Geology

Leveling the Landscape

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Science  09 Feb 2007:
Vol. 315, Issue 5813, pp. 739
DOI: 10.1126/science.315.5813.739a

It has been recognized for some time that humans are the most important geomorphic agent modifying our planet's surface, dwarfing the effects of deforestation, desertification, and erosion caused by other processes globally. However, the details of these activities are enlightening, and Wilkinson and McElroy compare the rates of erosion from human activity in different settings with natural processes and with long-term and short-term rates inferred throughout Earth's history. About 5 gigatons (Gt) of sediment per year are thought to have been deposited naturally by rivers during the past 540 million years. There has been high variation about this average, particularly since the Pliocene and during glacial times; the average flux today is about 21 Gt/year. Most of this material (about 80%) comes from mountainous regions where natural erosion rates are highest. For comparison, it is estimated that humans now move about 75 Gt of dirt and rock annually, mostly in low-lying or low-topography areas, and particularly near coasts. This difference implies that large amounts of human-derived detritus are being stored primarily on floodplains and in small stream networks on coastal plains immediately downslope from such areas. This flux greatly exceeds the movement of material by Pleistocene ice sheets. — BH

Geol. Soc. Am. Bull. 119, 140 (2007).

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