New York’s White Roofs Prove They’re Cool

A satellite photograph of New York City reveals a dark blot fronted to the north, west, and east by a sea of light green forest, and to the south by an actual sea: the pastel blue of the Atlantic Ocean. Into this blot, on hot summer days, soaks enough solar radiation to turn its denizens into sweaty, irritable, iced-latte swilling malcontents prone to cranking the air-conditioning full blast 24/7 while daydreaming about a weekend upstate.

Those who live in fear of sweltering July subway rides days may soon have a respite, from an unlikely source. A just published study by researchers at NASA and Columbia University has concluded that painting the surfaces of the city’s roofs white or a light color could potentially reduce its ambient temperature by 5 or more percent during hot summer months.

Miller-McCune has in the past reported on the curious phenomenon scientists refer to as the “urban heat island effect,” in which cities — dark jungles of asphalt, metal, and concrete — turn into heat reservoirs, soaking up the warmth of the sun. By failing to reflect solar radiation back into the atmosphere, they can end up more than 5 degrees warmer than surrounding areas.

Last summer, on the most sweltering day in New York — July 22, 2011 — the researchers discovered that a white-surfaced roof was 43 percent cooler than a typical black counterpart (which reached up to 170 degrees Fahrenheit). Not coincidentally, July 22 set a city record for electricity use, as miserable citizens twisted the dials of their air conditioners to “high.”

In 2007, NYC Mayor Michael Bloomberg signed into a law a program to reduce the city’s greenhouse emissions by 30 percent by 2030. According to the authors of the study, increasing the city’s “albedo” — the degree to which it reflects solar radiation — by brightening its surfaces is one of the quickest, cheapest, and most effective ways to achieve significant reductions.

The study compared the benefits of two methods of increasing reflectivity. Professionally installed white membrane coverings, which cost about $15 to $28 per square foot, were found to be more durable, but for 50 cents a square foot, the job could be done with white acrylic paint, with repainting expected every two years. This second, DIY method is being promoted by the city’s CoolRoofs program as a highly cost-effective way to cool the city and reduce its greenhouse gas emissions by lowering energy demand during hot summer months.

The cooling effect created by a sea of white-topped buildings will be especially beneficial as climate change intensifies, the authors of the study wrote. “Right now, we average about 14 days each summer above 90 degrees in New York. In a couple decades, we could be experiencing 30 days or more,” said co-author Cynthia Rosenzweig, a NASA scientist.

Worldwide, interest in the benefits of increasing urban albedo is growing among city planners. Scientists believe that if enough cities around the world embrace measures to brighten their cities — green roofs, anyone? — it could substantially off-set global carbon emissions. Researchers at Lawrence Livermore National Laboratory, in the San Francisco Bay area, have estimated that painting urban surfaces in warm parts of the world white or a light color could offset the carbon emissions of all 600 million of the world’s cars for 18 to 20 years — at a savings equivalent to at least $1 trillion worth of CO2 reductions.

After announcing a plan to install cool roofs on the Department of Energy and other federal buildings in the summer of 2010, Energy Secretary Steven Chu told said, “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.”