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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. Charcoal is a stable solid rich in carbon content, and thus, can be used to lock carbon in the soil for hundreds to thousands of years.

Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide. Biochar is a low-tech way to harness the vast power of plants to remove carbon dioxide from the atmosphere. In fact, some people believe the only real chance to fight global warming on the massive scale needed is via massive biochar projects. One advantage is that it is low tech: anyone can do it.

Biochar can be used to create useful energy and also Terra preta. Beneficial synergistic effects with soil microbial life and good agricultural practice can amplify the total carbon sequestration effect. (Current industrial agriculture depletes soil and is a significant emitter of greenhouse gases.)

The following figure, from:

shows how full-grown forests emit as much CO2 as the absorb, while forests combined with biochar technology can be used to sequester CO2:

Fig. from Johannes Lehmann

The next figure, from:

gives an overview of the sustainable biochar concept:

Fig. from Woolf et al, Nature Communications 1 (2010)

In a comment on Azimuth, John Furey writes:

I think the main atmospheric-carbon-removal benefit from char is from increased humification, which occurs in aerobic (vadose) soil.

Can we get evidence supporting or disproving this?

We can get both.

It depends on pre-loading the charcoal with nutrients. Otherwise it would suck up nutrients from surrounding humus, thus depleting it - until after some time it perhaps forms again: Perhaps more and better than before, depending on climate and soil. (This is a phenomenon well-known to gardeners applying carbon rich mulch. See carbon/nitrogen ratio.)

There’s an important paper mentioned in Terra preta:

David A. Wardle, Marie-Charlotte Nilsson and Olle Zackrisson, Fire-derived charcoal causes loss of forest humus, Science 320 (2008), p. 629.

It also addresses another discussion I had with John F: How stable is biochar in soil? Wardle’s pure charcoal did not decay in a decade. But the humus/charcoal mixture did.

But here’s my caveat: I guess they took standard BBQ char coal, which is relatively water repellent (thus attracts not that much microbes first). They might have gotten a different result if they had cooked the char coal before.

I need to read it (first need register to the sciencedings, but first need sleep…)

Here’s from their response to a comment by Lehmann and Lohi:

“We recently reported that fire-derived charcoal can promote rapid loss of forest humus in boreal forests (1). Lehmann and Sohi (2) raise two issues regarding the interpretation of our results. The first issue is that the accelerated mass loss in the mesh bags containing a mixture of humus and charcoal could have been due to decomposition of some unspecified labile component in charcoal. We believe that this explanation is implausible. First, our experiment also included mesh bags containing only charcoal that was placed in intimate contact with the surrounding humus for 10 years and which showed insignificant mass loss over that time—not a result that would be expected if there were a substantial labile pool of carbon (C) in the charcoal.”

– Martin Gisser


The following study from 2009 estimated the cost to sequester CO2 in the form of charcoal as ~$157 per ton of CO2, or about 104 euros per tonne:

The calculation assumes that commerical charcoal manufacture methods are used:

Bulk charcoal prices in the United Kingdom range from 310 to 425 euros (US$466 to US$640) per metric ton giving an average price of 365 euros (US$553). If we allow 35 euros (US$53) per metric ton for other costs like forest and production development, storage and security, 380 euros (US$572) appears a realistic esimate for the production and long-term storage cost per metric ton of charcoal. As CO2 comprises only 27.3% of carbon by weight, this is equivalent to an offset cost of approximately 104 euros (US$157) per metric ton of CO2 produced.

Questions: Using the current price of commercially produced bulk charcoal may not be very accurate.

Answer: The “fossil fool’s cost” of wood pellet char, if energy yield is discounted at the cost of equivalent heating oil is minus 378US$ or -278€ per metric ton, using German prizes of Feb. 2011. This is “nice” char well suited for agricultural application. See Experiments in biochar for details.

James Lovelock

In the Guardian, James Lovelock wrote:

I said in my recent book that perhaps the only tool we had to bring carbon dioxide back to pre-industrial levels was to let the biosphere pump it from the air for us. It currently removes 550bn tons a year, about 18 times more than we emit, but 99.9% of the carbon captured this way goes back to the air as CO2 when things are eaten.

What we have to do is turn a portion of all the waste of agriculture into charcoal and bury it. Consider grain like wheat or rice; most of the plant mass is in the stems, stalks and roots and we only eat the seeds. So instead of just ploughing in the stalks or turning them into cardboard, make it into charcoal and bury it or sink it in the ocean. We don’t need plantations or crops planted for biochar, what we need is a charcoal maker on every farm so the farmer can turn his waste into carbon. Charcoal making might even work instead of landfill for waste paper and plastic.

