Giant Footprint

Scientists estimate that 80 percent of Earth's land surface now bears the marks of human activities, from roads to crops to cell phone towers.

Under present land-use practices, studies show, society is seizing an ever-bigger share of the planet's biological resources to satisfy human demands. There is growing concern that the resulting environmental changes may seriously undermine the natural functions of terrestrial ecosystems. This could threaten their long-term capacity to sustain life on Earth by providing essential services such as food production, water and air filtration, climate regulation, biodiversity protection, erosion control and carbon storage.

"Ultimately, we need to question how much of the biosphere's productivity we can appropriate before planetary systems break down," Jonathan Foley and a group of co-authors caution in a paper published last July in the Proceedings of the National Academy of Sciences (PNAS).

Foley, director of the Center for Sustainability and the Global Environment at the University of Wisconsin-Madison, uses state-of-the-art computer models and satellite measurements to analyze links between land-use changes and environmental conditions around the world. This research has shown that agriculture is the dominant form of human land use today, with about 35 percent of all ice-free land now used to grow crops and raise livestock. That's up from only 7 percent in 1700.

The physical extent of land conversion for human activities is only part of the story, however. The intensity of such activities also matters a great deal: more intensive land use usually consumes more resources.

One of the best pictures so far of humanity's collective impact on terrestrial ecosystems comes from a new study, also in the July PNAS, by a team of European researchers. They compiled spatially explicit maps, in units of 6.2 square miles, indicating not only what types of local land use predominate around the world, but roughly how much biomass energy—or natural productivity—the various land-use practices consume. (The remaining biomass energy is available to support biological functions in all other trophic levels, or food webs, of ecosystems.)

"Our results show that humans, just one of 2 to 20 million species on the planet, use up 25 percent of the trophic energy available in all terrestrial ecosystems," says lead author Helmut Haberl of Klagenfurt University in Vienna. "That's quite a dramatic inequality. "

Patterns of human land use vary widely around the world, influenced by biophysical and socioeconomic conditions. Across large areas of Asia and sub-Saharan Africa, for example, subsistence agriculture and small-scale farms are still standard. But in general, there's a steady shift toward more intensive land use today, driven by rising living standards and population growth that fuel increasing demand for goods and services.

Modern farming offers a good example. In the last 40 years, global grain harvests doubled although total cropland expanded by only 12 percent. Squeezing more output from farmland is possible thanks to new grain varieties, chemical fertilizers, mechanization and irrigation. But the downside is greater environmental damage, including soil degradation, increased pesticide use and water pollution from nutrient runoff.

Another new study illustrates the kind of massive environmental damage that modern farming practices can induce over the long term. Published in PNAS in August, the report suggests that the planet's base of agricultural land may already be more fragile than society realizes. After compiling records from around the world, David Montgomery of the University of Washington in Seattle concluded that conventional plow-based farming methods are dramatically accelerating global soil erosion. It's happening, he says, at rates 10 to 100 times above the rates at which new soil is created.

"This means we could strip off topsoil in a couple hundred to a couple thousand years," says Montgomery . "The current rate of erosion is one that should worry civilization over the next couple of centuries, but the problem plays out so slowly it's hard for people to wrap their heads around it."

To counter the problem, Montgomery advocates wide-scale adoption of no-till agriculture. That approach forgoes the use of a plow to turn the soil, which leaves topsoil more susceptible to erosion; instead, farmers lightly churn crop stubble into the topsoil. Although no-till farming may require the use of pesticides and herbicides under some conditions, this would be outweighed over the long term by a number of benefits, says Montgomery. No-till farming, he asserts, would reduce erosion to rates closer to that of natural soil production. Other benefits include improved soil fertility and increased carbon storage as more organic matter accumulates in the soil.

For years, scientists have widely assumed that massive soil erosion from agriculture plays a significant role in altered levels of carbon in the atmosphere. Yet the exact nature of that link isn't well understood, and evidence from various studies has been highly contradictory. Some studies have concluded that global soil erosion from agriculture releases considerable amounts of carbon into the atmosphere; others found a sizable carbon "sink" effect.

An October report in Science refutes both those claims. Using a new method of analysis, an international team of scientists headed by Kristof Van Oost of Catholic University of Leuven in Belgium found that global agricultural soil erosion has a minimal effect on levels of atmospheric carbon. It does captures carbon, these researchers say, but only a fragment, in amounts well below some previous estimates.

Diana Parsell of Falls Church, Va., writes often on topics in science.