The bright prospect of biocharEnthusiasts say that biochar could go a long way towards mitigating
climate change and bring with it a host of ancillary benefits. But
others fear it could do more harm than good.
Nature Reports: Climate Change
21 May 2009
By Kurt Kleiner
Fournier wants to help save the planet, though in a most unlikely way:
by burning biomass. At the forefront of a carbon-sequestration
technology that proponents say offers a rare 'win-win-win'
environmental opportunity, Fournier's company Biochar Engineering in
Golden, Colorado, manufactures machines that turn biomass into
charcoal, or biochar.
Spread on soil, biochar can keep CO2 out
of the atmosphere while improving soil fertility and boosting
productivity. In addition, gases released in the charcoal-making
process can be used to make biofuels that are more sustainable than
those currently on the market. "Char happens to be the one thing that
represents a solution to all of these factors together. It's a unique
opportunity," Fournier says.
But while enthusiasts are pushing
to have biochar recognized as an official means of offsetting
greenhouse gas emissions, others remain cautious. At best we know too
little, say critics, and at worst using biochar to sequester carbon
could ultimately lead to unintended consequences, including the
destruction of virgin forests to make way for plantations.
certainly has potential," says David Wardle, a soil scientist at the
Swedish University of Agricultural Sciences in Uppsala. "But it's
premature to be already including it in carbon accounting. Maybe it
really is an answer. But we don't know that yet."
idea of using biochar for climate change mitigation is relatively new,
its origins extend back to the pre-Columbian era, when humans first
made terra preta — in Portuguese, dark earth — soils in the central
Amazon basin. According to archaeologists, the rich, black and fertile
terra preta was created by adding a mixture of bone, manure and
charcoal to the otherwise relatively infertile soil over many years.
The charcoal – believed to be the key ingredient – is 70 times more
concentrated in terra preta than in surrounding soils and is formed by
heating biomass in an oxygen-poor or oxygen-free environment. Some of
the charcoal in Amazon terra preta soils has persisted for thousands of
years, back to when people first started this practice. Its persistence
has attracted the attention of research scientists who think that it
could be used to lock away carbon for a similarly long time in the
future, keeping it out of the atmosphere as a greenhouse gas.
can get charcoals that are tens of thousands of years old, or even
older," says Chris Turney, a geographer at the University of Exeter and
director of the start-up Carbonscape. With headquarters in Blenheim,
New Zealand, Carbonscape is developing a unique approach to producing
biochar. The company is soon to launch in the United Kingdom. "If you
want a very simple method of fixing carbon in a relatively stable form
for potentially tens of thousands of years, charcoal is a good way of
doing it," Turney says.
Tonnes tucked awayThe recent surge
of interest in biochar as both a commercial venture and an academic
challenge was evident at a one-day workshop held last month at the
University of Edinburgh and sponsored by the UK Biochar Research
Centre. "When I wrote the grant proposal to fund this, I could find
only about four or five people in the UK who were interested," says
Stuart Haszeldine, the geologist and biochar researcher who organized
the event. "Now last week we were turning people away. We had 80 people
attend, and we could have had 150."
As a solution to escalating
emissions, biochar is certainly promising. Every year, human activity
results in the release of somewhere between 8 and 10 billion tonnes of
carbon dioxide. Of that, several billion tonnes are soaked up by the
oceans and land, leaving around 4.1 billion tonnes in the atmosphere.
number is dwarfed by the 60.6 billion tonnes of carbon that terrestrial
plants absorb during photosynthesis every year. A similar amount goes
back into the atmosphere through plant respiration. But if a fraction
of that carbon could be stored in the soil, it would mitigate climate
change to some degree. "Any organic matter that is taken out of the
rapid cycle of photosynthesis ... and put instead into a much slower
biochar cycle is an effective withdrawal of carbon dioxide from the
atmosphere," says Johannes Lehmann, a soil scientist at Cornell
University in Ithaca, New York, who has spent years studying terra
preta and biochar.
