THE IMPACT OF HUMANS ON CONTINENTAL EROSION AND SEDIMENTATION

Phanerozoic rates of subaerial denudation inferred from preserved volumes of sedimentary rock has irregularly increased over the ~542 million year span of Phanerozoic time to a Pliocene value of 53 m/My (16 Gt/y). Current estimates of large river sediment loads are similar to this value (21 Gt/y), and suggest that ~83% of the global river sediment flux is derived from the highest 10% of the Earth's surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation, and is proceeding at a rate of about 600 m/My (~75 Gt/y). Agricultural soil loss is largely confined to lower elevations of the Earth's land surface; ~83% of cropland erosion occurs over the lower 65% of the Earth's surface.

The conspicuous disparity between natural sediment fluxes suggested by data on rock volumes and river loads (~21 Gt/y) and anthropogenic fluxes inferred from measured and modeled cropland soil losses (75 Gt/y) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately down slope from eroding croplands over the history of human agriculture. Accumulation of post-settlement alluvium on higher order tributary channels and floodplains (mean rate ~12,600 m/My) is the most important geomorphic process in terms of the erosion and deposition of sediment that is currently shaping the landscape of the Earth. It far exceeds even the impact of Pleistocene continental glaciers or the current impact of alpine erosion by glacial and/or fluvial processes.

Conversely, available data suggest that since 1961, global cropland area has increased by about 11% while the global population has approximately doubled. The net effect of both changes is that per capita cropland area has decreased by about 44% over this same time interval; about 1 percent per year. This is ~25 times the rate of soil area loss anticipated from human denudation of cropland surfaces. In a context of per capita food production, soil loss through cropland erosion is largely insignificant when compared to the impact of population growth.