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 It's now out-of-date and its qualitative judgments have proven inconsistent and hardly reproducible.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.

Thursday, December 3, 2015

Arable Land Loss

Soil degradation is a serious problem. I care very much about environmental protection and the long-term sustainability of agriculture around the world.

This is why it upsets me that a demonstrably false claim is currently circulating in the news media.
"...nearly 33 per cent of the world's arable land has been lost to erosion or pollution in the last 40 years ..." (,
"... nearly 33% of the world’s adequate or high-quality food-producing land has been lost ..." (theguardian.come,
experts from the University of Sheffield's Grantham Centre for Sustainable Futures revealed that nearly 33 per cent of the world's arable land has been lost to erosion or pollution in the last 40 years

Read more at:
This claim was delivered to the news media by the The University of Sheffield's Grantham Centre for Sustainable Futures which is a political research and advocacy organization. They've circulated a pamphlet and press release which both repeat the "33%" statistic without any indication how the number was calculated.

I suspect they got the number from the FAO website which says:
"... 33 percent of the global arable land used to producing feed for livestock ..."
Or perhaps they misunderstood this description of FAO data:
"Agricultural land covers 33% of the world's land area, with arable land representing less than one-third of agricultural land (9.3% of the world's land area)."
Whatever their mistake, we can use the FAO data to find out for ourselves how world-wide arable land area has changed over time. By multiplying the fraction of arable land by the total land area of each country we get the graph below.

Arable land is not being lost. Click here for the raw data.
But changes in "arable land" over time isn't even useful for measuring soil erosion and degradation. At least, not according to the FAO.
"Data for Arable land are not meant to indicate the amount of land that is potentially cultivable."
The three main causes of soil degradation are overgrazing, deforestation, and mismanagement of arable land. We can do something about all three. Spreading demonstrably false claims about about catastrophic loss of arable land does nothing help. By spreading misinformation, the Grantham Centre for Sustainable Futures only fosters distrust by people who aren't fooled by their doomsday warnings.

If we want people to listen, a good place to start is with the truth. Not demonstrably false claims.

UPDATE: I heard back from a representative of the Grantham Centre, but I still have no idea where the number "33%" comes from.

Power your home with only 150 Indurains!

Today I saw an hilarious news article (more like covert advertisement, but whatever) with the fantastically bullshit headline Pedaling For A Hour Can Power Your Home For Twenty-Four Hours. This may be true if your home has only one dim light bulb. But for modern homes in the USA, it's demonstrably false.

The article advertises a bicycle that's hooked up to generator and battery. You can store charge from exercise and run your home on this power. Well, partially run your home on this power. Don't get me wrong. It's an awesome bike! I want one!

But the average American rural household consumes 93.6 million Btu of electric energy per year.

(93.6 million Btu per year) / (365 days) = 75 kWh per day

The most bad-ass professional cyclists in the world can output at most 0.5 kWh per hour.

So how many Miguel Indurains would you need to invite over to your American rural household each morning to spend an hour peddling in order to supply the average day's power consumption?

(75 kWh) / (0.5 kWh) =  150 Indurains

"We're here to power your home. Don't worry, this will only take an hour. Got any food, by the way...?"