Chatham House’s recent paper, Woody Biomass for Power and Heat: Impacts on the Global Climate, highlights how the use of wood for electricity generation and heat in modern (non-traditional) technologies has grown rapidly in recent years, and has the potential to continue to do so. EU member states’ national targets for renewable energy generation agreed in 2009 have helped ensure that the EU is now the world’s largest producer and consumer of wood for energy. And although other member states use wood more extensively for heat, the UK is the EU’s largest user for electricity generation, mostly sourced from the US and Canada.
Wood for energy often has a positive image: a natural product of growing forests. The biomass energy industry, which has grown rapidly on the back of government subsidies, likes to contrast it with dirty coal or oil. They point to the government’s sustainability criteria, which notionally guarantee a reduction of at least 60 per cent in greenhouse gas emissions compared to the fossil fuels the biomass replaces.
The problem with this happy picture, however, is that in fact biomass, when burnt, emits more carbon per unit of energy than most fossil fuels. The exact amount varies with the type of biomass and the type and age of the power plant, but figures from the Drax power station, Europe’s largest consumer of wood pellets, show that in 2013 it emitted about 13 per cent more carbon dioxide per unit of energy generated from biomass than from coal.
How is this consistent with meeting the government’s requirement for a 60 per cent reduction in emissions? Only by completely ignoring the carbon emitted when the wood is burnt; the sustainability criteria measure only supply-chain emissions from harvesting, processing and transporting the wood. (Direct land-use change – for example, clearance of the forest for agriculture or urban development – also falls outside the criteria, but biomass for energy generally originates from existing forests.)
This treatment of combustion emissions as zero – and thus, the awarding to wood the same kind of financial and regulatory support as other renewables such as solar PV and wind – is justified on the basis that the carbon contained in woody biomass is part of the natural forest cycle. The carbon released during combustion was absorbed by forest growth in the past and will be reabsorbed by forest growth in the future; in contrast, fossil fuels originate outside this cycle and their combustion adds carbon to the atmosphere.
But this argument rests on a basic fallacy. Carbon is carbon, wherever it comes from, and if you burn wood for energy, you increase carbon dioxide concentrations in the atmosphere (by more than if you had used fossil fuels), and thereby contribute to climate change. The fact that the carbon emitted was absorbed by growing trees in the past is simply irrelevant. After all, when it’s harvested you don’t have to burn it; you could use it for construction or furniture or window frames or a host of other uses, fixing the carbon in wood products rather than emitting it to the atmosphere.
It is true that continued forest growth will absorb carbon in the future, but the process is a long one, taking decades or even centuries if whole trees are harvested and burnt. Replacing large mature trees, with plentiful leaf cover absorbing large volumes of carbon dioxide, with small young ones mean that the rate of carbon uptake will be far lower for years. On top of that, the impact of harvesting itself releases soil carbon into the atmosphere, further accelerating climate change.
The impact on the climate of using sawmill or forest residues for energy rather than whole trees is undoubtedly lower, since these tend to be wastes from other industries which harvest trees for their own purposes, and do not imply any additional harvesting. Sawmill wastes which, if left to themselves, would rot and release their stored carbon into the atmosphere in a matter of months or years, are in many ways the ideal feedstock; it makes sense to use them for energy rather than leave them to decay. However, mill residues are already intensively used and there seems little room for expansion; a survey in the US in 2011 found that over 99 per cent of mill residues were already used, mainly for energy and wood products such as particleboard.
Forest residues are the parts of harvested trees that are left in the forest after log products have been removed, including stumps, tops and small branches, and pieces too short or defective to be used; these can amount to as much as 40–60 per cent of the total tree volume. Their impact on the climate if used for energy varies significantly. While the smallest pieces tend to rot and release their stored carbon into the atmosphere quite quickly, if left in the forest, they are generally not suitable for use for energy, as they contain too much dirt and ash to be burnt cleanly. Larger pieces are more suitable but take much longer to decay; burning them for energy instead of leaving them in the forest thereby increases carbon concentrations in the atmosphere for years or decades. And on top of that, a portion of the carbon and other substances contained in the residues is transferred to the soil as they decay; their removal from the forest for energy may reduce both soil carbon and the levels of the nutrients trees need to grow, again with negative impacts on the climate.
The biomass industry generally likes to claim that it uses mainly mill and forest residues, though on closer inspection the categories they report often contain whole trees, perhaps classified as ‘unmerchantable’ or similar. (This is not helped by the fact the categories used by Ofgem, for example, to whom UK biomass users have to report, are confusing and potentially overlapping.) Several independent studies, however, have concluded that the use of mill and forest residues is in reality substantially lower; pellet plants in the US – the UK’s main source of supply – in fact source about 75 per cent whole trees.
Setting aside these arguments about feedstock, however, can it be safely assumed that future forest growth allows us to treat biomass as carbon-neutral? If the trees would have grown anyway, even in the absence of the biomass energy industry, it cannot be assumed that their future absorption of carbon cancels out the carbon emitted when wood is burnt. If the rate of carbon absorption in forests remains the same whether or not some of the harvested wood is burnt, then clearly, the best outcome for the climate in the short and probably medium term is not to burn it, but to use it for wood products or leave it to decay slowly in the forest. This is not an academic argument: the current global rate of emissions of greenhouse gases is incompatible with the aims of the Paris Agreement and may risk triggering irreversible tipping points in the Earth’s climate system. We need to reduce carbon emissions now, not in several decades’ or centuries’ time.
The biomass industry likes to point to the expansion of US forests in recent decades to show that forests overall have been absorbing more carbon even while increasing volumes are burnt for energy – sometimes implying that this forest growth has been encouraged by the demand for energy. But in fact US forest expansion started in the 1950s, decades before European subsidies stimulated the expansion of the modern biomass industry. And there is little evidence of recent overall forest growth in the US southeast, the location of almost all the pellet plants supplying European demand. In any case, the point is not whether US (or European) forests are expanding, but whether they would have grown at a different rate if part of their wood had not been burnt for energy. If they would have grown at the same rate, or faster, in the absence of biomass energy use then it cannot be assumed that using wood for biomass is good for forests, or the climate.
Redirecting public money
There is no question that renewable energy policy and forest policy both have a critical role to play in the mitigation of climate change. But governments have limited resources to deploy in their support, which is why the Chatham House paper questions whether it is really a good use of public money to subsidise activities which release stored forest carbon into the atmosphere, thereby increasing carbon emissions and accelerating climate change.
I argue instead that support should be limited to those feedstocks which genuinely reduce carbon emissions over the short term – i.e. mill residues and post-consumer wood waste. This would not only have a positive direct impact on the climate but would also release more resources for genuine zero-carbon technologies, such as solar, wind or tidal – and perhaps also for programmes encouraging afforestation and the more extensive use of wood in buildings and products. Use it, don’t burn it.