By Professor Colin Prentice, AXA Chair in Biosphere and Climate Impacts
Further to previous posts on this blog regarding Owen Paterson’s recent speech to the Global Warming Policy Foundation, I would like to take this opportunity to correct his dismissive statement about biomass energy as a potential contribution to decarbonized energy production in the UK. This is what the former Environment Secretary said:
“Biomass is not zero carbon. It generates more CO2 per unit of energy even than coal. Even DECC admits that importing wood pellets from North America to turn into hugely expensive electricity here makes no sense if only because a good proportion of those pellets are coming from whole trees.
The fact that trees can regrow is of little relevance: they take decades to replace the carbon released in their combustion, and then they are supposed to be cut down again. If you want to fix carbon by planting trees, then plant trees! Don’t cut them down as well. We are spending ten times as much to cut down North American forests as we are to stop the cutting down of tropical forests.
Meanwhile, more than 90 percent of the renewable heat incentive (RHI) funds are going to biomass. That is to say, we are paying people to stop using gas and burn wood instead. Wood produces twice as much carbon dioxide than gas.”
There are two misconceptions here.
(1) It is extremely relevant that ‘trees can regrow’ – this is the whole reason why biomass energy is commonly accounted as being carbon neutral! To be genuinely carbon neutral, of course, every tonne of biomass that is burnt (plus any additional greenhouse gas emissions associated with its production and delivery to the point of use) has to replaced by a tonne of new biomass that is growing somewhere else. This is possible so long as the biomass is obtained from a sustainable rotation system – that is, a system in which the rate of harvest is at least equalled by the rate of regrowth, when averaged over the whole supply region.
Now it has been pointed out several times in the literature (e.g. Searchinger et al., 2009; Haberl et al., 2012) that if biomass is burnt for energy and not replenished (for example, if trees are cut down and the land is then converted to other uses), then it is not carbon neutral. Indeed, the carbon intensity of this form of energy production is at least as high as that of coal. Paterson may have been influenced by a report on this topic (RSPB, Friends of the Earth and Greenpeace, 2012) which drew attention to the “accounting error” by which energy derived from biomass might be classed as carbon neutral while actually being highly polluting. But this refers to an extreme scenario, whereby increased demand for forest products leads to no increase in the area covered by forests. In this scenario, biomass energy demand would have to be met from the existing (global) forest estate, drawing down the carbon stocks of forests and forcing builders to substitute concrete and other materials for wood. This would certainly be undesirable from the point of view of the land carbon balance; and carbon accounting rules should recognize the fact.
Nonethless, this extreme scenario is implausible. It assumes that the value of biomass as fuel would be comparable to that of timber (highly unlikely) and more generally that there would be no supply response to increased demand. In more economically plausible scenarios, the increased demand for biomass fuel is met by an increase in the use of by-products of timber production (which today are commonly left to decay or burnt without producing any energy), and by an increase in the amount of agriculturally marginal land under biomass production – including non-tree energy crops such as Miscanthus, as well as trees.
Paterson’s blanket dismissal of the potential for biomass production to reduce CO2 emissions is therefore not scientifically defensible. Sustainable biomass energy production is entirely possible, already providing (for example) nearly a third of Sweden’s electricity today. It could represent an important contribution to decarbonized energy production in the UK and elsewhere.
(2) It might seem to be common sense that planting trees (and never cutting them down) would bring greater benefits in extracting CO2 from the atmosphere than planting trees for harvest and combustion. All the same, it is wrong. The point is that just planting trees produces no energy, whereas planting trees for biomass energy production provides a substitute for the use of fossil fuels. There is an enormous difference. Indeed, it has been known for a long time that the total reduction in atmospheric CO2 concentration that could be achieved under an absurdly optimistic scenario (converting all the land that people have ever deforested back into forests) would reduce atmospheric CO2 concentration by a trivial amount, relative to projected increases due to burning fossil fuel (House et al., 2002; Mackey et al. 2013).
I thank Jeremy Woods (Imperial College) and Jonathan Scurlock (National Farmers Union) for their helpful advice on this topic, and suggestions to improve the text.
Haberl, H. et al. (2012) Correcting a fundamental error in greenhouse gas accounting related to bioenergy. Energy Policy 45: 18-23.
House, J.I., I.C. Prentice and C. Le Quéré (2002). Maximum impacts of future reforestation or deforestation on atmospheric CO2. Global Change Biology 8: 1047-1052.
Mackey, B. et al. (2013) Untangling the confusion around land carbon science and climate change mitigation policy. Nature Climate Change 3: 552-557.
RSPB, Friends of the Earth and Greenpeace (2012) Dirtier than coal? Why Government plans to subsidise burning trees are bad news for the planet. http://www.rspb.org.uk/Images/biomass_report_tcm9-326672.pdf
Searchinger, T. et al. (2009) Fixing a critical climate accounting error. Science 326: 527-528.
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