Most people don't realize it, but they are very effective agents in changing the shape of the Earth's surface. In fact, they are the most important geomorphic agents on the planet. Simply by eating food and living in built structures, they have contributed to a major Earth-altering process that has been going on for thousands of years: erosion. Scientists have never before identified the rate of man-made erosion -- until now.
According to new research by Syracuse University earth sciences professor Bruce Wilkinson, humans cause erosion at a rate 10-15 times faster than all natural processes. Using data gathered from around the world and the universal soil loss equation, Wilkinson was able to calculate and compare the speeds at which natural and human-related erosion processes take place. Wilkinson, a sedimentary geologist, presented his findings at the 118th annual meeting of the Geological Society of America, held Oct. 22-25 in Philadelphia. His paper, titled "The impact of humans on continental erosion and sedimentation," will be featured in an upcoming issue of the Geological Society of America's GSA Bulletin.
Before humans began moving earth, the main processes of erosion on the planet were wind, water and glacial movement. This gradual movement of sediment took hundreds, thousands and even millions of years. The main cause of man-made (or cropland) erosion is the practice of agriculture, followed by construction and mining. Since humans developed technology to facilitate these pursuits, the speed at which erosion has taken place has increased dramatically.
Wilkinson's estimates from the Phanerozoic eon (542 million years ago) indicate that around the world natural erosion took place at a rate of about 16 meters per million
years (16 m/m.y.), resulting in an accumulation of about 5 gigatons of sediment per year (5 Gt/yr). Between the Phanerozoic eon and the Pliocene epoch (5.3 to 1.8 million years ago), erosion is estimated to have increased irregularly to a rate of 53 m/m.y., (with an accumulation of 16 Gt/yr.) Current estimates of naturally eroded sediment loads in large rivers are close to this figure at about 62 m/m.y. (about 21Gt/yr).
Calculations of cropland erosion rates yielded much higher values. According to Wilkinson, current cropland erosion is proceeding at a rate of about 600 m/m.y. (about 75 Gt/yr), and much of this sediment accumulates as post-sediment alluvium that is being stored in shallow streams and reservoirs instead of being deposited into large rivers.
"To put that into context," says Wilkinson, "current annual amounts of rock and soil moved over the Earth's surface in response to human activities are about 18,000 times that of the 1883 Krakatoa eruption in Indonesia, about twice the volume of Mount Fuji in Japan, and are an amount of material that would fill the Grand Canyon of Arizona in about 50 years."
The other difference between natural and cropland erosion is the location in which it takes place. Natural erosion occurs at the planet's highest elevations. That means 83 percent of the global river sediment comes from the highest 10 percent of the Earth's surface. Cropland erosion, by contrast, occurs in the lower elevations. Eighty-three percent of this erosion occurs at the lower 65 percent of land surfaces.
Wilkinson's 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. He points out that global cropland has increased by 11 percent, while the global population has approximately doubled. The net effect of both changes is a 44 percent decrease in per capita cropland.
"Soil loss through cropland erosion is largely insignificant when compared to the impact of population growth," says Wilkinson. "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."