Lehmann and colleagues think that the
potential benefits could be huge. Of the more than 60 billion tonnes of
carbon taken up annually by photosynthesis, around ten per cent
eventually becomes available as agricultural residue such as corn and
rice stalks, or forestry residue such as branch and leaf litter, as
well as animal waste. If all 6 billion tonnes were put through
pyrolysis — the heating process that turns biomass into charcoal — 3
billion tonnes of biochar would be produced every year, reducing
atmospheric carbon emissions by the same amount1. That would offset a
substantial proportion of the 4.1 billion tonnes of excess carbon
dioxide that accumulates annually in the atmosphere.
biochar manufacture has the added benefit of creating liquid fuel as a
useful by-product, there's even greater potential for mitigating
climate change than from sequestering CO2 alone. According to Lehmann's
calculations, a third of a tonne of biofuel could be produced for every
tonne of biomass used. If those biofuels replaced fossil fuels — in
transport, for example — it would reduce carbon emissions by an
additional 1.8 billion tonnes per year.
Tim Lenton, professor of
Earth-system science at the University of East Anglia, UK, recently
rated biochar as one of the best technological fixes for cooling the
planet. According to Lenton's analysis of 17 geoengineering options2,
biochar has the potential to sequester almost 400 billion tonnes of
carbon by 2100 and to lower atmospheric carbon dioxide concentrations
by 37 parts per million. Advoctaes, including Lehmann, admit that the
real numbers will probably be much smaller. Haszeldine, for instance,
says that 1 billion tonnes of carbon sequestered per year by 2030 is a
reasonably conservative estimate of biochar's potential. "Even if it's
only 500 tonnes of carbon a year, it's useful," says Haszeldine. "If
it's a million or a billion tonnes a year, that's significant."
biochar-making technologies use heat produced by the biomass itself to
form the charcoal. But Turney, the Exeter professor and Carbonscape
director, is backing a slightly different approach, one that uses
industrial-scale microwaves. He says the idea was inspired by cooking
accident in his teenage years, when he put a potato in the microwave
for 40 minutes and it turned into charcoal. Although using microwave
technology has the disadvantage of requiring electricity, the process
will result in twice as much carbon being stored in the soil as is
emitted as greenhouse gas.
A much lower-tech approach is to
promote the use of charcoal-making cook-stoves to the roughly 2 billion
people who rely on wood for fuel. The cook-stoves, produced by a number
of companies, use wood or other organic materials as fuel and burn only
the gases and oils, leaving charcoal behind. The result is a
cleaner-burning flame that gives off less smoke, and the leftover
biochar can potentially be applied to soil.
Fournier of Biochar
Engineering says that he became involved in biochar because of its
mitigation potential. But he thinks its value as an addition to the
soil will ultimately drive its production. Right now his company
manufactures relatively small biochar units for researchers, capable of
making 50 kilograms of biochar per hour. He says, however, that the
real market will probably be in medium-sized units that can produce 250
to 300 kilograms per hour but are still small enough to be packed into
a standard cargo container and shipped anywhere in the world. Fournier
expects individual farmers or local entrepreneurs to begin buying the
units and using them to make biochar for agricultural purposes, with
co-production of energy a secondary benefit. These small operators
might decide to forego biofuel production altogether, says Fournier,
and concentrate on making biochar. The extra heat generated by the
char-making process could be used for warming a building or for
industrial processes, however, and possibly for producing electricity.
charcoal for agricultural use is now selling for about US$500 per
tonne, that same tonne of charcoal, at current prices, is worth only
about US$50 if sold for offsetting emissions. Even if the price of
carbon offsets rose to US$100 per tonne of CO2, that tonne of biochar
would still be worth only US$350 in offsets, says Fournier. In fact, he
says, the economics of biochar will be determined by a combination of
its value as a soil additive, as a carbon offset measure and as an
Pressure to plantBut some worry that once
production becomes profitable, pressure will mount to use land
specifically for biochar plantations. "The level at which they are
promoting this could result in enormous land-use change issues," says
Rachel Smolker, a 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
"It would require huge areas of land
to be turned into plantations," warns Smolker. Carbonscape, for one,
has suggested that forests might be planted, harvested for charcoal,
and then replanted. For instance, the company says, if the 200 million
hectares of forest in the United States that are harvested for timber
were instead used for biochar, replanted, and harvested again, each
rotation would reduce atmospheric carbon dioxide levels by ten parts
per million. Others, such as Lehmann, have proposed replacing winter
wheat crops with switchgrass that would be turned into biochar.
and Almuth Ernsting, who works with Biofuelwatch, a UK environmental
organization, are specifically concerned that a market for biochar
would encourage the destruction of tropical forests, much as a market
for biofuel has encouraged forest destruction for palm-oil plantations.
