I often see this statistic….or at least numbers like it…..and I’ve always another question I want answered:
Yet all terrestrial life depends on a thin layer of earth, normally just six to 10 inches thick, for survival – and the world loses at least 30 billion tons of it a year, mostly from the drylands that provide nearly half of the world’s food.
OK, yes, topsoil washes away. We know it’s been doing this forever. Over thousands of years by looking at river deltas, over millions and hundreds of millions by the existence of sedimentary rock.
So we do know that it goes away, off somewhere instead of being in those nice fields. That it has been doing this forever also leads us to the necessary conclusion that there is some process which creates it. Here I’m thinking of Darwin’s observation about earthworms and the stony field on the estate. Or it might be humus that collects in what soil is left etc.
What we want to know therefore is the balance: OK, so 30 billion tonnes a year is gently floating downstream each year. How much is being created each year? And I’ve never actually seen a number for that. P
perhaps my Google Fu isn’t good enough.
Does anyone know? It wouldn’t surprise me at all to find out that the balance is negative, that we are eroding that topsoil on balance. But I do think that the net figure is likely to be more useful than the gross one which we’re continually presented with.
Update, in the comments, so, is 30 billion tonnes a lot or a little?
Hmm, zeros tend to go missing when I do this sort of thing. But cropland (ie, pasture plus arable) is apparently some 4.7 x 107 km 2……. 47 million I make that. Call the topsoil 10 cm, there’s 100,000 cm to a kilometre, thus there’s 47million / 10,000 km 3 of topsoil. Am I wrong yet? 4,700km 3 of topsoil.
1 km 3 of water weighs 1,000×1,000×1,000 tonnes, or one billion tonnes. Earth is, to a reasonable approximation, 2x density of water (erm, might be a bit high, mining assumption is that whatever is 1.8x). Call it 9,000 billion tonnes of topsoil among friends.
Gross loss is 30 billion tonnes a year. 0.3% of all topsoil is lost in any one year.
Which leaves us with our final thought. Do we think that the regenerative capacity of topsoil is greater than that? Alternatively, does it take more than three centuries to replace what is washed away?