Wood biomass - carbon impact a matter of scale
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A 50-year-old tree is cut down in the forest, ground into chips, trucked to a power plant and burned to generate electricity. All of the carbon stored in that tree has just been released into the atmosphere. It will take another 50 years before a tree that size grows back to replace it, sequestering the same amount of carbon. Fossil fuels also were used to harvest, haul and process the tree - and each of those activities released more carbon into the atmosphere. How can this sort of activity possibly help reduce net atmospheric carbon anytime soon?
Well, it certainly can; and the biggest mistake researchers make when studying the carbon impact of biomass is the scale of their analyses. Several researchers quite literally do not see the forest through the trees.
In order to properly assess the carbon impact of biomass energy one must look at a larger regional or even global scale. It would take decades for a seedling from the ill-fated tree described earlier to sequester the amount of carbon released by its parent. However, its dozens of neighbor trees are still growing and, in doing so, will continue to sequester carbon. It may still take these dozens of trees years to make up for the carbon emitted by our ill-fated tree, but working together they can certainly help speed up the process. As we scale up from tree to stand to landscape, etc. we see that the carbon impact of forest biomass energy is lessened as the scale of analysis increases.
In "Carbon 101: Understanding the carbon cycle and the forest carbon debate" Bowyer et al. (2012) very carefully explain the differences in life cycle analysis (LCA) of wood biomass at the stand, parcel and landscape levels. They conclude, "The stability of carbon stores resulting from balanced forest management is even more evident at the landscape scale. At this scale the amount of carbon stored in the forest remains essentially the same, even though every year a portion is harvested. The stability of carbon stocks is attributable to growth of trees across the landscape which offsets the small portion of trees harvested in any given year."
On an even larger scale, in "Economic Approach to Assess the Forest Carbon Implications of Biomass Energy," Daigneault, Sohngen and Sedjo (2012) used a dynamic model to estimate the impact of forest biomass energy on global timber production and carbon stocks over the next 50 years. Their study showed expanded demand for biomass energy increases timber prices and harvests, but reduces net global carbon emissions because higher wood prices lead to new investments in forest stocks.
In conclusion, in order to fully understand the carbon impact of biomass energy one must scale up their analysis in order to see the forest through the trees. With that, I will say forest biomass that comes from well managed, sustainable sources does not increase atmospheric carbon. Renewable energy from wood biomass, which comes from regions (such as North America) where forest growth exceeds removals, provides a significant reduction in atmospheric carbon over fossil fuel sources.
Article and sketch by Seth Walker - Associate Bioenergy Economist with RISI. He can be reached at swalker@risi.com or +1.781.734.8992.
Bowyer, J., Bratkovich, S., Frank, M., Howe, J., Stal, S., Fernholz, K. 2012 "Carbon 101: Understanding the carbon cycle and the forest carbon debate" Published by Dovetail Partners Inc. http://www.dovetailinc.org/reportsview/2012/responsible-materials/pjim-bowyerp/carbon-101
Daigneault, A., Sohngen, B., Sedjo, R. 2012. "Economic Approach to Assess the Forest Carbon Implications of Biomass Energy." Environmental Science and Technology
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