Biochar: can charcoal really stop global warming?

A lump of charcoal
Can pyrolysed biomass ploughed into soil really avert the worst of climate change?
Biochar - the charcoaled remains of agricultural waste - is being hailed as a huge opportunity to reduce the levels of carbon dioxide in the atmosphere. But is the science sound, and do we have enough waste to go around?

In Australia, opposition leader Malcolm Turnbull has condemned the government’s climate policy as inadequate. Not because the targets are so low that, if agreed globally, they would almost certainly put us on track towards a greater than 2°C rise in global temperature. Nor because all of the targets could potentially be met not from real emissions reductions, but instead from ‘offsets’ abroad. Rather, his party (which, while in power, refused to take any action against climate change) now advocates that the country can achieve major reductions in emissions without any restrictions on coal burning. Instead they claim that targets could be met by applying large quantities of fine-grained charcoal to the soil. 

When biomass is exposed to high temperatures with little or no oxygen present (a process called pyrolysis) it yields two types of fuel that can be used for heat and power or be refined into transport biofuels. It also produces charcoal. Somewhere between 12 and 50 per cent of the carbon is retained in this charcoal, depending on the process. The more energy gained, the less charcoal produced. Bioenergy and ‘biochar’ (the name for charcoal used in soils) can be produced in small stoves, but a growing number of companies are building industrial-sized pyrolysis plants. 

Advocates claim that grinding charcoal down and ploughing it into the soil can dramatically reduce atmospheric CO2 levels. The carbon in biochar, they claim, will be permanently buried underground. Biomass is classed as 'carbon neutral' because emissions caused by burning are theoretically offset by photosynthesis as the plants regrow. Biochar enthusiasts take it a step further, claiming it is actually ’carbon negative’ i.e. would actually draw down CO2 from the atmosphere if large plantations of trees were grown, charred, buried and the process repeated again and again. However, this ignores the fact that monocultures, whether of trees or crops, are major sources of emissions, because they drive the conversion of carbon-rich natural ecosystems, and because they require high energy inputs as well as fertilisers which are linked to emissions of nitrous oxide, a greenhouse gas 300 times as power as carbon dioxide.

What about charring waste products? The problem is that ‘residues and wastes’ - meaning the removal of deadwood, tree stumps and roots, twigs and undergrowth - can themselves cause emissions as forests are depleted, and too often ecosystems that play an essential role in regulating the climate are degraded.

Keeping carbon in the ground

Let's suppose we could find a sustainable source of biochar. Would the carbon stay where we put it? Advocates claim that the carbon in biochar will remain in soils for thousands or tens of thousands of years, that it will make soils more fertile and reduce nitrogen runoffs.

Some seem to regard biochar as the answer to most current crises. According to the Biochar Fund, which works in Central Africa, it offers an answer to climate change, hunger and deforestation. The manager of an Australian company, Crucible Carbon, is enthusiastic:
‘When I first heard about biochar the hairs went up on the back of my neck,’ he said. ‘This is the best news on climate change I've ever heard.’

So how well-founded are the claims made for biochar’s climate and soil credentials? Claims are largely based on two types of evidence: the first one is the highly fertile and carbon rich soils which have been found in Central Amazonia, called terra preta or ‘black earth’.  Terra preta was created by indigenous farmers between 500 and 2500 years ago by adding charcoal - along with a highly diverse combination of  organic residues including river sediments, animal bones, kitchen waste and manure - to the soil. Their techniques were developed over a long time frame, and were adapted to particular local soils and climate conditions.

A second line of evidence comes from laboratory and short-term greenhouse studies. These confirm that charcoal is an important factor in the soil fertility and the stability of carbon in terra preta.

However the results from modern biochar vary widely. This is not surprising because soils are diverse, complex and dynamic ecosystems. Laboratory and greenhouse studies with controlled conditions cannot reveal the full picture. 

In medicine, no new drug could be released without clinical studies.  The equivalent for agriculture are field studies. These are worryingly scarce for biochar, particularly any long-term studies which look both at impacts on soil carbon and soil fertility. To date, one field study looking at both carbon and soil fertility in one area in Amazonia has been published and offers mixed results. No other such field study has been published.

Greenhouse studies suggest significant differences in the effects of biochar according to the temperature at which biochar is produced, the type of biomass used, the type of soil and the crops to which it is added. In some cases, fresh biochar boosts plant growth, in others it stunts growth. Some biochars encourage beneficial fungi, others discourage them. What is true for biochar from rice husks used on sandy tropical soil probably won't be true for biochar from conifers on humus-rich soil in the UK.  Without a wide variety of published field studies, impacts of different biochars on different soils simply cannot be predicted.

Biochar, with added extras

What is certain, though, is that charcoal on its own cannot feed plants, even if in some cases it can make fertilisers more effective. Compost and other organic amendments or synthetic fertilisers are also needed. Biochar companies are therefore looking at combining charcoal with fossil-fuel based fertilisers. Eprida, for example, is pioneering a fertiliser made from charcoal which has been used to scrub nitrogen-rich gases from coal power stations.

