Biochar prevents carbon from returning to the atmosphere through oxidation. Kobus Venter, my associate in South Africa, just posted an equation for calculating how much carbon is in biochar after several hundred years. But I think it’s more useful to show how much carbon dioxide is prevented from reappearing in the atmosphere over that same time period, so I adapted his work to find out how much CO2 is removed from the air by making one tonne of biochar.

About 200 lbs. of coarse biochar (est. dry weight, approx. 0.9 yards) after two weeks and a good rain to wash off the ash, equivalent to 460 lbs. of carbon dioxide diverted from the carbon cycle.

First, we calculate how much carbon is in our biochar. Carbon is not the only element in biochar. Lab results from Restord, a UK biochar startup cited by Kobus, have shown that biochar is c. 85% carbon. So, we multiply our biochar weight by 0.85.

Next we need a C to CO2 conversion. When carbon is oxidized, the addition of two oxygen atoms to a carbon atom increases its mass 3.67 times. One tonne of carbon in biochar equates to 3.67 tonnes of CO2 that does not return to the atmosphere. Therefore, we multiply by 3.67.

We then need to account for how much of that biochar will stay locked in the ground over time. Standards vary, but a conservative estimate is that 80% will still be here hundreds of years from now.

To be thoroughly precise, we need to account for the embodied emissions in the process, such as steel smelting and fabrication for a kiln. This will vary of course, depending on your kiln’s amortized carbon footprint and other factors, such as transportation. Restord worked theirs out to be c. 180kg CO2e (carbon dioxide equivalent) per tonne of biochar made (0.18 tonnes).

(0.85 x 3.67 x 0.8) - 0.18 = 2.3 tonnes per tonne of biochar

So whether you’re using tonnes, tons, pounds, or any other unit of weight, the ratio of atmospheric CO2e to biochar is about 2.3, varying according to the embodied emissions involved in the so-called externalities.