Why Some Cities Are Getting Drier as Skyscrapers Rise

By Christina Larson, Environment: Yale Magazine. Posted February 17, 2009.

Left, an image from the NASA Landsat satellite shows areas of vegetation (red) and fallow agricultural regions (white) in Shenzhen in 1973. Right, 28 years later, new urban regions (dark blue grids) and new urban development (bright white) proliferate.

Marshall Shepherd, a professor of atmospheric sciences at the University of Georgia and an editor of the Journal of Applied Meteorology and Climatology, approaches the question from a background in meteorology. He has studied rainfall patterns near Atlanta, Ga. -- a region recently pinched by water shortages -- and focused, among other things, on how air circulates around urban terrain. "Visualize wind blowing straight over wheat fields in Kansas," he says. "Now imagine low-level winds blowing around city skyscrapers. Because of the structure of buildings in a city, the air is more turbulent." The combination of turbulent air, elevated temperatures and other factors can trigger more dramatic weather patterns, including more frequent and abrupt rainstorms over and downwind of cities. "What appear to be random thunderstorms around cities aren't so random at all," he says.

In the case of China's Pearl River Delta, where dwindling precipitation is the overarching trend, an additional factor may be at work: pollution. Rapid development has brought billowing smog, also known as the presence of airborne "aerosols," which affects the way clouds "seed" to form rain. Some aerosols are necessary to trigger showers, but an oversaturation of particles impedes the formation of raindrops. Daniel Rosenfeld, a professor at the Institute of Earth Sciences at Hebrew University in Jerusalem, is the lead author of a paper on pollution and precipitation that was published in the journal Science in September. Over a 50-year period, his data show that rainfall over the mountains near Xi'an, a congested city in central China, has decreased 20 percent. "This is a serious problem for areas where water availability is scarce," he says, noting that many of today's fastest-growing regions, including much of the Middle East, northern China and Africa, are especially susceptible.

While scientists focus on different aspects of the feedback loop between cities and climate, they agree that planners have some control over the outcomes. Rosenfeld urges tighter emissions controls; new research "should act as a red light," or warning, to all of those responsible for controlling the amounts of pollution we release into the atmosphere." Shepherd recommends evaluating how urban areas influence rainfall when deciding where to build reservoirs.

Seto offers more out-of-the-box proposals that are not so much refinements of existing planning practices as they are new ways to envision a metropolis. Today's cities, seen from an airplane, she notes, are shaped like a dinner plate or, perhaps, a sprawling amoeba. As such, the urban area is a contiguous paved region that extends outward from a defined center. But a new city -- in China and elsewhere, new cities are being built virtually from scratch -- could be mapped as a series of concentric circles or as a checkerboard, alternating office parks with forest parks. This could augment or disperse the heat island effect, the ground's ability to retain moisture and the impact of pollution. Or, a city could be designed with several small centers -- think of a constellation of mini-downtowns -- with residential areas, business districts and public transportation planned accordingly. "If you have to lay down 1,000 square kilometers of urban development," she asks, "where are you going to put it?"

One of the remarkable things you notice if you travel around the world today is that older buildings -- Chinese courtyards, Venetian canals, Dutch windmills, Iranian wind towers -- all look different. But newer structures, with glass and steel exteriors and air-conditioned lobbies, all look relatively the same. At one time architecture was adapted for local topography and climate; today this is far less true.

The same pattern of regional adaptation once held true for urban planning writ large. Planners took advantage of resources at hand and sought particular fixes to distinct problems, mindful of each region's natural margin for error. "The ancient Greeks were masters of matching the buildings, squares and streets of the city to its topography," wrote MIT's Anne Whiston Spirn in her classic history of urban planning, The Granite Garden. "New York City owes the distinctive skyscraper skyline of Manhattan Island to the strength of the underlying bedrock and its proximity to the surface."

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Christina Larson is a journalist focusing on international environmental issues. Her reporting has brought her to seven provinces across China, as well as cities and villages in Cambodia, Vietnam, Thailand, and Greece. Her writing has appeared in The New York Times, The New Republic, Christian Science Monitor, China Environment Series, and The Washington Monthly, where she is a contributing editor.

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