Visualization of approximate amount of wood that would have to be charred and buried annually to offset carbon emissions of one United States resident.  (Picture credit)

A popular idea at the moment to address climate change is biochar - essentially taking organic materials, charring them, and burying them in the soil.  As the Wikipedia explains:

Biochar is charcoal created by pyrolysis of biomass, and differs from charcoal only in the sense that its primary use is not for fuel, but for biosequestration or atmospheric carbon capture and storage.[1] Charcoal is a stable solid rich in carbon content, and thus, can be used to lock carbon in the soil. Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide (CO2) and other greenhouse gases (GHG). Carbon dioxide capture also ties up large amounts of oxygen and requires energy for injection (as via carbon capture and storage), whereas the biochar process breaks into the carbon dioxide cycle, thus releasing oxygen as did coal formation hundreds of millions of years ago. Biochar is a way for carbon to be drawn from the atmosphere and is a solution to reducing the global impact of farming (and in reducing the impact from all agricultural waste). Since biochar can sequester carbon in the soil for hundreds to thousands of years[2], it has received considerable interest as a potential tool to slow global warming. The burning and natural decomposition of trees and agricultural matter contributes a large amount of CO2 released to the atmosphere. Biochar can store this carbon in the ground, potentially making a significant reduction in atmospheric GHG levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity and reduce pressure on old growth forests.

Giving credit were due, this practice was invented by pre-Columbian Amazon tribes, and Wade Davis, in his book The Wayfinders reports that an area in the Amazon the size of France may once have been managed in this way.  James Lovelock is on record with the idea that this is the last best hope of humanity.

Now, the biofuel story has given me a bit of a horror of ideas that sound cool to environmentalists, are fine on a small scale, but are a disaster when scaled up by industrial society.  So I wanted to do a few quick back-of-the-envelope calculations of the limits of this approach.

US carbon emissions currently run about 1.8 billion tonnes of carbon each year, and the US population is about 300m, so emissions per person are 6 tonnes/year of carbon.  To get a feeling for this, let's express it in wood terms, approximately.  Let's figure wood is basically carbohydrate with a formula of CH₂O, so that 6 tonnes of carbon corresponds to 6 * (12+2+16)/12 = 15 dry tonnes of wood.  The density of the softwoods that are mainly used commercially in the US are around 1/2 tonne/m³, so we would need to char and bury about 30 m³ of wood to offset the emissions of each person in the United States - roughly a medium sized tree.  Each year.

30 m³ is about 480 board feet of lumber (gross - much of a tree would only be good for firewood).  There's about 3000 board feet of lumber in a 1000 sq foot US house, for comparison.  So a typical American family's share of carbon emissions is equivalent to all the lumber in their house every three years or so (figuring on a family of four in a 2000 sq foot house).

Looking more globally, here is an estimate of the amount of the carbon flowing into the global economy through the major channels of wood and agricultural products (from this TOD story):

Note the y-axis scale.  The data are a few years out of date, but you can pretty much see where the trend was going.  Next, here's fossil fuel carbon emissions:

As you can see, the entire carbon inflow from the biosphere into the economy currently is only about one third of the fossil fuel throughput.  So in order to also offset carbon emissions entirely with biochar, we'd need to char and bury an amount of carbon three times larger than current off-farm usage of biological products in the economy.

Given that the existing level of take that humanity makes on the biosphere is pretty impactful, what you don't want to do is create some kind of general payment incentive for commercial operations to char and bury carbon.  That would be disastrous and lead to bulldozers wiping out tropical forests on a huge scale in order to pile them up, char them and bury them.

What would be potentially more reasonable is an incentive, on existing farmland only, to do biochar of agricultural residues.  That might be environmentally beneficial on the whole (improve the soil in-situ, without incentivizing spillovers onto marginal soils or tropical forest ecosystems) though the interaction with current no-till agricultural practices should be thought about carefully.  You might expect to get, very roughly, about the same amount of carbon in the residues as there is in the food supply - a couple of gigatonnes globally.  Given about 1.5 billion hectares of arable land globally, or 4 billion acres, 2 gigatonnes of carbon is about 1/2 ton of carbon/acre, which is 1.25 tonnes/acre in carbohydrate terms, which is consistent, for example, with current estimates of harvestable corn stover of 1-1.5 dry tons/acre.  Globally, it could be somewhat more or somewhat less, but that's the ballpark.

So a few gigatonnes of biochar carbon sequestration would be very useful, but it's not a panacea for even the current level of 8.5 gigatonnes of fossil fuel emissions, let alone the growth trajectory.