In Brazil’s Amazon basin, farmers have long sought out a special form of fertiliser — a locally sourced compost-like substance prized for its amazing qualities of reviving poor or exhausted soils. They buy it in sacks or dig it out of the earth from patches that are sometimes as much as three meters deep. Spread on fields, it retains its fertile qualities for long periods.
They call it the terra preta do indio — literally, "the dark earth of the Indians". Dense, rich and loamy, this earth forms a stark contrast with the thin, poor soils of the region. (It seems a paradox, but rain forest soils have low fertility. This is why farmers who cut down the forest for agriculture have to keep on felling — after a few years of cropping, yields collapse and they have to move on.) Patches of terra preta extend for many hectares in some places but until recently, no one really knew what the mysterious dark earth was. Some guessed it was volcanic, or the sediment of old lakes, or the residue of some long-rotted vegetation. Few imagined that it was man-made.
Terra preta, modern analysis has proved, is one of the last remaining traces of pre-Columbian agriculture in the Amazon basin. It was made more than 2,500 — and perhaps as long as 6,000 — years ago by people living by the river. These cultures survived and supported complex agriculture, despite poor soil, by making their own earth. They used dung, fish, animal bones and plant waste — the usual suspects. But the key ingredient in terra preta, and what gives it its dark colour, is charcoal.
"It’s wonderful stuff," says Simon Shackley, a social science lecturer at the University of Edinburgh. "We started to get to know about it when Dutch scientists began to look at it in the 1960s. They found these dark soils in this area of very poor soil, where it was being put on fields like compost. It’s really the product of slash-and-burn agriculture, and other organic waste, incorporated into the soils over hundreds or even thousands of years — and it does appear to be fertile indefinitely, which is really a very odd thing."
This ancient product of the Amazon is now the subject of intense scrutiny by climate-change scientists. The tenacity of the charcoal of terra preta — retaining its fertilising properties over centuries — has given them an idea. Charcoal is a form of carbon, the burnt remains of plant and animal material. If it can stay intact in the earth for so long, without being released as carbon dioxide (CO2) gas, why not lock up more carbon in the earth in this manner?
Scientists have begun to refer to the charcoal made from plants for the purpose of storing carbon as "biochar". The theory is that biomass — any plant or animal material — can be turned into charcoal by heating it in the absence of oxygen. By taking CO2 out of the atmosphere, the impact on climate change could be huge.
Soils naturally contain large quantities of carbon, from decayed vegetation. But this carbon is relatively unstable, in climate terms; soils give off CO2 when they are disturbed — by ploughing for example — making them as much a carbon source as a carbon sink. So the idea of trying to lock up carbon in soils has found little favour among climate scientists. Indeed, it has even gained a bad name, as farmers have sought to cash in by claiming that their fields should qualify for the carbon credits intended to provide financial support to projects such as wind farms or solar-power plants.
What is different about biochar is that the stability of the charcoal should make it possible to lock away the carbon it contains for hundreds of years. The carbon is mineralised, so it’s very resistant to breaking down. What’s more, the ancillary benefits — not just its soil-improving characteristics, but certain byproducts of its manufacture — should be enough to make it economically attractive.
When it’s made, about a third of the biomass is turned to char, a third is turned to syngas that can be burned to generate electricity, and a third into a crude oil substitute that could be very useful in making plastics, though it would be hard to use as a transport fuel. Tim Flannery, the eminent Australian explorer and naturalist, argues that these properties of biochar "allow us to address three or four critical crises at once: the climate-change crisis, the energy crisis, and the food and water crises", because putting biochar in the soil not only fertilises the soil, but also helps it to retain water.
Just how much could biochar do to change the world’s carbon balance? There is little doubt of the enormous amount required. Every year, human activities — burning fossil fuels, cutting down forests, converting grassland to crops and so on — contribute eight to 10 billion tonnes of carbon to the atmosphere. Most of that carbon does not go on to damage the climate — the world has a natural carbon cycle, by which carbon dioxide in the atmosphere is absorbed and re-emitted by "carbon sinks" of vegetation, soils, the seas and other natural processes. But these processes are being severely overloaded, so the carbon content of the atmosphere is rising. At present, it stands at about 387 parts per million, certainly higher than at any time in the last 650,000 years and probably in the last 20 million.
According to the Global Carbon Project, between 2000 and 2007, the land and ocean carbon sinks — such as forests, and plankton in the ocean — removed about 54%, or 4.8 billion tonnes a year, of the carbon that humans pumped into the atmosphere. That leaves a carbon surplus of about four billion tonnes or so per year, which we need to find ways to reduce or absorb. Moreover, the amount absorbed by natural sinks is declining as land and oceans warm, meaning every year we must either work even harder to remove carbon from the air, or stop emitting it.
Even as governments talk of a "low-carbon economy", global greenhouse-gas emissions are rising fast. According to the Intergovernmental Panel on Climate Change (IPCC), the world authority on climate science, emissions must peak in the period 2015 to 2020 if we are to avoid the most catastrophic effects of climate change. On present projections, that will be impossible — unless a way can be found to make available cheap, easy methods of removing carbon dioxide from the atmosphere, and of generating clean electricity in ways that can be adopted around the world much more quickly than current renewable technologies.
According to some early estimates of biochar’s potential, this wonder substance alone could achieve all the carbon reductions necessary to prevent further global warming. Johannes Lehmann of Cornell University and others calculated that biochar could remove between 5.5 and 9.5 billion tonnes of carbon from the air each year. But those estimates relied on heroic assumptions about the ability to make biochar easily around the world, says Shackley. "There has been a tendency to withdraw from some of the very large figures lately," he notes. "Now, I would say people are talking more about something in the range of one to two billion tonnes a year."
This may seem disappointing in comparison with previous grandiose claims, but it still represents an impressive potential contribution from a single method, Shackley says. "It’s certainly not trivial," agrees Tim Lenton, professor of earth systems science at Britain’s University of East Anglia. "It might be a good-sized slice of what we need, and it has sizeable side benefits – it’s win-win." If other carbon-reducing techniques — such as preserving and regrowing forests, increasing the share of energy from renewables, and the push for energy efficiency — were pursued simultaneously, the world could make the cuts needed in our "carbon budget" to stave off climate disaster.
This potential, and the unique and sometimes mysterious qualities of biochar, are making it one of the most exciting new areas of climate-change research. The idea of sequestering carbon through biochar has gained some heavy-hitting scientific backers, such as James Lovelock, the maverick scientist whose Gaia hypothesis has come back into vogue.
Scientists at Cornell University, led by Lehmann, are working on ways to sequestrate carbon in biochar-enriched soil. In the United Kingdom, a biochar research centre has been set up at the University of Edinburgh; other European countries are following suit, and research projects are under way in countries from Canada to Australia. A few companies are in the early stages of trying to find ways to commercialise biochar production.
Copyright The Financial Times Limited 2009