Last Monday in my series about the climate crisis, I posted about the potential of reversing the carbon cycle: grow plants to remove carbon from the atmosphere and put it back into the ground. Here now are some numbers that show the viability of this approach. Let’s start with a bit more data on greenhouse gases. I will focus here on CO2 because it stays around in the atmosphere the longest and accounts by far for the largest percentage of warming.

First, how does the carbon cycle relate to the ppm (parts per million) number quoted for CO2 in the atmosphere? We are emitting about 9 GtC/year, that is 9 gigatons (billons of tons) of carbon © per year. But 5 gigatons of that is absorbed by the oceans and through increased biomass already, for a net addition to the atmosphere of 4 gigatons. To convert that into a ppm measure we first ned to go from C to CO2, i.e. from carbon to carbon dioxide. Assuming for simplicity that all our C emissions are in the form of CO2, we get to 4 * 3.67 = 14.7 GtCO2/year (why 3.67? that’s the relative atomic weight of CO2 compared to C). Now to 1ppm to the atmospheric count requires 7.8 GtCO2, which means our back of the envelope calculation suggests we are adding 14.7 / 7.8 = 1.9 ppm per year. That’s pretty close to the actually observed increases of about 2.5ppm per year in the last few years measured at the Mauna Loa station.

Second, how much carbon can we capture per acre of farmland? Biomass consists of about 45% carbon. Assuming a yield of 10 tons per acre, which is achievable with certain grasses, that means a single acre could capture as much as 4.5 tons of carbon annually. Now of course it is crucial that this carbon doesn’t get back into the atmosphere, so one cannot just let this biomass decay or worse yet burn it. Instead, one needs to use a pyrolysis or similary type process to compact the biomass and wind up with residue such as biochar and biocrude. These processes have not historically been optimized for storage of the resultant material but rather for things like biofuel production. To be conservative for now I will assume that only one third of the carbon can be captured that way (this would be just the biochar at low temperature pyrolysis), yielding 1.5 tons of carbon/acre per year. I am assuming that the entire rest of the carbon is rereleased back into the atmosphere in some way (for instance the burning of biofuel and syngas).

Third, how much agricultural area could we use for this? There are about 900 million acres of farmland in the US. Some of it is used for export, some for feed for milk cows and much of it is used highly inefficiently compared to what can be accomplished with vertical farming. So for the sake of this analysis I will assume that we can free up 400 million acres for carbon sequestration. That is a big number and definitely will take a lot of time but I just want to get at ballpark numbers here. We are then talking about 400 * 1.5 = 600 million tons of sequestered carbon per year or 0.6 GtC/year. That is 12% of the current global net addition of 5 GtC/year. Annual US emission are 5.7 Gt of CO2 which is 1.55 Gt of C, so this could do away with 0.6 / 1.55 = 38% of US net addition.

Of course another way to look at how much land would be required to sequester all 5 GtC / year. There are 48 million square kilometers of farmland globally. There are 247 acres per square kilometer for a total of 11.8 billion acres. At 1.5 tons/acre it would take 3.3 out of the 11.8 billion (nearly one third of all farmland). Now you might think that sounds completely implausible, but I am not trying to argue that we would or should do exactly this at the current yields suggested above. Simply that even without heroic assumptions one winds up in the right ballpark. There are parts of the world where biomass yields can be 30 or even 40 tons/acre. At 30 tons/acre the math is 2.5 out of 11.8 billion acres for carbon sequestration.

Fourth, you might ask where would we actually store this stuff? The best answer here would be existing mines. The output is a lot denser then the original biomass (thankfully). I estimate based on coal that 1 ton of biochar fist in 1 cubic meter of space. That means that 1 years worth of 600 million tons of sequestration could be stored in 0.6 cubic kilometers. Again this seems like a conservative estimate as it might be possible to further condense the carbon. Also, for comparison, this is about 2x the amount of trash produced in the US annually, which I suspect is a lot less dense.

So what is to be concluded from all of this? Well, there is always the possibility that I have made a major research or calculation error – if you find one please point it out. In the absence of that though it shows that plants and plant based solutions can play a major role in fighting the climate crisis. That’s of course not a substitute to also decarbonizing the electric grid, transportation and habitation but it will make a huge difference.

PS Thanks to Tom O’Keefe for pointing out a calculation error which I have now fixed, making plant base removal on agricultural land look better (only requiring about one third of available farmland).