Hot Times in the City: Part 2

Readers: This is the second installment of a two-part report on Urban Heat Islands. Part one focused on the problem of overheated cities. Part two now looks at the people and projects that are creating viable solutions to these growing problems, rather than waiting for federal legislation.

As we saw in part one, the problem of Urban Heat Islands (UHIs) is a chronic environmental crisis that could get much worse in coming decades.

This fall the mood was gloomy at the 17^th meeting of the United Nations’ international panel on climate change, known as COP-17, in Durban, South Africa. While climate science remains a political football in the U.S. congress, scientists and nearly 200 member countries at COP-17 discussed new data that the world could warm as much as 10.8F above pre-industrial levels by 2100, making UHIs over most cities climb well into the 100s+˚F. This follows last May’s U.N. panel findings that severe heat waves, which now occur every 20 years, could increase to every two years worldwide. “A hotter, moister atmosphere is an atmosphere primed to trigger disasters,” Michael Oppenheimer, the principal author of the report told the New York Times in late November. “As the world gets hotter, the risks get higher.”

So it’s no surprise that many ordinary people around the world are not waiting for politicians to solve the dangers of climate change, but are taking action themselves. Interestingly, two leading methods to cool down UHIs don’t involve complex technology, massive development costs or expensive materials. In fact, these simple, natural methods need little maintenance and, once installed, work for free.

The Great White Hope

White roofs reflect heat back into space and thus cool temperatures. Credit: Brian Gratwicke/Research.gov

Since the invention of air conditioning black tar shingles have been the U.S. roofing material of choice. Like street blacktop, these shingles absorb nearly 90 percent of the sun’s rays, heating up to 180˚F or higher during the hot summer months, and intensifying the effects of UHIs. Slate roofs aren’t much better, and radiate heat well into the night. As a result, homes and office buildings have to use more air conditioning to keep their upper floors cool. That’s like running your air conditioning and your furnace at the same time, wasting money and energy. Ironically, the simplest, cheapest and most obvious way to cool roofs and other city surfaces down was discovered thousands of years ago by Arabic peoples in the Middle East: paint them white.

Author and green-design guru Jerry Yudelson says light-colored adobe buildings in desert countries reflect solar energy and disperse absorbed heat quickly at night.

“If you look at the architecture and design of most hot places of the world,” he says, “its all about narrow streets, shading, vegetation, using water to help cool the microclimate.”

Environmental visionaries have touted white roofs since the 1980s, but the idea only began to catch on in the U.S. around 2000 as new findings about climate change renewed interest in keeping the planet cool. A 2001 report from The Department of Energy’s Oak Ridge National Laboratory found that white roofs reflected nearly half of the light and heat from the sun, and cut about 35 percent from home air conditioning bills. Since then scientists and pundits including former President Clinton, the Obama Administration’s Energy Secretary Steven Chu and former Vice President Al Gore have endorsed white roofs as an economical first step toward reducing UHIs and saving energy. But how much good would a white roof really do?

“One at a time makes no difference,” says Yudelson, “But if you look at the aggregate, it could have a significant impact over time on climate change.” In fact, a 2008 study from Lawrence Berkeley Laboratories stated that permanently installing reflective materials on urban roofs and sidewalks throughout the temperate and tropical areas of the Earth could (in theory) offset about 44 billion tons of CO2. Since the average car emits about 4 tons of CO2 annually, that would be the equivalent of taking Earth’s 600 million or so cars off the road for 18 years.

side-bar-summer-afternoon-experiment Groups, coalitions and non-profits have also sprung up advocating the benefits of high albedo (see sidebar) city surfaces, inspiring a growing number of homeowners and businesses across the U.S. to get white roofs and save lots of green during the summer months. But Yudelson says white or light-colored roofs are not for everyone everywhere. As many recent studies have confirmed, white roofs keep things cool all the time, summer and winter, which may be fine for buildings in high-sunshine states like Texas or Arizona, but can mean higher heating bills for northern cities such as New York. So, what kind of roof can keep heat out during the hot months and in when it’s cold? Once again, Mother Nature has an answer.

Green Acres

In the quiet Washington D.C. suburb of Chevy Chase, renovation is progressing on a fairly ordinary home on 36^th street. But amid the screech of band-saws and the pounding of nails, a trio of workmen are hoisting trays of colorful sedum plants onto the roof using a crude metal lift. The men connect the 12-inch square trays like interlocking jigsaw pieces on the house’s flat rear roof. Soon it resembles a shag carpet of green and orange hues, some 4 inches thick. This “green roof” is the next step in heat island mitigation, and the vanguard of a bold new vision for the urban landscape.

