The conversion to biomass energy has played a key role in reducing our dependence on fossil fuels. But is this renewable energy source really as green as we first thought?
08 Jan 2020 | Kate Ravilious | physicsworld.com
However, those calculated (Co2) savings rest on a few key assumptions: first, that the carbon released when wood pellets are burned is recaptured instantly by new growth; second, that the biomass being burned is waste that would have released carbon dioxide naturally when it rotted down. But are those assumptions right?
Advocates of biomass energy claim that when forests are harvested sustainably, and the timber industry thinnings are used as fuel, the smokestack emissions are cancelled out by the carbon absorbed by forest regrowth. However, some scientists say that this carbon accounting simply doesn’t add up. “Wood bioenergy can only reduce atmospheric CO2 gradually over time, and only if harvesting the wood to supply the biofuel induces additional growth of the forests that would not have occurred otherwise,” says John Sterman, an expert on complex systems at Massachusetts Institute of Technology (MIT) in the US. The time needed for the regrowth to mop up the additional CO2 is known as the “carbon debt payback” time, and it is this that is hotly disputed.
Sterman – who is keen to point out that his bioenergy research is independent, funded neither by the forestry or bioenergy industry, nor environmental groups – says he was initially optimistic about biomass energy. “The climate crisis is so dire that when we began our work, I dearly hoped that wood would prove to be part of the solution,” he says. But the more he looked into it, the more concerned he became.
Using a lifecycle analysis model, Sterman and his colleagues calculated the payback time for forests in the eastern US – which supply a large share of the pellets used in the UK – and compared this figure to the emissions from burning coal. Under the best-case scenario, when all harvested land is allowed to regrow as forest, the researchers found that burning wood pellets creates a carbon debt, with a payback time of between 44 and 104 years (Environ. Res. Lett. 13 015007). “Because the combustion and processing efficiencies for wood are less than coal, the immediate impact of substituting wood for coal is an increase in atmospheric CO2 relative to coal,” Sterman explains. “This means that every megawatt-hour of electricity generated from wood produces more CO2 than if the power station had remained coal-fired.”
Sterman stresses that he is not advocating a return to burning coal. “Coal and other fossil-fuel use must fall as soon and as fast as possible to avoid the worst consequences of climate change. [But] there are many ways to do that, with improving energy efficiency being one of the cheapest and fastest.”
However, biomass energy advocates say that Sterman’s carbon debt is a fallacy, created by assessing the forest stand by stand (referring to a group of trees planted at the same time and then harvested a few decades later) rather than viewing it at the landscape level. “What actually happens is that one part of the forest is harvested (typically 3–4%) while the rest of it grows (typically net growth after harvesting is about 0.7 to 1% per year), supported by active forest management,” says Stevenson in London.
But Sterman argues that the opposite is actually true. “Harvesting one part of a growing forest does not cause trees miles away to grow even faster,” he says. “The trees harvested for bioenergy would have continued to grow, thus removing more CO2 from the atmosphere. The faster a forest is growing, the greater the future carbon storage is lost.”
It had been assumed that young trees mop up more carbon than old ones because they are fast-growing, but recent studies have revealed that ancient woodland growing in temperate regions takes up more CO2 than young plantations. This is because in some cases, growth accelerates with age and CO2 absorption is approximately equivalent to biomass (Nature 507 90). “Far from plateauing in terms of carbon sequestration at a relatively young age as was long believed, older forests (for example over 200 years of age without intervention) contain a variety of habitats, typically continue to sequester additional carbon for many decades or even centuries, and sequester significantly more carbon than younger and managed stands,” researchers write in the journal Frontiers in Forests and Global Change (2 27).
From growth to rot
But even if old trees are continuing to draw down CO2, what happens when a tree dies? Current carbon accounting assumes that all the carbon from dead wood is released back into the atmosphere again. Removing forest thinnings and burning them to produce energy is therefore viewed as better than leaving them on the forest floor to rot. Indeed, Biomass in a Low-carbon Economy – a report produced in November 2018 by the UK Committee on Climate Change – states that “Unharvested, the maintenance of these carbon stocks in perpetuity is essential to ensure that the sequestered carbon does not re-enter the atmosphere.”
However, Sterman argues that this fails to take account of the entire system. “We need to consider the carbon stored in the soil too. Removing and burning ‘waste’ wood lowers the source of carbon for forest soils. This allows soils to become net sources of carbon to the atmosphere as bacterial and fungal respiration continue to release soil carbon into the atmosphere,” he says.
Mary Booth, an ecosystem ecologist and director of the Partnership for Policy Integrity in Pelham, Massachusetts, shares Sterman’s concerns. In 2017 she used a model to calculate the net emissions impact – the difference between combustion emissions and decomposition emissions, divided by the combustion emissions – when forestry residues are burned for energy. “It is the percentage of combustion emissions you should count as being ‘additional’ to the CO2 the atmosphere would ‘see’ if the residues were just left to decompose,” she explains. Her calculations revealed that even if the pellets are made from forestry residues rather than whole trees, combustion produces a net emissions impact of 55–79% after 10 years (Environ. Res. Lett. 13 035001). Even after 40 years her model shows that net emissions are still 25–50% greater than direct emissions. Like Sterman, Booth concludes that it takes many decades to repay the carbon debt, and she concludes that biomass energy can’t be considered carbon neutral in a timeframe that is meaningful for climate-change mitigation.
Booth was so concerned by what she found that she co-ordinated a lawsuit against the EU in March 2019 (eubiomasscase.org), challenging its treatment of forest biomass as a climate-friendly renewable fuel. “Our position is that policies should count biogenic carbon emissions, and burning forest wood for fuel should not be eligible for renewable-energy subsidies,” says Booth. Currently she is waiting to hear if the court will accept the case.
But even if living trees can claw back these carbon-dioxide emissions relatively quickly, there is a danger in front-loading our emissions in this way. “Regrowth is not certain,” says Sterman. “Forest land may be converted to other uses such as pasture, agricultural land or development. And even if it remains as forest, wild fire, insect damage, disease and other ecological stresses including climate change itself may limit or prevent regrowth, so that the carbon debt incurred by biomass energy is never repaid.”
Taken from the January 2020 issue of Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.