Carbon debt is not a reason to pull the plug on biomass electricity

14709789_sThis is a guest post by Dr Raphael Slade, research fellow at Imperial College’s Centre for Energy Policy and Technology.

To meet the 2020 renewable energy target the UK is going to need biomass, and lots of it. The Department for Energy and Climate Change is hoping for an additional 20-38TWh of biomass electricity. This will require around 12-23 million dry tonnes of biomass, most of which will be imported as pellets from North America and burnt in converted coal-fired power plants.

NGOs and the renewable industry at loggerheads
As imports increase there has been mounting opposition from Greenpeace, the RSPB and Friends of the Earth. The problem they’ve identified is that, if a 60 year old tree is cut down and burnt and it takes 60 years for another one to grow, the amount of carbon in the atmosphere increases in the short term. In other words, burning trees now creates a ‘carbon debt’ that will take many decades to repay. They argue that the way in which biomass carbon is accounted for under IPCC guidelines also significantly overstates the benefits of large scale bioenergy.

It follows from the NGOs’ reasoning that, to meet near term carbon targets, we should avoid turning saw-logs, that could otherwise be used for furniture or construction, into pellets and should prioritise using residues such as sawdust. (The argument that furniture is a form of carbon sequestration only makes sense if you think the total stock of furniture is increasing. If, for every new chair sold, an old one ends up in landfill or being burnt there is no change in the carbon stock and no sequestration).

NGOs fear that, as the market for pellets expands, the consumption of saw-logs and pulp-logs for energy will inevitably increase unless prevented by more demanding sustainability criteria. This argument threatens to undermine political support for policies such as the Renewables Obligation which subsidises bioelectricity production.  It has also infuriated the Renewable Energy Association which last month accused them of spreading “misinformation” and using arguments that are “half-baked”.

Understanding Carbon debt
The idea that a carbon debt might be incurred as a result of forest management is not new. Whether a debt arises depends on how a forest is managed and the balance of impacts between natural disturbance (wind, fire, pests) and human disturbance (harvesting). Forest Research categorises carbon debt into three types:

1. The first is the single tree example above. In this case the debt only appears because of the scale at which the calculation is done. If, instead of a single tree, a population of different age trees is considered, and each year only the annual growth increment is harvested, no debt arises.

2. The second occurs where the intensity of harvest is increased, reducing the total mass of trees across the entire forest. This might occur if harvesting is stepped up from every 100 years to every 60 years. In this case, the reduction in the average mass of the trees would result in a one-off carbon emission, which would need to be set against the benefits of energy production. This type of debt is most likely to occur when unmanaged forest is brought into management, but it also assumes that nothing is done to increase the productivity of the forest, such as increased replanting.

3. The third type of debt is where there is a choice between continuing to manage a forest or stopping and allowing it to carry on growing and sequestering carbon. If it is decided to keep managing the forest, then the opportunity cost of the decision is the carbon sequestration foregone. The problem with this argument is that, as a forest matures, the rate of carbon sequestration declines and it becomes more vulnerable to the natural disturbance that would result in some carbon being released anyhow. In the absence of a real opportunity to stop existing forest management this scenario is, in any case, hypothetical.

Using whole trees for bioenergy is mad but not necessarily bad
Using saw-logs and pulp-logs for bioenergy makes little economic sense as co-produced low grade wood is cheaper and likely to remain so. Defying the economics and turning  saw-logs into pellets would only generate a carbon debt if the logs come from an unmanaged forest; or if there are indirect impacts such as changes in the level of forest production elsewhere; or if there are changes in the use of wood substitutes such as concrete.

Indirect effects are very difficult to quantify and build into policy, as the brouhaha over indirect land-use change and biofuels for transport has shown. Consider the following alternatives:

  • Increased pellet use provides a market for low-grade wood. This lowers the cost of producing saw-logs. Sawn timber prices fall and we build more houses from wood rather than concrete. Overall this has a carbon benefit.
  • Increased pellet use increases demand for wood indiscriminately. Saw-logs are converted into pellets. The price of timber rises and we stop building wooden houses and use concrete instead. Overall this has a carbon penalty.

Working out whether the carbon benefits predominate is the realm of general equilibrium modelling, itself often a world of pain and a source of conflicting policy recommendations.

Sustainability certification might prevent the indiscriminate use of wood for pellets, but schemes are difficult to implement and are by no means a silver bullet.  A precautionary approach might be to slow the rate at which the pellet market develops, but this prolongs the use of coal and oil. Using pellets for combined heat and power would be more energy efficient but, unlike Scandinavian countries, the UK has very little district heating infrastructure and building a large scale network is likely to be slow and expensive.

Pinning our hopes on a technological breakthrough in carbon capture and storage risks delaying action now and is a gamble that may never pay off.

The RSPB recommends promoting anaerobic digestion; this makes sense, but is not an alternative to large scale power production, it is something that needs to be done in parallel. A utopian low carbon energy system wouldn’t include burning pellets in old
coal-fired power stations, but we need to be sure that the alternatives can deliver before pulling the plug.

No easy answers
The main driver for increasing global pellet production is policy designed to promote renewable energy and save carbon. The issue of carbon debt arises because the timeframes for policy and forest management decisions are dramatically different. Whether a carbon debt arises depends on how the biomass pellets are produced and the choice of timeframe.

Guaranteeing that pellets are produced sustainably is not easy. To find out whether there is a sustainable way forward for the pellet market, best practice and certification standards need the opportunity to co-evolve. Unfortunately a “let’s give it a go and see if we can make it work” policy is unlikely to be popular with anyone.


  • In the meantime – I would like to know what the correct figure for kgCO2/kWh of heat produced should be? Probably significantly more than the Carbon Trust currently specifies.

  • Pingback: Making a biomess: a very British conundrum | Post-Carbon-Living

  • Of course there are always going to be extreme ideas and actions on topics to do with energy sources – on both sides of the argument. I think the right approach is to obtain a balance; to include different energy solutions within a portfolio of varied energy sources.

  • Interesting comparison with concrete. Imperial College developed more benign cement (lower carbon dioxide emissions for manufacture). It could be argued that a concrete house is more durable than a timber (including the environmental impact of the paint) and therefore not as damaging.

  • Is the concept of a carbon debt being presented too simplistically? If trees are allowed to stand, I understand that some carbon sequestration into the soil will occur. However, if the wood is unmanaged, dead branches and old trees will fall to the ground and rot. Some of the emissions from this rotting process will be methane – so, is there a methane credit for managing the wood and using it for pellets, furniture or construction? There are also wider issues related to biodiversity with leaving wood to rot and the methodologies adopted for managing the wood.

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