recent report is thought to be more accurate; it’s based on a general circulation model that includes measurements for cloud cover in the 100 global cities studied.
The report is a follow-up to a 2008 study, also written by Arthur Rosenfeld, Hashem Akbari and Surabi Menon, and it corroborates the importance of the "albedo effect" — a measure of how well swaths of territory on the surface of the Earth reflect sunlight and heat.
“Everybody in the climate change business is very aware of albedo cooling,” Rosenfeld told SolveClimate. “We rely on ice in the Arctic ocean, which is disappearing. We rely on glaciers in Iceland and Greenland and so on. So everyone concerned with global warming, and who has access to climate circulation models, will make a run and see how much you can change the albedo of a city by whitening its roofs. And remember we’re getting more urban roofs per year.”
While proving that whitening up roofs really would deliver cooling and emissions reductions is important, the study’s authors are quick to remind policy makers that this is only one small solution of the many necessary to combat climate challenge.
“Two years worth of emissions is huge, but compared to what we need to do, it’s just a dent in the problem,” said Akbari , the former head of the Berkeley Lab Heat Island Group and now Hydro-Quebec Industrial Research Professor at Concordia University in Montreal. “We’ve been dumping CO2 into the atmosphere for the last 200 years as if there’s no future.”
Moreover, the “two years’ worth” is based on changes to pavement as well, which aren’t currently happening. If the roofs alone were to change, according to Rosenfeld, the study predicts an offset of about 31 Gigatons of carbon. Nonetheless, from an adaptation standpoint, white roofs could be fairly critical.
“What this does is offset the urban heat island from most cities,” Rosenfeld said. “Now that’s a big effect. There’s another paper from NCAR in Boulder that uses our input—which is pretty optimistic and assumes everyone complies—and gets something like 5 to 7 degrees of cooling for the city involved. As global warming sets in and cities become larger, every city is an urban summer heat island, and these heat waves are beginning to be a problem. Some would say this is worth doing for just that part.”
Cool roofs also have the benefit of being cheap and easy to implement; people can simply make the switch when it’s time for a new roof, or opt for white instead of black when they’re building. There is no cost difference. “Instead of having to convince 3 billion or so urban dwellers to change their individual spaces, you have to convince only the policy makers,” Rosenfeld said. “In the U.S. most states have standards for new buildings.”
In California, for example, Title 24 has stipulated since 2005 that all commercial buildings with flat roofs must have white roofs. The state also implemented a building code in 2008 that required residents of its four hottest zones to have “cool roofs,” which are not necessarily white, but painted colors that are less heat-trapping than black (red, blue, green). The decision was made to give residents more color options, according to Rosenfeld, because many US homes have sloping roofs and the color white is not always aesthetically pleasing on such homes.
In addition to white roofs on buildings and residences, Rosenfeld hopes to see more vehicles go for white tops in the near future. On a recent trip to India he spoke with policy makers about painting the country’s blue-roofed trains that have no air conditioning white on top. “I think I convinced them,” he said.
Rosenfeld said California is getting ready to mandate cooler colors for government fleet vehicles as well, to reduce the air conditioning load on vehicles. “Really all vehicles should have a white top and then whatever color you want underneath,” he said.
As unlikely as it might be to get all Americans to agree to that color scheme, it’s even tougher to get people to embrace the idea of “cool” pavement, according to Rosenfeld. Not because the public has some sort of affinity for black asphalt, but because they just don’t care one way or the other.
“It’s the tragedy of the commons,” Rosenfeld said. “Cool roofs are easy because you can sell people on the idea that they’ll be more comfortable in their buildings or homes and their electricity bills will be lower, but they don’t see any direct benefit from changes to the pavement so that’s going to take a longer time.”