11 February 2008

Anthropogenic erosion

Many people probably realise that a wide range of geological processes acting on the surface of our planet cause sediment erosion and movement: Rain water draining in a gully in a field, or waves slowly eating away coastal cliffs, for example. Landslides, aeolian (wind) activity and glaciers also erode sediment. 

Humans also move sediment (for example construction and mining activities). But how much? More or less than the 'normal' geological processes acting on the surface of the earth? These questions formed the basis of a couple of papers by Hooke (On the history of humans as geomorphic agents. Geology 28 (2000): 843-846) and Wilkinson (Humans as geologic agents: A deep-time perspective. Geology 33 (2005): 161-164. doi:10.1130/G21108.1), and they just blew my mind. 

Wilkinson looked at how much the deep-time sediment flux was, using the volume of surviving continental and oceanic sedimentary rock through the geological epochs from the Lower Cambrium to the Pliocene. As we go back in time less and less sedimentary rock remains, because there has been more time to erode it, and by fitting a curve to the remaining volume of sedimentary rock it is possible to estimate the cycling rate of the sedimentary rocks. At present 0.14% +- 0.06% of all sedimentary rock is being eroded every million year - equivalent to a lowering of the surface of the earth of 24+-11 m every million year.  This number is the rate we would expect without humans also acting on the surface of the planet.  So now on to the humans.

Human sediment-moving activities can be divided into direct (for example mining and construction) and indirect (for example agriculture and forestry) activities. In his paper, Hooke showed that movement of rock and soil during construction accounted for ~30% of all sediment transported by humans, the balance being made up by erosion from agricultural activities. The US Department of Agriculture estimates that development of pastureland results in a soil loss of 400 m every million year, while cropland tillage results in a soil loss of up to 1400 m every million year. By estimating the global area being tilled and the global area used as pastureland, Wilkinson estimated that the erosion resulting from these practices was equivalent to a lowering of the earth's surface at a rate of 360 m every million year - more than 10 times faster than the deep-time erosion rate obtained from natural processes. Humans are, therefore, much, *much* more efficient in eroding sediment than mother Nature herself! 

As amazing as that may seem, an even bigger surprise (to me) arose when Wilkinson then computed the historical rates of erosion caused by humans, and compared that to the deep-time erosion rates (the 24 m per million year above).  Hooke provided data on the per capita annual amounts of soil and rock movements from construction and agriculture for the last 5000 years.  By multiplying with the population estimate he could then estimate the total erosion due to human activity.  Wilkinson's comparison with the deep-time erosion rates showed that the two curves crossed each other around the end of the viking age (approximately year 1000).  This means that for the last 1000 years humans have on an annual basis eroded more sediment than all other natural processes acting on the surface of the planet combined.

That's wild! I first read about this 3 years or so ago and I'm still amazed by it.  Who would have thought that our ancestors just by digging around with primitive shovels, dragging wodden ploughs through the earth and planting some crops could erode as much sediment as all other natural processes combined? And this was in year 1000! Today it's 10 times as much! Wow!

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