The Physics Police

The Physics Police

Friday, December 4, 2015

Agricultural Soil Loss

Yesterday I posted about a demonstrably false claim is currently circulating in the news media.

This started when the University of Sheffield’s Grantham Centre for Sustainable Futures put out a press release saying:
"...nearly 33 per cent of the world’s arable land has been lost to erosion or pollution in the last 40 years ..."
So I contacted Mark Sinclair, the author of the press release asking how they reached this conclusion.

He let me know the intention here was to highlight soil loss, rather than land loss per se as I addressed in my post yesterday. He acknowledged that this distinction has "perhaps been lost in the drafting". I'll say.

He also shared with me the conclusion above comes from a book called Dirt by David Montgomery.
"So far in the agricultural era, nearly a third of the world's potentially farmable land has been lost to erosion, most of it in the past forty years."
As we shall see this is a fantastical exaggeration!

The book cites something call the GLASOD project which in 1991 surveyed 250 scientists across the world and found that about 15% of agricultural land was degraded. This was a map of subjective perceptions, not an objective measure of land degradation. It's now out-of-date and its qualitative judgments have proven inconsistent and hardly reproducible.

Since this was a snapshot in time it can't be used to assess the rate of soil degradation. So how did Montgomery come up with 40 years? In reference to a paper (full text) by Bruce Wilkinson which again reports only the rate of loss.
"...mean soil losses are therefore equal to 885 m/m.y. in the areas under cultivation ..."
That indeed equals about 1.39 inches per 40 years. But that's a rate of erosion today. Not an historical account of the past 40 years.

Even if we ignore this, GLASOD reports 1.96 billion hectares or 15% agricultural land was degraded which is not "nearly a third".

But agricultural land is a small subset of arable land. So only 0.246% of arable land was degraded.

Furthermore, most of the land classified by GLASOD as degraded was only degraded by amounts "light" to "moderate". The book claims land was "lost" as in "could no longer support crops" which corresponds only to the most severe categories "strong" and "extreme".

This brings the total down to 305 million hectares or 0.0381% of worldwide arable land.

That's 866 lower than reported by Montgomery and thereby the Grantham Centre press release!

Soil erosion is a real problem. To address it and make positive change we start with the truth.

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