December 1, 2006
Humans Said to Have Huge Impact on Erosion
SYRACUSE, N.Y. -- Mother Nature may be a force, but nothing like humans when it comes to causing erosion, says a Syracuse University scientist.
Humans cause erosion at a rate 10 to 15 times faster than any natural process, according to new research by Bruce Wilkinson, a sedimentary geologist.Scientists have long identified humans as the primary agents altering the shape of the Earth's surface. Wilkinson said his study gauged the rate of man-made erosion and compared the speeds and differences under which natural and human-related erosion occur.
By using data gathered from around the world and the universal soil loss equation, Wilkinson determined that global erosion is occurring at a rate of about 75 gigatons a year - a gigaton is equal to a billion tons.
"To put that into context," Wilkinson said, "current annual amounts of rock and soil moved over the Earth's surface in response to human activities are ... an amount of material that would fill the Grand Canyon of Arizona in about 50 years."
Wilkinson presented his findings at the 118th annual meeting of the Geological Society of America, held in late October in Philadelphia. His paper, titled "The impact of humans on continental erosion and sedimentation," will be featured in the January-February issue of the Geological Society of America's GSA Bulletin.
Roger LeBaron Hooke, a prominent researcher now at the University of Maine in Orono, first wrote a decade ago about the impact of humans as the most potent force in shaping the planet.
At that time, Hooke estimated humans moved about 45 gigatons of sediment annually, but he looked only at construction and mining, but did not include agriculture.
Hooke "basically agreed" with Wilkinson's results, although he said the study underplayed the impact of precipitation in natural erosion.
Before humans began moving earth, wind, water and glaciers accounted for most erosion.
But nature was in no hurry. The movement of sediment took millions of years.
The main cause of man-made erosion is agriculture, followed by construction and mining.
Where humans once used sticks and stones, they have since developed technology that dramatically accelerated the speed of erosion, Wilkinson said.
Wilkinson's estimates from the Phanerozoic eon - about 542 million years ago - indicated that natural erosion took place at a rate of about five gigatons of sediment per year.
Between the Phanerozoic eon and the Pliocene epoch - about 5.3 million to 1.8 million years ago - erosion increased to about 16 gigatons per year as continental glaciers plowed across the Earth's surface and then retreated. Current estimates of natural erosion stand near 21 gigatons per year.
There's also a major difference in where the erosion is occurring, Wilkinson said.
Natural erosion occurs at the planet's highest elevations. Wilkinson said about 83 percent of the global river sediment comes from the highest 10 percent of the Earth's surface.
Human-induced erosion, by contrast, occurs in the lower elevations. Eighty-three percent of this erosion occurs at the lower 65 percent of land surfaces.
Wilkinson said his findings are significant not only to the field of geology, but to concerns about sustainable living as well.
The data indicate that given the continuing population growth on the planet, the soil loss caused by erosion will present a serious challenge to meeting the food needs of a growing population.
Global cropland has increased by 11 percent since 1961, while the global population has approximately doubled, Wilkinson said. The net effect of both changes is a 44 percent decrease in per capita cropland.
"Erosion by itself is not necessarily a crisis, but when you have more and more people, and less land on which to grow food, then you have a real problem," he said.
Reports in recent years show erosion rates decreasing in the United States, which has invested billions in improved farmland conservation practices, said Mark Nearing, a soil scientist and erosion expert for the U.S. Department of Agriculture. However, those figures do not take into account recent climate changes that are again accelerating erosion, he said.
Although he had not read Wilkinson's study, Nearing concurred that human-induced erosion is "an order of magnitude" greater than natural rates.
"There is no doubt that our (U.S.) erosion rates are unsustainable in the long run," said Nearing, adding that the rates in Latin America, Southeast Asia and Africa are much worse.
Geological Society of America: http://www.geosociety.org
Soil Science Society of America: http://www.soils.org