This last week there have been a few reasonably sensational articles in the news about using biomass for heating and power. These include Most wood energy schemes are a ‘disaster’ for climate change by the BBC, and £450m lost over failed green power programme by the Times. They have both been sparked by a paper published by Chatham House which essentially suggests that the use of biomass as a fuel is not sustainable, because of adverse effects on the climate (‘worse than coal’ apparently) when they are burnt.

Since this report seems to have sparked a wider debate in the media about the relative carbon benefits of biomass fuels, I thought that it might be helpful if I pointed out some of the ways that this is argued, to shed a bit of light on the subject.

First things first: trees absorb carbon as they grow, and typically release it when they die (through decomposition or combustion) though this is not automatically the case, and trees which are harvested for long lived timber products or which are left in low oxygen conditions (such as bogs) may retain carbon for many years. Taken in isolation, burning wood may look like a bad thing as it immediately releases carbon dioxide but this is certainly not the whole story.

The argument that is currently taking place is essentially about how we account for this carbon, where it has come from, where it is going, how fast, and crucially, what would have happened without our intervention?

Area and timescales

The perspective we take in terms of areas and timescales has a strong influence on how sustainable the use of biomass appears.

In terms of area, a felling may look devastating in terms of its impact on small areas, or single trees may appear insignificant in terms of its impact on a regional or national forest area. Further complexity is revealed when we take account of displaced forestry activity outside the immediate area (leakage). As well as other possible knock on effects, such as changes in the permanence of forests or changes in species composition.

Equally if we look only at the year in which the tree is felled and burned- the carbon balance may seem very poor, but if we look at a forest over the length of a rotation (often 50 to 100 years) we may actually find that the overall balance is favourable. In general, shorter timescales tend to appear less favourable (forests take a long time to regrow) but we do need to remember that fossil fuels (the most common alternative) take many millions of years to form – so even the slowest growing tree species will outperform them in terms of how renewable they are by several orders of magnitude.

This dynamic change in growth rates in forests depending on site conditions, and the duration over which we measure them, means that it is possible to influence how the statistics appear depending on where we draw the boundaries to measurement. For example: Graphs 1 and 2 below show extremes of this where the timeframe is either limited to the most destructive phase of the rotation, or stretched to incorporate a full rotation into the future, which (while it may be an optimal outcome) cannot be guaranteed.

Graph 1: A (vastly simplified) single tree carbon stock over 2 rotations. The measurement duration (orange box) gives a bad impression of biomass because it omits the regenerative phase.
Graph 1: A (vastly simplified) single tree carbon stock over 2 rotations. The measurement duration (orange box) gives a bad impression of biomass because it omits the regenerative phase (click for enlarged graph).

 

Graph 2: The same single tree carbon stock, looking at a full rotation (100 years). The measurement duration (orange box) gives an over-optimistic view, because it incorporates a full regeneration over the next rotation (which of course may not actually happen)
Graph 2: The same single tree carbon stock, looking at a full rotation (100 years). The measurement duration (orange box) gives an over-optimistic view, because it incorporates a full regeneration over the next rotation, which of course may not actually happen (click for enlarged graph).

In reality these represent two possible scenarios and a vigorous debate is currently taking place about the appropriate measurements, and how we set up the accounting methodologies to ensure that our actions actually provide the benefits we want. This is broader than a simple biomass issue – discussions over how to quantify the modification of carbon stock during land use change and other land management activities are ongoing (for example see Timmons et al., 2016; Röder and Thornley, 2016; Buchholz et al., 2016).

Comparisons

To determine whether burning is a net benefit to human efforts to curb climate change we need to decide what we are comparing it to. This is a case of quantifying the carbon absorbed and released during the growth, harvesting, processing, and combustion of a biomass fuel and then comparing it with a baseline scenario.

Baselines (or ‘counterfactuals’) are assumed substitutes or other courses of action which would take place in the absence of biomass use for fuel (see Ter-Mikaelian et al., 2015). For example, a common element in many of these studies is the use of an equivalent amount of fossil fuel if the biomass was not available. Other factors might include the use of different parts of the tree for different purposes, assumptions about disturbance, land use change, forest growth rates and carbon held within the soil.

A great deal of work has been done by the scientific community to examine these assumptions, and the debate is far from over. There is significant variation in the results obtained by different studies looking at the net carbon balance of biomass utilisation, and these results seem to be influenced by a range of variables (Buchholz et al., 2016). For example, whole tree harvesting for biomass typically results in a less advantageous carbon balance, whereas removal of dead material in areas where forest fires occur naturally will usually result in a substantial carbon gain.

Conclusion

It is disappointing to see that a body as well respected as Chatham House have published such a one-sided document which doesn’t represent the full picture of forests and carbon or indeed make any reference to the complexity of the subject. Mr Brack makes a lot of assertions about the relative efficiencies, and the types of feedstock used to fuel biomass installations, but since there are no references to other sources within this section of the paper (Brack, 2017 sec. Is biomass carbon-neutral? p9) it is difficult to see where they come from, and whether they are valid or balanced.

While it is understandable (and indeed positive) that Mr Brack, with his background in research relating to environmental crime and illegal logging is sensitive to these issues, it does appear that in this document he has rather overstated the case against biomass fuels without providing adequate supporting evidence. It’s hard to see this as a well-reasoned argument, because it looks so much more like a simple appeal to emotion.

While I don’t think anyone is trying to say that biomass is the complete answer to renewable electricity generation, it may provide part of an answer, using particular feedstocks over specific timescales. The complexity of this field, how we balance our requirement for electricity, with our need to store carbon and manage forests to retain other functions is vast. Wood for biomass fuel is one of a wide range of different products that we take from forests, and the incorporation of this new product type into the marketplace does not remove the need to maintain sustainable forest management.

However, because of the ongoing debate, it looks as though we have to maintain the position that “Bioenergy systems can cause both positive and negative effects and their deployment needs to balance a range of environmental, social and economic objectives that are not always fully compatible” (Creutzig et al., 2015).

Or in other words: I think you’ll find it’s more complicated than that….

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Will Rolls has been working in the fields of forestry and biomass energy since 2005, including several years at the Biomass Energy Centre, the UK government’s hub for bioenergy technology transfer. Will is currently studying for a PhD on biomass and carbon at Leeds University.