Sprawling over croplands.

Urban sprawl has been grabbing headlines because it lengthens commutes, creates heat islands, and alters local weather (SN: 3/27/99, p. 198). Less attention has focused on what the cities sprawl over. Satellite surveys now indicate that the best croplands are disproportionately giving way to cement and asphalt.

At NASA's Goddard Space Flight Center in Greenbelt, Md., Marc L. Imhoff has been probing the impacts of urban development on photosynthesis. He hunted down developed areas in the United States by examining nighttime illumination as recorded by a military satellite. Imhoff mated these data with daytime satellite images recording surface greenness, an index of plant cover. His group then overlaid these two maps on a third. Prepared by the United Nations, it ranks a location's soil on a 9-point scale denoting its potential to support crops.

Overall, just 3 percent of U.S. surface area turns out to be urban by Imhoff's definition--that is, inhabited at a density of at least 10 people per hectare, which is about the area of a football field. However, sprawling urban development has been encroaching on some of the best soils.

In California, one of the nation's biggest agricultural producers, 16 percent of the best soils now underlie urban areas, as do nearly 9 percent of the next-best soils. Another 55 percent of the state's best soils reside in areas not classified as urban but "already seen as lit [at night] by satellites," Imhoff observes. No more than a tenth as populated as urban regions, these zones run along transportation corridors that are showing early signs of development. "They will be urbanized," Imhoff predicts.

His group's analyses "not only confirm something people had suspected," Imhoff says, "but add a measure of quantification to help people understand [sprawl's] impacts." For instance, Miami's spread onto area grasslands has cut the region's annual photosynthesis productivity by an amount equal to 22 days of plant growth, he found. "It's like turning the lights out in a greenhouse for 22 days," Imhoff says.

Nor is sprawl strictly a U.S. phenomenon. Rapid urban development has preferentially targeted agricultural lands in China's Pearl River Delta--that nation's fastest-growing region. High-resolution satellite images from 1988 and 1996 show that the urban area more than tripled over the period. Karen C. Seto of Boston University reports that some 1,500 square kilometers of this sprawl consumed farmland--almost four times the amount of natural land cover lost to urban development.

With collaborators from the Chinese Academy of Sciences, Seto visited 150 sites and determined that her team's satellite analysis had been 93.5 percent accurate in tracking urban sprawl. That's rewarding, she says, since aerial detection of urban expansion in China can be difficult. With far smaller patches being built up than tend to occur in North America, very fine-resolution satellite data proved a necessity.

Imhoff says that society can't afford to lose its best agricultural land. However, development on nonfarm lands could hike biodiversity threats, warns Andrew Dobson of Princeton University, because natural lands tend to harbor more species than cultivated fields do. But moving agriculture out of the path of urban development is no panacea, he argues, because most of the best land is already farmed.