Researchers contend that large-scale forest bioenergy is neither sustainable nor greenhouse-gas neutral (updated with link to full study)
21 April 2012
|Land management trade-off: maximizing productivity vs. carbon stocks. NPP= net primary production. Schulze et al. Click to enlarge.|
Large-scale bioenergy production from forest biomass is unsustainable and will increase greenhouse gas emissions, according to a group of international researchers in an invited analysis in the journal Global Change Biology/Bioenergy.
Some 60-70% of the global increment of woody biomass would be needed to produce 20% of current global primary energy supply, they note, and argue that such an increase in biomass harvest would result in shorter rotations, younger forests, lower biomass pools, depleted soil nutrient stocks and a loss of other ecosystem functions.
Depleted soil fertility requires fertilization that would increase GHG emissions, and deterioration of current biomass pools requires decades to centuries to be paid back by fossil fuel substitution, if paid back at all, they contend.
Thus, early suggestions that a large-scale forest biofuel industry would be greenhouse-gas neutral or even reduce greenhouse emissions “are based on erroneous assumptions,” according to the authors from the Max-Planck Institute for Biogeochemistry in Germany, Oregon State University, and other universities in Switzerland, Austria and France. The work was supported by several agencies in Europe and the US Department of Energy.
The general assumption that bioenergy combustion is carbon-neutral is not valid because it ignores emissions due to decreasing standing biomass and contribution to the land-based carbon sink. The notion of carbon-neutrality is based on the assumption that CO2 emissions from bioenergy use are balanced by plant growth, but this reasoning makes a ‘baseline error’ by neglecting the plant growth and consequent C-sequestration that would occur in the absence of bioenergy production, and it ignores the fact that fossil fuels are needed for land management, harvest and bioenergy processing.
Recent life cycle assessments cast doubt on the existence of emission savings of bioenergy substitution from forests. In the Pacific Northwest United States, policies are being developed for broad-scale thinning of forests for bioenergy production, with the assumed added benefit of minimizing risk of crown fires. This includes forests of all ages and thus timeframes of biomass accumulation. However, a recent study suggests that more carbon would be harvested and emitted in fire risk reduction than would be emitted from fires. Furthermore, policies allow thinning of mesic forests with long fire return intervals, and removal of larger merchantable trees to make it economically feasible for industry to remove the smaller trees for bioenergy. These actions would lead to even larger GHG emissions beyond those of contemporary forest practices.
Increased GHG emissions from bioenergy use are mainly due to consumption of the current carbon pool and from a permanent reduction of the forest carbon stock resulting from increased biomass harvest. When consumption exceeds growth, today’s harvest is carbon that took decades to centuries to accumulate and results in a reduction of biomass compared to the current biomass pool.—Schulze et al.
Among the concerns they detail are:
The general assumption that bioenergy is carbon-neutral is not valid.
The reduction of biomass and lost carbon sequestration by forests could take decades to centuries to be “paid back” by fossil fuel substitution, if paid back at all.
There are significant concerns about the economic viability of biofuels, which may require government mandates or subsidies.
A higher demand for biomass from forests will increase prices for the biomass, as in Germany where they have already increased in price 300-600 percent from 2005 to 2010.
An emphasis on bioenergy production from forests could lead to shorter rotation lengths, questionable management practices and increased dependence on wood imports.
Negative impacts on vegetation, soil fertility, water and ecosystem diversity are all possible.
Fertilizer use, another important source of greenhouse gas emissions, could increase.
The use of fossil fuels in the Industrial Revolution allowed previously degraded forests to recover in much of Europe and the US, while industrial-scale use of forests for biomass would likely reverse this trend.
If biofuels are desired, the researchers said, a better alternative would be to produce them on lands that once were forested but now are not, although that runs the risk of competing with food and animal forage production.
Society should fully quantify direct and indirect GHG emissions associated with energy alternatives and associated consequences prior to making policy commitments that have long-term effects on global forests. Reasonable alternatives for reducing GHG emissions on the order of the proposed bioenergy substitution include increased energy efficiency and reduced waste of energy via technological improvements and behaviour modification. There is a substantial risk of [sacrificing] forest integrity and sustainability for maintaining or even increasing energy production with no guarantee to mitigate climate change.—Schulze et al.
Ernst-Detlef Schulze, Christian Körner, Beverly E. Law, Helmut Haberl And Sebastiaan Luyssaert. Large-scale bioenergy from additional harvest of forest biomass is neither sustainable nor greenhouse gas neutral. GCB Bioenergy doi: 10.1111/j.1757-1707.2012.01169.x
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