Human "Footprint" Seen on 83 Percent of Earth's Land

Hillary Mayell
for National Geographic News
October 25, 2002

Scientists have produced the first map that traces human influence on the natural world, and the numbers are big. Overall, 83 percent of the total land surface and 98 percent of the areas where it is possible to grow the world's three main crops—rice, wheat, and maize—is directly influenced by human activities.

"Yes, humans have a huge influence on the Earth's ecosystems," said Eric Sanderson, a landscape ecologist with the Wildlife Conservation Society and one of the co-authors of the study.

"But instead of being discouraged or depressed," he added, "we want people to understand they can actually make choices; that it's possible to live with wildlife in ways that allow us to make a living and at the same time coexist with wildlife."

The map was designed to illustrate the extent of human influence and identify opportunities for conservation. "What can't be measured can't be managed," said Marc Levy, a data specialist at Columbia University's Center for International Earth Science Information Network (CIESIN).

The figures are not a surprise to scientists, said Sanderson.

"Wildlife biologists and ecologists are really pretty discouraged about the inability to get the message out," he said. "In the United States, people have the idea that there are these really big wild spaces still out there. It's not true. One study showed that 20 percent of the continental U.S. land mass is within 500 meters of a paved road"—an area equal to about five and a half football fields.

"I think we're really missing the boat on this," said Sanderson. "It's going to be the most important issue in the 21st century, yet no one is talking about it." The study by Sanderson, Levy, and colleagues is published in the latest issue of the journal BioScience.

Mapping Human Impact

Trying to measure the extent of human impact is not new. As far back as 1864, author and naturalist George Marsh posed the question, asking to what degree the processes of nature were threatened by human activities. Biologists and ecologists have been trying to make that measurement ever since.

The authors of the study used four variables to measure human influence: population density, access from roads and waterways, electrical power infrastructure, and land transformation.

"There's a lot of evidence that these individual factors have effects, but this is the first time someone's tried to put them all together as one index, to try and see what the biological effects are," said Sanderson. "What we tried to do is actually map human influence and the gradients of human influence."

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