“Cool Roofs Might Be Enough to Save Cities from Climate Overheating” from Scientific
American By David Biello (2/14/2014)
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 inProceedings 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 landin 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
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.
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.
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