Ernsting estimates that sequestering even a relatively modest 1 billion
tonnes of carbon a year would require that 500 million hectares of land
be devoted to biochar plantations3. By comparison, there are an
estimated 1.5 billion hectares of tropical forest remaining in the
But demand for biochar plantations needn't lead to the
destruction of forests, argues Turney. Although he believes it would
make the most economic sense to use agricultural and forest waste for
biochar, he says that if plantations were needed they could be situated
on land that has already been deforested. In fact, he says, biochar
production might actually provide an incentive to reforest the
estimated 900 million hectares of degraded land worldwide. "The whole
point is to suck up carbon, not to start pillaging native vegetation
that's already out there," says Turney. Fournier also agrees that
destroying forests for biochar plantations would be a perverse effect,
but he thinks that international agreements and certification could
prevent that from happening.
That isn't Smolker's only concern
though. The hope is that once the carbon is stored in the soil, it will
stay there for many thousands of years. But although terra preta shows
that's possible, it is not known whether all soils will benefit from
biochar application, or even how long modern manufactured char will
persist. "You can't assume that modern biochar behaves like terra
preta," says Smolker. Soil scientist David Wardle reported in Science
last year that, in Swedish forests at least, charcoal may cause carbon
to disappear from the soil much more quickly than expected4. Wardle and
his team left mesh bags containing either humus, charcoal or a mixture
of both on the forest floor and recorded how much mass was lost from
each over a ten-year period. They found that the mixtures of humus and
charcoal lost more mass than the controls of humus and charcoal alone.
Wardle thinks that the charcoal promoted microbial breakdown of the
humus, accelerating the release of CO2 back into the atmosphere. It's
also possible that some microbes could degrade biochar directly.
Although the black carbon that makes up the bulk of biochar is thought
to be biologically unavailable to most microbes, research suggests that
some microbes might be able to metabolize it. If so, it would be less
stable in soil than currently thought5.
issue is to what extent modern-day biochar application will fulfil the
promise of terra preta in improving soil fertility. Research by
Lehmann6 suggests that in most cases the addition of charcoal improves
soil productivity, and although the reasons for the increased fertility
still aren't entirely understood, several things seem to be going on.
First, the biochar itself contains some nutrients such as phosphorus,
potassium and zinc. But the biochar also seems to help the soil retain
some nutrients that would otherwise leach out, as well as helping it to
retain water. In addition, biochar might encourage soil microbes that
increase crop productivity. And the productivity gains seem to continue
to increase even when very high levels of carbon have been added to the
soil — up to 140 tonnes per hectare in sandy, weathered soils, and up
to about 50 tonnes per hectare on average.
Proceed with cautionBut
without more research, says Smolker, it's wrong to assume biochar can
be safely applied to soil on a large scale. "I think there's potential
that this could backfire and worsen the climate situation," she says.
Alan Robock, a climate scientist at Rutgers University, also worries
that methods to sequester carbon, including biochar production, could
distract attention from the need to reduce emissions. "The people who
created the problem like the idea. They can keep using the atmosphere
as a sewer and let other people clean up the mess," he says.
biochar researchers agree that the technology needs more study and that
the most important thing is to reduce emissions in the first place.
"Biochar is not a silver bullet for sequestration," Lehmann says. "We
cannot continue the emissions that we generate today and anticipate
that any technology or combination of technologies could compensate."
Nevertheless, it's possible that biochar could help mitigate those
emissions, he says.
"Part of what our group will be trying to do
is to contribute to that work, and monitor and review where all this
has got to," Haszeldine says. "We want to make sure we're not making a
1. Amonette, J. et al. in American Geophysical Union Fall Meeting 2007, abstract U42A-06; http://tiny.cc/biochar1
2. Lenton, T. M. & Vaughan, N. E. Atmos. Chem. Phys. Discuss. 9, 2559–2608 (2009).
3. Ernsting, A. & Smolker, R. Biochar for Climate Change
Mitigation: Fact or Fiction? (Biofuelwatch, 2009);
4. Wardle, D. A. Science 320, 629 (2008). | Article | PubMed | ChemPort |
5. Hamer, U., Marschner, B., Brodowski, S. & Amelung, W. Org. Geochem. 35, 823–830 (2004).
6. Lehmann, J. & Rondon, M. in Biological Approaches to Sustainable
Soil Systems (eds Uphoff, N. et al.) 517–530 (CRC Press, 2006).