Cities eat away at Earth's best land

December 18, 2002

URBANISATION is eating away at the planet's most fertile and productive land. Although only 3 per cent of land has been built on in the US, the resulting loss in plant growth is enough to offset the gains made by agriculture as shown by a groundbreaking study. If this pattern is repeated in developing countries, the loss of agricultural land could have a serious effect on food supplies.

Forests and cropland also absorb large amounts of the greenhouse gas carbon dioxide. When that land is urbanised, these "carbon sinks" are lost and the carbon is released into the atmosphere. These effects have been hard to quantify because there was no accurate map of urban areas. But now Marc Imhoff of NASA's Goddard Space Flight Center in Maryland, and his colleagues have shown the drastic effects of urbanisation.

To gather their data, Imhoff and his team used a weather satellite that normally maps moonlit cloud cover at night. But on clear, moonless nights, the satellite picks out city lights, oil flares, forest fires and even lightning strikes. The NASA team used this information to map the mainland US states except Alaska according to three categories of land use: urban, urban periphery and non-urban.

They then calculated how active the vegetation is in each region using data gathered by satellites that map the red and near-infrared frequencies reflected by the chlorophyll of photosynthetic plants. Combining this information with weather conditions such as temperature, humidity and rainfall gave them the amount of plant growth or "net primary productivity" (NPP) in each region.

The researchers found that in the mid-1990s, urban areas cost the US 40 million tonnes of carbon per year, or 1.6 per cent of the country's total pre-urban NPP. That's enough to cancel out the 1.8 per cent contribution to NPP made by agriculture.

Some 29 per cent of the total land area of the US is devoted to agriculture. Yet the urbanised areas that account for the decline occupy just 3 per cent of the land. "This means that the very best soils are indeed the ones that we are urbanising," says Imhoff.

Cristina Milesi of the University of Montana in Missoula agrees. Her own study, presented to the Annual Geophysical Union meeting in San Francisco earlier this month, used similar satellite-based techniques to show that urbanisation in the south-eastern US has risen from 4.5 per cent of total land area in 1992 to 6.4 per cent in 2000, far above the national average of 3 per cent. "We are replacing something that was extremely productive from an agricultural point of view with lawns, golf courses and a few scattered trees," says Milesi. "We are eating up all the prime land."

According to Imhoff's calculations, urbanisation is reducing the amount of dry vegetation in the US by 91 million tonnes each year. If it was all edible, that would be enough to feed around 450 million people. Of course, much of it is inedible for humans. "But it gives you an idea of the biological significance [of urbanisation] for the food web in general," says Imhoff.

While the US has the land to feed its people, the same is not true of developing and heavily populated countries such as India and China. "If you are a developing country, you might want to take a closer look at which lands are being developed," says Imhoff. He suggests trying to shift some of the development onto lands that are less productive. "There's still a chance not to make the same mistake [as the US]."
New Scientist issue: 21/28th December 2002


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