Incidentally, in making charcoal this way, there is a by-product of biofuel that the farmer can sell. If we are to make this idea work it is vital that it pays for itself and requires no subsidy. Subsidies almost always breed scams and this is true of most forms of renewable energy now proposed and used. No one would invest in plantations to make charcoal without a subsidy, but if we can show the farmers they can turn their waste to profit they will do it freely and help us and Gaia too.

There is no chance that carbon capture and storage from industry or power stations will make a dent in CO2 accumulation, even if we had the will and money to do it. But we have to grow food, so why not help Gaia do the job of CO2 removal for us?

Rachel Smolker

Rachel Smolker is biologist and anti-biochar activist who helped organize a petition in April signed by 143 non-profit groups protesting what they called a “charred earth policy”. The petition came as a reaction to an effort by 11 African countries and biochar proponents to have the United Nations consider biochar’s eligibility as an official means for nations and companies to offset their emissions under international regulations. See:

Woolf, Amonette, et al.

From their comprehensive article on sustainable biochar:

Land-use changes that incur high carbon debts and biochar production using technologies with poorly controlled emissions lead to both large reductions in avoided emissions and excessively long carbon-payback times, during which net emissions are increased before any net reduction is observed. Biochar production and use, therefore, must be guided by well-founded and well-enforced sustainability protocols if its potential for mitigating climate change is to be realized.

The nice image above illustrates what wrong can be done: Felling a living tree and turning it into char coal releases a substantial amount of CO2 and other GHG. This carbon debt may take decades to redeem, for a new tree has to grow to replace the old one. This is counter productive given the rapidity of climate change. So it is important to char biomass that would decompose quickly if left alone, like straw and other agricultural waste.


What are the biggest biochar projects currently in operation, or planned?

Carbonscape has developed microwave pyrolysis. This has the advantage that the heat is more evenly supplied to the biomass than with surface heating or hot gas. It would also allow the required exothermic energy to be provided from sources such as wind or solar rather than using a proportion of the syngas that is made.

What are the best ways to get large numbers of farmers to create biochar without damaging the environment? What sociological, political or economic research has been done on this question?

What is the fastest feasible rate at which we could ramp up biochar production? What would the effect on carbon dioxide concentrations be? What studies have been carried out on these questions?

What about the bark beetle infested forests? Let them decay and slowly release their carbon - or turn them into char and quickly release a substantial amount of carbon?

Degradation of biochar and biochar versus black carbon

Biochar (char coal) seems not a very well-defined substance, neither physical nor terminological. Depending on the charring temperature it contains a huge diversity of more or less biodegradeable components, like tars and oils.

Biochar is sometimes equated with black carbon, which e.g. in climate science is something different (soot). From Black carbon, Wikipedia:

Black carbon or BC is formed through the incomplete combustion of fossil fuels, biofuel, and biomass, and is emitted in both anthropogenic and naturally occurring soot.

The mean residence time of biochar is claimed to 1000 – 4000 years. (See J. Lehmann and S. Joseph, eds., below)?.

The following paper found 100 year C losses of 3−26% and biochar C half-lives on orders ranging from 10^2 to 10^7 years, generally increasing with increasing charring temperature. Biochar lability was found to be strongly controlled by the relative amount of more aliphatic and volatile components:

Whereas the following authors:

found that black carbon amended in agricultural soil degraded a lot in 30 years. (However, “physical export played a role but was not quantified separately and warrants further research.”)

It has been found by some researchers that the better effect of adding biochar to soil is to increase humification:

The following paper:

says that “the BC pool in marine sediments is significant and is considered to comprise 12–31% of sedimentary organic carbon (SOC) in the deep ocean”.


Good places to start include:

For various viewpoints on biochar, try:

A quote from the abstract of the above paper:

We find that explicitly designing a biochar system around ‘true wastes’ as feedstocks combined with safe system drivers could minimize unwanted land-use impacts and leakage. Applying baselines of biomass decomposition rather than total soil carbon is effective and supports a longer crediting period than is currently standard. With biochar production introduced into bioenergy systems, under a renewable biomass scenario, the change in emissions increases with higher fuel use, instead of decreasing. Biochars may have mean residence times of over 1000 years, but can be accounted for more effectively using a recalcitrant and labile fraction.

The actual paper is more interesting than this!

Additional references:

Highly recommended by Martin Gisser. Incl. critical examination of Biofuelwatch. Plus, first engl. translation of parts of Carvajal’s diary of Orellana’s first Amazon exploration 1542. (When I get the book back, I plan to do some overhaul of this page…)

category: carbon