Erosion is one of the most environmentally destructive processes, leading to loss of biodiversity in plants and animals, as topsoil with all its nutrients washes and blows away,  ending with stony ground that cannot sustain plants or store water. When water is not stored, plants need continuous rain, making them drought-prone, and rainfall runs off fast, causing flooding downstream, collapse of hillsides, and silting up of dams (although seasonal inundations with topsoil and its nutrients have been part of traditional cropping in many areas).

I sometimes ask classes to think about the question of ‘how far is the human race from extinction?’; answers come back in terms of the months of food we have stored now, the random time until the next huge volcano erupts or asteroid hits us, the years until there is not enough food with current unsustainable production methods, or a few decades before wars over water or hunger lead to our extinction. My answer to the question: we are about 15 cm or 6 inches from extinction, the depth of the topsoil that sustains most plant and microbial life.

Erosion has been with us from the start of agriculture: traditional slash-and-burn agriculture was able to grow crops for only a few years before plots lost topsoil and nutrients, so they were abandoned without production for several decades before restarting the cycle. Landscape-wide erosion caused huge areas of limestone to be exposed in the Burren area of Ireland, now interestingly conserved as a very special ecosystem but with minimal biological productivity compared to the temperate rain forests that preceded the limestone pavements. In the US, the dustbowls of the 1930s, with many plowed (ploughed) and fallow fields displacing deep-rooting perennial grasses.