Unlike their white counterparts, green roofs do more than simply deflect heat, they do what living plants do best: absorb the sun’s rays, soak up CO2, and breathe out oxygen as a bi-product. That’s a good recipe for a rooftop because the greenery processes solar energy (heat and light) into nutrients through photosynthesis, preventing the roof from getting hot. In winter the blanket of soil and plants on a green roof keeps heat from escaping (see diagram Anatomy of a Green Roof) and protects the roof structure from harmful ultraviolet radiation. Bacteria in the plants’ roots also digest pollutants like benzene, formaldehyde and trichloroethylene out of the air, breaking toxic molecules down or locking them into the soil.

Green roofs perform another crucial function that white roofs can’t: control rainwater runoff. In recent years scientists have shown that superheated UHIs, coupled with higher levels of smog, can actually help create thunderstorms, or make existing ones worse. More frequent downpours can stress city sewer systems as rainwater gushes off impervious stone buildings and into storm drains, which usually lead to water treatment plants.

When too much water roars down the sewers, it can overwhelm these plants’ capacity to hold water, and untreated Wet Weather Overflow (WWO) water is then released into local rivers. WWO water is often a toxic soup of sewer waste and street chemicals containing disease-causing bacteria (e.coli and enterococci) and chemicals like phosphorus and nitrates. Rivers carry this witches’ brew of pollutants downstream into estuaries and the sea, spurring dangerous algal blooms (europhication) and the creation of underwater Dead Zones (hypoxia) that not only kill aquatic life like fish and crabs, but also costs local fishing industries millions of dollars in lost productivity.

One solution is to control rainwater before it hits the streets, and according to Peter Ensign, the Executive Director of the non-profit DC Greenworks, green roofs excel at doing just that. “A 10,000 square-foot green roof that gets about 40 inches of rain per year translates into about 15 gallons (of rain) absorbed per square foot or 150,000 gallons of water per year,” says Ensign. “That’s 150,000 gallons of water that is not … flowing down our streets carrying oil and gas into our rivers.”

side-bar-summer-afternoon-experiment2 Some 410 miles to the North in New York City, climatologist Stuart Gaffin has been studying the science of green roofs since 2003, measuring exactly how they work, and why they can help improve the environment. Gaffin, who works for Columbia University’s Center for Climate Systems Research, says the data from his studies gives scientific heft to green roofs, because they accurately measure the costs and benefits of different green roof strategies, giving policymakers hard numbers that can’t be ignored. To him it’s a bold new way to use one of the greatest underused resources found in cities today: roof top real estate. “This is the biggest greening opportunity in the New York’s history,” he explains. “We have a billion square feet or more of roof space, equivalent to 30 sq miles: That’s 22 times the size of Central Park!”

In addition to Gaffin’s 10 experimental projects, green roofs can also be found throughout the city’s five boroughs, on private apartment buildings, the top of the Museum of Modern Art, Queens’ Con Edison power plant in Long Island City and even Silver Cup television studios, where The Sopranos is filmed. Mayor Michael Bloomberg has embraced the idea in his 2010 PlaNYC initiative to install green roofs, porous sidewalks and other sustainability projects. The 20-year plan aims to cut 40 percent of the city’s rainwater runoff, clean 90 percent of its waterways, and save taxpayers more than $2 billion – in addition to shrinking New York’s chronic UHI.

Studies say green roofs alone can save up to 40 percent of energy bills for people in more forested regions of the country. According to the the U.S. Environmental Protection Agency, green roofs run between $10 and $25 per square foot to install, only slightly more than a conventional roof, and an investment which quickly pays for itself. The proof of this comes from science.

Researchers at the University of Michigan studied a green roof more than 21,000 square feet in size, and found that over its 20 year lifetime, the roof saved its owners more than $200,000, three quarters of which came from reduced energy needs. Seattle’s downtown Justice Center is a green-roofed building that is home to the city’s municipal courts and police headquarters. Coupled with other heat-reducing features, the Justice Center’s verdant toupee alone reduces some 445 tons of CO2 emissions each year, at a savings of $17,817 annually. Ensign says the U.S. green roof industry grew some 35 percent last year with more than 8.5 million square feet of green roofs installed nationally.

Jerry Yudelson sees the integration of ecological systems like green roofs as part of a larger trend of sustainable, green methods solving many of our current environmental problems, without political clout from Washington. Green technology could turn cities into pioneers of urban agriculture, he says, reducing greenhouse gases, heat islands and pollutants as an added benefit.

“You’d have a lot more room for community gardens and community-supported agriculture,” he says. “And you’d have healthy, outdoor jobs for people who want them. It’s a win-win scenario for everybody.”

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References

World Energy Outlook: Press Quotes

New York Times: Environment: U.N. Panel Finds Climate Change Behind some Extreme Weather Events

Build That Green: Hot Summer — Cool Roof

Telephone interview with Jerry Yudelson by J.Tidwell, Sept. 2011

Berkeley Lab: News Center

Coachella Valley Green

Colorado State University Extension Plants “Clean” Air Inside Our Homes

The Guardian: Plants working to combat pollution

American Meteorological Society

U.S. Environmental Protection Agency: Wet-Weather Flow