Posted June 21, 2010, at 4:19 p.m. CST

There has been much press coverage of our study about using forest biomass for energy in Massachusetts. This study was commissioned and funded by the Massachusetts Department of Energy Resources. Many of the resulting press articles have oversimplified the results. Indeed, a key lesson of the study is that understanding the greenhouse gas (GHG) impacts and benefits of using wood for energy is more complex than most people have assumed, and that a life-cycle assessment is needed in order to assess these GHG costs and benefits.

Here we seek to provide some additional clarifying comments about the study given the substantial press coverage that followed the release of our report on June 10. The study can be downloaded from www.manomet.org. We encourage interested parties to read the report, or at least the Executive Summary, to understand first-hand what the study concludes.

One commonly used press headline has been ‘wood worse than coal' for GHG emissions or for ‘the environment.' This is an inaccurate interpretation of our findings, which paint a much more complex picture. While burning wood does emit more GHGs initially than fossil fuels, these emissions are removed from the atmosphere as harvested forests re-grow. As discussed in more detail below, the timing and magnitude of the recovery is a function of forest productivity, land management choices, and technology and fuel characteristics. To help stakeholders and policy makers gain a more accurate and complete understanding of the study results, we revisit some of the key points found in the report.

• First, the study addresses only the carbon cycle implications of biomass harvested from actively managed, natural forests. We do not analyze woody biomass from other sources, for example biomass plantations, land clearing, tree work and landscaping wastes, or construction waste. These materials can be important potential sources of biomass-ones that likely have very different carbon cycle implications than biomass from natural forests-and merit careful and separate consideration in biomass policy development.

• Second, we do not analyze the impacts of non-GHG pollutants emitted from energy generation facilities (e.g., particulate matter, NOx, SO2, or other hazardous air pollutants such as mercury). Emissions of these pollutants vary considerably between wood and fossil fuel energy systems, and are an important consideration in determining the relative merits of biomass and fossil fuels.

• Third, we clearly state that the study is specific to the forest and energy situation in Massachusetts. While the study methodology is transferable to other regions of the country, the specific results of our analyses, particularly the carbon cycle implications, cannot be readily applied to states where the biophysical characteristics of forests, forest management practices and energy sector differ significantly from Massachusetts.

• Fourth, based on the results of our economic analysis of potentially available wood supplies, we concluded that, overall, biomass harvests in the state would include a mix of logging residues (tops and limbs) and low-quality whole trees or logs (pulpwood and low-grade saw logs). The relative proportions of these materials in the biomass feedstock have an important effect the timing of GHG impacts and benefits to the atmosphere. We pointed out that these proportions will be different in other situations or states, and that conclusions about the impacts on the atmosphere will necessarily be different. Each state or situation (or even specific biomass facility) would need to do its own analysis to properly evaluate the GHG costs or benefits.

• Fifth, there has been some confusion about whether our assessments of GHG implications are based on a ‘life-cycle' analysis of biomass and fossil fuel carbon emissions. In fact, the study considers the ‘upstream' costs of producing and transporting both biomass and fossil fuels, and the stack emissions from burning these fuels. Capture of carbon in growing forests is also part of our life-cycle framework.

• Sixth, the study makes no recommendations regarding the development of specific policies to address GHG emissions from biomass. The intent of the study is simply to provide the best possible information and analysis of the carbon cycle implications to Massachusetts decision makers as they develop biomass energy policies for the state. These decision makers will need to carefully weigh the relative importance of nearer term increases in GHG emissions against longer-term benefits. The study did show that using wood for energy generally results in greater emissions of GHGs per unit of energy than using fossil fuel. These differences are a function of the lower embedded energy content of wood relative to fossil fuels, inclusion of emissions from upstream production and transportation of fuels, as well as differences in the efficiency of the various energy generation technologies. We called the excess emissions from burning biomass for energy the carbon debt. But because trees can grow back, this debt can be paid off and a carbon dividend can be achieved as GHG levels are reduced to levels lower than they would have been had only fossil fuels been burned.

The length of time it takes to pay down the debt and realize dividends depends on four factors:

1.The lifecycle of the wood (e.g., logging debris, whole trees, trees vulnerable to catastrophic events) in the absence of the biomass energy opportunity.

2. The type of energy that will be generated (heat, electricity, combined heat and electricity), because different types have different efficiencies and thus different CO2 emissions profiles.

3. The type of fossil fuel being displaced (coal, oil, or natural gas), because different fuels have different emissions profiles.

4. The management of the forest-management can either slow or accelerate forest growth, and therefore recovery of carbon from the atmosphere.

Unless these factors have been assessed, as they have in our report for Massachusetts, it is not possible to estimate the time it would take to pay off the debt or the magnitude of the carbon dividends-making it difficult to draw conclusions about GHG implications of using wood. For example, when the wood used to fuel an energy facility is all, or nearly all, logging debris that would have decomposed in the forest anyway, the debt period can be relatively short, even for large-scale electricity generation where biomass replaces coal.

Conversely, fueling an electricity generating facility with mostly whole (live) trees will likely incur a longer carbon debt period (up to several decades) before GHG benefits are realized. Thermal uses of wood generally have a shorter debt period than electricity generation with wood. Renewable energy policy makers who seek to reduce GHG emissions by using wood for energy will be well served by assessing these four factors for the specific energy and forestry contexts of their state or region.

Finally, there are many other considerations besides GHG emissions when making energy policy-these include energy security, air quality, forest recreation values, local economics, other environmental impacts besides just GHG emissions, and quality of place, among others.

We hope these comments help to more accurately present the major findings of this study and to better inform policy makers and stakeholders. We welcome and invite feedback on our study, as well as improvements or corrections to our approach.