Criterion 5 describes stocks and flows (pools and flux) of carbon in forested ecosystems (Indicator 5.22) and forest products (Indicator 5.23) along with avoided carbon emissions from the use of forest biomass for energy (Indicator 5.24). As such, the criterion provides valuable information regarding the current and potential role of forest management efforts in offsetting or otherwise mitigating carbon emissions from fossil fuels and associated sources. It also provides an indication of how broadscale ecosystem processes may mitigate or exacerbate carbon balances, and thereby climate change, in the long term.
Indicator 5.22 relies directly on FIA forest inventory data. The process by which these data are translated into carbon stocks for various components (live biomass, forest soils, and so on) involves a number of assumptions and modeling techniques, which continue to be developed and refined over time. The inclusion of carbon stocks in forest soils, which were omitted in the 2003 report, is a major innovation in the current report.
According to Indicator 5.22, forested ecosystems in the United States currently contain an amount of carbon equivalent to more than 165 billion metric tons of CO2, a figure close to 27 times the 5.9 billion tons of CO2 emitted nationally every year through the burning of fossil fuels and similar sources. Live trees and forest soils account for the bulk of forest-based carbon stocks. In terms of flows, forests sequester approximately 650 million metric tons of additional CO2 every year, offsetting close to 11 percent of total U.S. annual carbon emissions. This rate of sequestration has been relatively stable for several decades, reflecting the long-term increases in forest volume described in Criterion 2.
Indicator 5.23 measures carbon stored in forest products, underlining the important fact that many long-lived forest products continue to sequester carbon long after the trees that supplied their raw materials have been harvested. The indicator shows that a carbon equivalent to around 8 billion metric tons of CO2 are currently stored in long-lived forest products and in discarded forest products in landfills. Annual rates of sequestration are approximately 100 million tons, substantially less than 650 million tons annually sequestered by forests but still a significant number. As in the case of Indicator 5.22, Indicator 5.23 measures broad processes, although in this case social rather than ecological. Major variations are not likely in the short term, except in so much as they are driven by major changes in overall economic activity (such as the recent recession). Over the long term, the indicator will provide information on major shifts in consumption patterns and their relative effects on carbon stocks and flows, and thereby the role of forest products in global carbon balances.
Using forest biomass to produce energy is another means by which forests may help mitigate greenhouse gas (GHG) concentrations in the atmosphere. Indicator 5.24 measures avoided carbon emissions resulting from the replacement of energy from fossil fuels with that generated by the use of forest biomass. Although this process releases the carbon stored in the biomass, it is assumed that the subsequent regrowth of forests will sequester an equivalent amount over time and, thus, the process is considered to be carbon-neutral (at least in the long run). This is a simplification of a complex argument, but it is nonetheless broadly accepted that replacing fossil fuel consumption with energy from forest biomass will result in reduced carbon emissions in the long term.
The indicator shows that annual production of energy from the combustion of wood in the United States is around 2,100 trillion BTUs (British Thermal Units) (about 2 percent of the 101 quadrillion BTUs consumed in 2007). When converted to avoided carbon emissions, this number translates to between 100 and 200 million metric tons of carbon depending on the energy source used for comparison. Contrary to what one might expect, this number has been slightly falling since the mid-1990s, but the result is less surprising if one considers the fact that the use of fire wood for heating purposes has been declining for decades and that the wood products industry has long used wood residues and byproducts to generate energy as part of its production processes. Consequently, Indicator 5.24 may be tracking developments in these more traditional uses more than measuring the emergence of a nascent bioenergy sector. To the extent that forest-based bioenergy becomes more important in the future, this trend may be reversed in subsequent reporting cycles.