Outdoing Mother Nature

Wearing down. Severe soil erosion in a wheat field near Washington State University.

Move over Mother Nature, another force is outdoing you. Humans now move an order of magnitude more earth than all natural processes combined, according to a new study.

The idea that human activity erodes soil from Earth's surface faster than it accumulates is not new. For example, plowing loosens soil, allowing water to carry it away into river deltas and ocean basins, where it eventually turns into sedimentary rock. In the absence of agriculture, most of this soil would be kept in place by the roots of forests and grasslands. But the size of the disparity between human-driven and natural erosion has remained a matter of debate.

So geologist Bruce Wilkinson of the University of Michigan, Ann Arbor, compared the rates of erosion caused by human activity, including construction, plowing, and development of pastureland, with prehistoric rates of erosion. To determine natural rates of erosion, Wilkinson added up the total volume of sedimentary rock that has accumulated over the past 500 million years. After correcting for the fact that some of the rock was eroded away again and recycled, he found that before man came along, natural erosion lowered Earth's surface an average of about 24 meters every million years. If the erosion from human activity is averaged out over the entire planet surface, it is equivalent to about 360 meters per million years--15 times the natural rate, reports Wilkinson in this month's issue of Geology.

The rate has ominous implications: With nearly every bit of arable land already under the plow, current agricultural practices are unsustainable, says Wilkinson. He believes man probably overtook nature as the bigger eroder near the end of the first millennium. At present rates, says Wilkinson, the soil humans erode would fill the Grand Canyon in 50 years.

Wilkinson is the first to attempt a comparison between natural and anthropogenic erosion rates in this way, says geologist Paul Bierman of the University of Vermont. "The most intriguing part of this study is to be able to look back over 500 million years of Earth history," he adds.

Related sites
Bruce Wilkinson's research
Abstract of study
Paul Bierman's research

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