Unlike fossil fuels, which have the disadvantage of releasing carbon into the biosphere that can never be recaptured, wood chips or pellets, biofuels, and other forest energy products are renewable, and the carbon released in energy production can be recycled indefinitely through a regenerated forest. In contrast, the burning of fossil fuels does not take place in a closed loop system, meaning the additional carbon released into the biosphere cannot be reabsorbed into the fossil fuel system but will result in increased atmospheric concentrations unless it is offset though carbon sinks, such as oceans or expanded forest cover. This fundamental difference is posing a challenge to the US Environmental Protection Agency (EPA) in its formulation of the Greenhouse Gas Tailoring Rule, which regulates various types of carbon emissions from energy production. The question has become, to what extent might a categorical exclusion from regulation be applied considering that all or most of the emissions are ultimately recaptured in subsequent biomass growth?
One approach for examining this question is a life-cycle assessment (LCA) of the carbon emissions associated with bioenergy. Such assessments are commonly used to evaluate the environmental implications of various products and activities, including the net effect on carbon and other greenhouse gas (GHG) emissions. However, a number of different LCA approaches exist, and they can generate very different findings.
At one end of the spectrum is an analysis with narrow, fixed boundaries—such as a particular forest—that looks at a single product over its lifetime while assuming that markets, prices, and investments are unchanged by the use of the product. At the other end of the spectrum is an LCA that incorporates changes in markets, prices, and investment levels directly attributable to the use of the forest products.
Comparing two case studies in the literature shows how the results can diverge. One influential study—undertaken by the nonprofit conservation group Manomet—used an LCA to examine the amount of carbon released and subsequently resequestered from the wood harvested from a single forest site for bioenergy. The study was commissioned by the state of Massachusetts to help inform the legislature about the feasibility of substituting wood biomass for coal in some of its electrical power generation. A concern of the legislature was meeting near-term emissions reduction targets.
The study found that in the near term, the carbon released by the wood biomass was somewhat greater than the carbon that would have been released by the coal it replaced. However, after a period of 32 years of regrowth, the carbon sequestration by the regenerating forest would have completely recaptured all the excess carbon emitted earlier. Furthermore, additional tree growth beyond 35 years would generate net positive sequestration compared to the situation where wood would not replace fossil fuels.
In another LCA study, I used a forward-looking model to examine a multi-site forest system. This approach incorporates changes in market conditions, including associated changes in harvests and investments in forest stocks. We found that as more bioenergy is demanded and enters the market, companies respond by investing in forest expansion to meet anticipated future demand. Thus, even in the short-term, the stock of the forest system will be maintained or expanded, thereby eliminating most or all of the short-term net carbon emissions while generating net emissions reductions over time.
The different results of various LCAs have generated confusion among regulators. But even with their differences, the entire range of studies indicates that declines in forest carbon sequestration associated with increased harvests for bioenergy would be offset by new forest growth given sufficient time. This clearly suggests that EPA should treat biomass emissions separately from the emissions of fossil fuels.
Manomet Center for Conservation Sciences. 2010. Biomass Sustainability and Carbon Policy Study. NCI-2010-03. Manomet, MA: Manomet Center for Conservation Sciences.
Sedjo, Roger. 2013. Comparative Life Cycle Assessments: Carbon Neutrality and Wood Biomass Energy. Discussion paper 13-11. Washington, DC: Resources for the Future.