Policy commentary

The Carbon Footprint of Wood for Bioenergy

Jun 1, 2012 | Roger A. Sedjo
This week, the series is relaunched with an exploration of a controversial topic—carbon emissions from biomass energy. Proponents claim that biomass is an abundant source of energy with no net carbon emissions—a closed carbon loop. Critics argue that that biomass use will directly or indirectly lead to greater carbon emissions, and that relying on it as a green energy source would therefore be a mistake. RFF Senior Fellow Roger Sedjo examines each side. 
Biomass energy, together with solar and wind, is expected to play a major role in the substitution of renewable energy sources for fossil fuels over the next several decades. The U.S. Energy Information Administration (EIA) forecasts increases in the share of biomass in U.S. energy production from 8 percent in 2009 to 15 percent by 2035. Of course, a major reason for the use of renewables is concern about greenhouse gas (GHG) emissions. Although biogenic materials, (wood, grasses, and plants, for example) do release carbon when combusted for energy, the general view—supported by the Intergovernmental Panel on Climate Change (IPCC)—has been that carbon emitted from biological materials can be viewed as in a closed loop, wherein the subsequent plant regrowth simply recaptures the carbon emitted earlier. Biogenic material, including wood, has therefore been viewed as “carbon neutral,” meaning that its use for energy generates zero net carbon emissions.

Recently, however, this view has been challenged on two accounts. First, a static accounting view of a forest (see Manomet 2010 in Further Reading) argues that the use of wood for bioenergy will result in a decrease in the forest stock and in an associated net reduction in sequestered carbon—meaning that the carbon in the forest will be transferred to the atmosphere, at least for a period of time. A second argument, advanced by Searchinger et al. in a 2009 article (see Further Reading), is that the use of biogenic material for bioenergy will lead to changes in land use associated with release of large volumes of GHGs. This occurs because, the argument goes, bioenergy use will raise demand, leading to increases in crop prices and providing incentives for natural land systems to be converted to crop production. This argument has been used particularly for biofuel production with respect to corn, corn prices, and ethanol.

The Searchinger et al. argument is not likely to be relevant to biomass used for electrical power, because the low-value wood and grasses used for this purpose cannot compete on the same lands as higher-valued crops. Rather, low-value biogenic feedstocks used for electric energy (for example, wood waste, more densely planted forests with excess residuals available for biomass energy, or woody biomass plantations) are typically established on lands poorly suited for cropping, such as abandoned or marginal agriculture lands, steeper slopes, or flood plains. This means that prime croplands are only minimally disturbed, hypothesized land use changes to croplands don’t occur, and neither do the associated carbon releases.

Debate about the carbon neutrality of biomass came to a head with the release of two letters sent to Congress. The first, from 90 scientists on May 17, 2010, questioned the treatment of biomass energy as carbon neutral, arguing that such treatment could undermine legislative goals to reduce emissions. Another letter, from 110 scientists on July 20, 2010, took the opposite view and expressed concern over equating biogenic carbon emissions with fossil fuel emissions. This was contemplated in the U.S. Environmental Protection Agency’s (EPA) Tailoring Rule, which governs EPA’s permitting process for GHG emissions from large facilities. The letter noted the significant role biomass could play for energy production in the United States.

A dispute has since ensued over the carbon neutrality of biomass energy and the nature of regulations that may be applied. Should biomass energy be treated like a fossil fuel, the carbon emissions of which are viewed as irreversible and are regulated as such? Or, does the fact that biomass is renewable and hence, when regenerated, offsets its own emissions imply a different treatment for biomass emissions as suggested by the IPCC? The static accounting view appears to be a stronger argument, at least for bioenergy in the electricity industry, but a number of factors provide grounds for skepticism.

The use of bioenergy is a two-way street, affecting supply as well as demand. Given an expectation of a significant increase in the use of biogenic energy, such as forecast by the EIA, it is clear that bioenergy harvests from forests will reduce stored forest carbon and release carbon into the atmosphere. It is also clear, however, that an anticipation of higher levels of future demand for wood biomass will encourage increased investments in forests for biomass energy. Indeed, to the extent that these investments (think forest expansions, new plantings, and improved, faster-growing seedlings) need be undertaken in anticipation of future demand not yet realized, forest expansion will precede harvests and carbon will be newly captured, thereby preceding its future release. Because these investments are trigged by anticipated increases in future demand, the expansion of forests and harvests is additional to what would have been the “business-as-usual forest.”

Most commercial forests in the United States are managed and increasing percentages involve investments in tree planting. Forest management allows for increased forest intensity, as with the denser planting of tree seedlings and more frequent pre-commercial thinnings. Should these thinnings, which traditionally have not been undertaken or have simply been turned to waste, now have value as biomass, they become economically important. Hence, market incentives will promote investments to increase forest stocks as part of traditional commercial forestry as well as through biofuel-dedicated forest plantings, often in anticipation of their use. A static analysis would not capture these anticipatory market adjustments. Indeed, an increasing demand for biomass for energy creates a self-generating incentive for markets to invest in its production. For wood, this means that higher biomass prices also create incentives for increased investment in forests and therefore increasing capacity for carbon sequestration.

Although some may question the notion of investments in anticipation of future demand, the entire sector of commercial forestry, which must wait 20 to 50 years before a tree is ready to be harvested, is predicated on making investments today in anticipation of future, as-yet-unrealized demand. Over the last 40 years, for example, almost 50 million acres of commercial forest have been planted in the United States and forest stocks rose even as harvests experienced their peak levels. Commercial forests, which provide the vast majority of industrial wood produced in the United States, involve investments based on a tree’s expected future market value, as investors anticipated future wood markets. These forests are managed largely on a sustainable basis and respond to market forces. So, too, higher biomass prices can be expected to both result in reductions in forest carbon through larger harvests and also to offset increases in forest carbon emissions as forest managers invest in biomass for the future to substitute for fossil fuels, through a combination of forestland expansion and increased forest management.

Forest dynamic modeling studies have shown that earlier static analysis fails to incorporate supply responses as well as demand changes into their carbon neutrality analysis. Thus, static analysis ignores the self-regulating feature of markets for renewable resources and commercial forests and the associated carbon sequestration adjustments. Specifically, dynamic analysis examines how an increase in the demand for wood as bioenergy affects investments in forestry and the volumes of carbon in the forest stock. It shows under what conditions that stock might increase or decline. Also, a dynamic approach of the entire multi-stand commercial, multiple-forest system captures the interconnectedness of the forest system both spatially and intertemporally. Indeed, decisions on a site are not independent of activities on other sites, both in time and space as market signals provide coordinating information.

This analytical approach demonstrates conceptually what the tree growing experience of the late 20th century illustrated: that when responding to an anticipated substantial increase in future demand, the forest stock will rise as fast or faster that harvest draw-downs. Again, this result reflects the response of managers to increase the area (and intensity) of forest production, thereby offsetting the carbon released in the harvest for feedstock for bioenergy production. This finding demonstrates that wood bioenergy production can be carbon neutral in a host of situations, and particularly when demand is anticipated to increase substantially into the future, as is the current case in the United States.

Further Reading

Searchinger, T., S.P. Hamburg, J. Melillo, and W. Chameides. 2009. Fixing a Critical Climate Accounting Error. Science 326.

Manomet Center for Conservation Sciences. 2010. Biomass Sustainability and Carbon Policy Study. NCI-2010-03. Manomet, Massachusetts: Manomet Center for Conservation Sciences.