There was a big debate about biochar in last week’s Guardian, between George Monbiot, who thinks it’s being sold as a “miracle mass fuel cure“, and defenders of biochar, including James Lovelock, who agrees that “it would be wrong to plant anything specifically to make charcoal” but that biochar has net benefits if it’s made from agricultural wastes.

Monbiot says what’s being proposed amounts to “turning the planet’s surface into charcoal”. But is it really? I calculated just how much biochar would really be needed to store all the excess carbon dioxide in the atmosphere…

• The total weight of the atmosphere over each square metre of the Earth’s surface is 10 tonnes. (A handy figure to remember, that.)
• Carbon dioxide is now present in the atmosphere at 385 parts per million by volume (ppmv), equivalent to 582 parts per million by mass (ppmm).
• This is 35% higher than pre-industrial levels (284 ppmv = 429 ppmm).
• So the excess CO2 which needs to be removed is 153 ppmm.
• Conveniently, 1 tonne = 1 million grams. So one part per million of 10 tonnes is 10 grams. The 153 ppmm CO2 we need to remove comes out at 1530 grams per square metre of the Earth’s surface.
• By mass, CO2 is 30% carbon and 70% oxygen. So if all that excess carbon dioxide were fixed as charcoal (which is more-or-less pure carbon), it would come to 459 grams per square metre of the earth’s surface.
• The density of charcoal is 208 kg/m3. So this hypothetical global layer of charcoal would be just 0.459 / 208 = 0.002m, or 2 mm thick.

Not a great deal, if you think about it. But in reality we aren’t talking about the whole earth’s surface, but only about the arable land, because that’s (a) where most of the crop wastes are, (b) where the people are to do the charring, (c) where people are interested in improving the soil, and (d) they are totally man-made ecosystems anyway, so if we need to modify land on a planetary scale, that’s the place to start.

The Earth’s surface area is 51 gigahectares (nice unit!) of which 1.36 gHa are arable (by George M’s figure) - that’s 2.7% of the total. So:

If all the “excess” carbon dioxide in the atmosphere were converted into carbon and spread across all the earth’s arable lands, there would be 17kg of charcoal per square metre, in a layer 8cm thick.

That’s not an unfeasible notion. The Gardening with Biochar FAQ mentions biochar application rates of around 5kg/m2. On the other hand, photos of Terra Preta soils show black layers that are many centimetres thick, so they must contain far more than 17 kg/m2 of carbon.

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