Why Some Cities Are Getting Drier as Skyscrapers Rise

It might seem like a cruel irony: While the growth of cities worldwide requires more water resources, urban growth itself may be a factor in creating a drier, or different, regional climate.

Take China's Pearl River Delta region, which in recent years has gone from being a regional backwater to being the center of the global manufacturing universe. Three decades ago, this area at the southern tip of Guangdong province, where the river spills into the South China Sea, was a relatively quiet spot, where farmers waved off mosquitoes buzzing in capacious rice paddies. Today the region, strategically situated just north of Hong Kong, is the industrial hub of the world's most prolific manufacturing nation -- home to some 50 million people and thousands of factories churning out toothbrushes, toys, computer parts and just about everything else that can be packed in shrink-wrap and shipped around the world. The region's new soundtrack is a constant whir of factory machinery, loading dock whistles and construction crews building ever more roads and apartment blocks on the outskirts of town.

In Shenzhen, one of its busiest port regions, nearly everyone is from somewhere else -- from other cities in China, from foreign companies and, especially, from the countryside. More than 95 percent of the workers on the assembly line are estimated to have flocked from nearby villages, a familiar pattern across China, where millions of people each year move to the nation's fast-growing cities. As they settle into factory dormitories and new high-rises, then turn on the faucet for cooking, showering and laundry, demand for water rises. Yet in precisely the same years that skyscrapers have soared and the sky has thickened with smog, rainfall in the region has declined. Why?

To untangle the connection, a team of interdisciplinary researchers compiled readings from 16 meteorological stations in the region, which they compared with maps charting urban growth, derived from NASA satellite data. Their study, published last year in the Journal of Climate, found that between 1988 and 1996, urban land cover in the Pearl River Delta increased 300 percent -- the equivalent of paving an area the size of Rhode Island in less than a decade. Meanwhile, during the dry winter months (the subtropical region's summer is influenced by the Asian monsoon cycle), rainfall declined. The team created a statistical model linking urban growth with winter rainfall; they found that each percentage point in growth correlated with a decrease of 2.44 milli-meters in rainfall.

A growing body of research, conducted in China and elsewhere, now shows that the way a city grows can have the effect of holding an umbrella, or in some cases turning on the sprinklers, over a city. Though debate remains over which factors (land cover conversion, urban topography and pollution) are most significant, scientists agree on the underlying principle: not only are cities impacted by their regional climate, they also shape it.

"Cities are modifying their own climates," says Karen Seto, associate professor in the urban environment at F&ES and a co-author of the study. "If you want rain, you need to start thinking about the way a city grows."

It might seem as though Shenzhen and other fast-growing urban areas -- from Dubai, to Bangalore, to Lagos -- sprung into being overnight. But to understand how cities influence climate, it's helpful to look closely at the process of converting fields to factories.

Seto's research focuses on what scientists call "land cover conversion" or, what Joni Mitchell crooned, to "put up a parking lot." When a forest, prairie or wetland -- or even a sand dune or arctic tundra -- is replaced by asphalt and concrete, the ground's ability to absorb and retain moisture changes. Simply put, water seeps into soil (to different degrees depending on the soil) but washes off pavement. At the same time, sidewalks and other urban surfaces absorb sunlight, whereas natural foliage reflects it. This contributes to the familiar "urban heat island" effect, where temperatures in a city exceed those in the nearby countryside. Of course, not all newly built parking lots have the same effect. "The types of landscapes that were converted [to pavement] have an impact," she says. "What areas were lost? And what was that land previously? Was that land formerly devoted to agriculture, or forest, or desert? Did the trees and ground absorb a lot of water, or not?" she asks, noting that planners do have a choice where to locate new interstates and residential communities.