Crickets chirp and bees buzz from sedum flower to flower atop the post office in midtown Manhattan during a visit to the 9th Avenue facility on a perfect New York City fall day. On a sprawling roof that covers most of a city block a kind of park has been laid, sucking up carbon dioxide and other air pollution, filtering rainfall, making it less acidic.
Such verdant roofs may form part of an effective strategy for both cooling buildings and helping combat climate change, according to new research published in Proceedings of the National Academy of Sciences (PNAS) on February 11. Other solutions cited in the study include white roofs that reflect more sunlight back to space or hybrid roofs that combine aspects of white and green, or planted, roofs.
A large enough number of such roofs could "completely offset warming due to urban expansion and even offset a percentage of future greenhouse warming over large regional scales," says sustainability scientist Matei Georgescu at Arizona State University, who lead the research. That conclusion contradicts previous findings by researchers from Stanford University, who found that reflective roofs actually might increase global warming.
Roofs have quickly become a sticky subject, touted by some, such as former U.S. Secretary of Energy Steven Chu, as one solution to global warming. That's because cities are hot places. As buildings replace forest or grasslands, the local temperature rises—the so-called urban heat island effect. The amount of urban land in the U.S. is predicted to increase by as much as 261,000 square kilometers by 2100, a more than doubling from the current 250,000 square kilometers, according to models developed by the U.S. Environmental Protection Agency.
That translates to an area nearly the size of Colorado, containing at least one house for every hectare of land. By the end of this century, according to the new research, some "megapolitan" regions of the U.S. could see local average temperatures rise by as much as 3 degrees Celsius, in addition to whatever global warming may do.
For example, New York City has some 100 square kilometers of rooftop, most of it tar or other black roofing materials, absorbing heat and helping to make buildings—and the city as a whole—hotter in summer. Some city roofs have measured temperatures as high as 70 degrees C, thanks to their being black, with concomitantly high cooling costs, not to mention local temperatures raised by a degree or more Celsius.
As part of a project known as Cool Roofs, volunteers in New York City have been painting black roofs white and have so far covered more than 500,000 square meters of roof, though that's less than one percent of the possible area. The U.S. Department of Energy suggests such reflective rooftops can keep a given roof 30 degrees C cooler than surrounding traditional rooftops. Even better, according to new research from Lawrence Berkeley National Laboratory, white roofs are the cheapest roofing option based on a study of 22 commercial roofing projects. The lab’s research confirms the Arizona findings that white roofs reduce global warming, proving three times more effective at countering climate change than even green roofs, thanks to all that reflected sunlight.
But the new research published in PNAS suggests that such white roofs would have different impacts in different places. So, in New York City any energy savings on air-conditioning in the summer are counterbalanced by increased heating usage in winter (although this can be addressed with optimal roof design or roofs with adjustable reflectivity). And white roofs can reduce precipitation as well, by reducing the amount of warm, humid air rising and, thus, the number of clouds and eventual rainfall. "Adaptation to urban-induced climate change depends on specific geographic factors," Georgescu adds, noting that white, reflective cool roofs work well in California, but could reduce rainfall from Florida up the U.S. east coast, for example. "What works over one geographical area may not be optimal for another," he says.
Green roofs may be a better fit for New York City, for example, because they provide better insulation during winter, along with cooling benefits in the summer. Water evaporation from the plants lowers overall temperatures—and releases more humidity into the air. And, they offer ancillary benefits like green space for weary urban minds.
Here's how a green roof works: a layer of felt retains water for hardy plants like sedum or certain grasses, covered by about an inch of "soil"—really a man-made substrate composed of porous shales and clays. The key is to have a strong roof, strong enough to hold its weight in water. But although expensive, the $5-million planted roof offers benefits beyond combating climate change, compared with white roofs. In New York City the rain falls at a pH of around 4.2—very acidic—but after filtered through a green roof it is 6.2 or higher (the higher the pH, the less acidic). The evaporative cooling provided by the roof also reduces local temperatures by around 2 degrees C.
The PNAS findings do not jibe with some other research, however. In 2011 Stanford University researchers found that white roofs would provide some local cooling but at the expense of more global warming, largely because such cooling means less hot air rising and therefore fewer clouds forming. That lack of clouds would cause more warming downwind. The reflected sunlight also hits more of the soot and other pollutants commonly found in the atmosphere above cities, which also increases warming, according to the computer model simulation. The Stanford scientists suggested roofs covered in photovoltaic panels would do a better job, by producing electricity that then obviates the need for more fossil fuel–burning power plants.
Many scientists disagree with the Stanford analysis, including yet another team of Lawrence Berkeley researchers who found that white roofs on every building globally could offset the effect of some 44 billion metric tons of greenhouse gas emissions, or more than a year's worth of global climate changing pollution. And at least one Stanford climate modeler suggests that local cooling has never been shown to create global warming. "I do not deny that such a case is possible," climate modeler Ken Caldeira of Stanford wrote in 2011, when the original research suggesting white roofs could worsen climate change was published. "I am highly skeptical that it would be a common occurrence," added Caldeira, who had a white roof installed over part of his own home for cooling purposes.
Regardless, the space for reflective or even green roofs is limited. Urban areas cover less than 1 percent of the globe (although that number is likely to increase in coming decades), and less than half of that area is roof- or road-top, amenable to whitening. It also fails to capture the complexity of an urban environment, such as how replacing trees with buildings affects the water table and wind speeds. "Urbanization affects not just surface albedo," says urban environment researcher Karen Seto of the Yale School of Forestry and Environmental Studies, who was not involved in any of the research. The new PNAS study "is an innovative first step, but limited in terms of what impacts they're looking at," she adds.
In the meantime, black roofs remain a human health risk. In the deadly Chicago heat wave of 1995 those living on the top floor of a building with a black roof were most likely to die, according to subsequent analysis. "Black roofs should be outlawed," geochemist Wade McGillis of the Lamont–Doherty Earth Observatory at Columbia University told me during my visit to the post office green roof. "If you're going to put up a roof, don't put up black."