Many advocates downplay such ‘industrial’ practices and speak of converting only agricultural and forest wastes and residues to charcoal.  Yet, as environmentalist Vandana Shiva has pointed out, humus from the decomposition or composting of ‘wastes and residues’ is ‘living carbon’, while charcoal is ‘dead carbon’ that cannot normally be digested by essential soil organisms. She points out that organic farming has been shown to increase the amount of carbon in soil and that soil fertility depends on ‘living carbon’ in humus. Replacing humus with charcoal on a large scale could have negative consequences.

Evidence for the long term climate benefit of biochar is so far weak. It is known that 1-20 per cent of the carbon in biochar will turn into carbon dioxide soon after it is ploughed into the soil. What about the rest? Given the lack of opportunity for long term studies of modern biochar, it makes sense to look for evidence from similar situations. One of these is the charcoal created during wildfires.

If 80 per cent or more of wildfire charcoal produced since the ice age had been retained, soils worldwide would be far richer in charcoal than they are. Where has it gone? Some of it may have simply been eroded and washed into the ocean, but analyses of marine sediments do not support this hypothesis. A study in Western Kenya estimated how much charcoal remains in the soil from a series of previous wildfires. Seventy-two percent had been lost within two or three decades.

Also, there’s no guarantee that something won’t work out how to eat the pure carbon and then breathe it back out as CO2 - effectively rendering the whole process pointless. One laboratory study showed that certain microbes can live on black carbon and turn it into carbon dioxide. The species appear to be rare now, but they might well thrive if charcoal is added to vast areas of soil. At least as worrying is the evidence that charcoal boosts levels of soil microbes that decompose the organic carbon in humus, turning much of that into carbon dioxide. All of this suggests that a lot of the carbon in charcoal and soils can still find its way back into the atmosphere, even though the processes are not fully understood.

What goes in must come out

Another concern is pollution: any toxins in wood or crops are concentrated in ash and charcoal, whether they are pesticide residues or heavy metals from background air pollution. In fact, the first commercially sold biochar, EternaGreen, is made not just from biomass but also from old tyres and municipal solid waste, the burning of which is known to produce highly toxic compounds. Even biochar from untreated wood is not always 'clean': Norwegian scientists recently warned that wood ash contains such high levels of cadmium, zinc and lead that it should not be used as fertiliser and should qualify as toxic waste.

And finally, there is the question of how to incorporate biochar into the soil without making climate change worse. Biochar will almost certainly require tillage which disrupts soil structures and causes the loss of soil organic carbon. Also, fine biochar particles can be blown away by wind.  In a recent trial in Canada, preliminary results suggest that 30 per cent of the biochar applied to a field was blown away as dust during application. Windborne dust can travel across large distances, for example from the Sahara to the Amazon Basin. Already, dust is implicated in increased snow melt on North American mountains and faster melting of the Arctic ice. Black dust could significantly worsen this impact, since it will retain more heat. Charcoal has reportedly been used in Japan to help melt snow and lengthen the planting season – but this effect would be far less desirable on a warmer planet.

Moving fast

In spite of the concerns and unknowns, it is not just the Australian opposition party that proposes large-scale commercialisation of biochar. During the United Nations negotiations for a post-2012 climate agreement, proposals to include biochar into the Clean Development Mechanism (a financial system for funding climate change projects in the less-industrialised world) are being debated. Biochar would then become yet another way in which European or US companies could ‘offset’ the burning of coal, gas and oil and thus avoid having to reduce emissions.

Including biochar in a new climate agreement raises the spectre of large new industrial plantations. The Chair of the International Biochar Initiatve (IBI), Professor Lehmann, has claimed that biochar could sequester between 5.5 and 9.5 billion tonnes of carbon annually– that means pyrolysing a quantity of biomass containing twice as much carbon as is released from the burning of fossil fuels every year. Harvesting biomass for such large quantities of charcoal would require a minimum of 500 million hectares of plantations: about one and a half times the size of India, possibly much more. Claims that there are massive quantities of ‘wastes’ available for such purposes are unrealistic and other industries, such as biofuel and biogas companies are competing for what ‘wastes’ there are.

A new report by the United Nations Environment Programme warns that ‘the impacts of large-scale biochar production on biodiversity and long-term agricultural sustainability (e.g. nutrient depletion) are unknown’. In other words, large-scale biochar production could potentially start to replicate the monocultures planted for conventional biofuel, which involve deforestation, the displacement and eviction of rural communities and indigenous peoples, and increased hunger as food production is displaced.

A United Nations website lists the charcoal-rich terra-preta soils in Amazonia next to other biodiverse and sustainable farming systems developed by indigenous peoples and rural communities around the world. What links those different systems is biodiversity and adaptation to specific local conditions. In parts of Amazonia, charcoal was one element in sustainable farming systems. Isolating this element, industrialising it and promoting it as a 'one size fits all' global solution could lead us down a familiar path.

150 organisations worldwide recently signed a declaration urging caution regarding proposed large-scale biochar use – you can read the declaration here.

Almuth Ernsting is a researcher with Biofuel Watch.

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