Emissions of Greenhouse Gases in the United States 2003

6. Land Use Issues

Overview

Land use and forestry issues are important to national and global inventories of greenhouse gases in two ways:

• Vegetation can “sequester” or remove carbon dioxide from the atmosphere and store it for potentially long periods in above- and below-ground biomass, as well as in soils. Soils, trees, crops, and other plants may make significant contributions to reducing net greenhouse gas emissions by serving as carbon “sinks.”
• Humans can alter the biosphere through changes in land use and forest management practices and, in effect, alter the quantities of atmospheric and terrestrial carbon stocks, as well as the natural carbon flux among biomass, soils, and the atmosphere.

Land use issues are of particular interest to the United States because U.S. forests and soils annually sequester large amounts of carbon dioxide. Much of the forest land in the United States was originally cleared for agriculture, lumber, or fuel in the hundred years prior to 1920. Since then, however, much of the agricultural and pasture land has reverted to forest land, increasing its ability to sequester atmospheric carbon dioxide.

The amount of carbon being sequestered annually is uncertain, in part because of an absence of data and difficulties in measuring sequestration. Moreover, in addition to technical uncertainties, there are also policy and accounting questions about the aspects of the biological carbon cycle that would be included in national inventories as anthropogenic emissions and removals.

The revised guidelines for national emissions inventories published in 1997 by the Intergovernmental Panel on Climate Change (IPCC) stipulate the inclusion of carbon sequestration through land use and forestry in national greenhouse gas inventories as an offset to gross greenhouse gas emissions from other sources.¹¹⁸ The U.S. Environmental Protection Agency (EPA),¹¹⁹ based on data generated by the U.S. Department of Agriculture, estimates annual U.S. carbon sequestration for the year 2002 at 690.7 million metric tons carbon dioxide equivalent, a decline of approximately 27.9 percent from the 957.9 million metric tons carbon dioxide equivalent sequestered in 1990 (Table 32). In 1990 land use change and forestry practices represented an offset of 15.7 percent of total U.S. anthropogenic carbon dioxide emissions, but by 2002 that amount had declined to 10.0 percent.¹²⁰

Land Use Change and Forestry Carbon Sequestration

The EPA’s estimates for carbon sequestration from land use change and forestry in 2002 include four main components: (1) changes in forest carbon stocks (600.8 million metric tons carbon dioxide equivalent or 87.0 percent of the total), (2) changes in agricultural soil carbon stocks (21.2 million metric tons carbon dioxide equivalent or 3.1 percent of the total), (3) changes in carbon stocks in urban trees (58.7 million metric tons carbon dioxide equivalent or 8.5 percent of the total), and (4) changes in carbon stocks in landfilled yard trimmings and food scraps (10.1 million metric tons carbon dioxide equivalent or 1.5 percent of the total).¹²¹

The EPA’s estimates for carbon sequestration in forests are based on carbon stock estimates developed by the U.S. Forest Service, U.S. Department of Agriculture (USDA), employing methodologies that are consistent with the Revised 1996 IPCC Guidelines. The USDA estimates of carbon stocks in urban trees were based on field measurements in ten U.S. cities and data on national urban tree cover, again employing a methodology consistent with the Revised 1996 IPCC Guidelines. Estimates for sequestration in agricultural soils were based on changes in carbon stocks in mineral and organic soils resulting from agricultural land use and land management, as well as emissions of carbon dioxide resulting from the use of crushed limestone and dolomite on soils. Methodologies drawn from the IPCC guidelines were used to derive all components of changes in agricultural soil carbon stocks. The EPA estimates for carbon stocks in landfilled yard trimmings and food scraps are based on the EPA’s own method of examining life-cycle greenhouse gas emissions and sinks associated with solid waste management.¹²²

The EPA’s carbon flux estimates, with the exception of those from wood products, urban trees, and liming, are based on periodic surveys of U.S. forest land and soils, conducted on a less frequent basis. Carbon fluxes from forests (except wood products) and from agricultural soils (except liming) are collected over 5- or 10-year intervals and averaged annually for years between surveys. Each State is surveyed independently and at varying times, thus the estimates for carbon fluxes from forest carbon stocks differ at the national level from year to year. Forest soils, which are surveyed on a regional scale, have fluxes over multi-year periods, with large discontinuities in-between intervals. Agricultural soils exhibit a pattern similar to that of forest soils. The most current national forest and land-use surveys were completed for the year 1999, thus carbon flux estimates from forests and agricultural soils are derived in part from modeled projections for future years. Data on carbon fluxes from urban trees, collected over the period from 1990 through 1999, were applied to the entire time series.¹²³

Changes in Forest Carbon Stocks

In the United States, the most significant pressures on the amount of carbon sequestered through forest lands are land management activities and the continuing effects of past changes in land use. These activities directly affect carbon flux by shifting the amount of carbon accumulated in forest ecosystems.¹²⁴ Land management activities affect both the stocks of carbon that can be stored in land-based carbon sinks, such as forests and soils, and the flows, or fluxes, of carbon between land-based sinks and the atmosphere.

Forests are multifaceted ecosystems with numerous interrelated components, each of which stores carbon. These components include five forest carbon pools and two wood products carbon pools:

• Forest carbon pools:
• Trees (living trees, standing dead trees, roots, stems, branches, and foliage)
• Understory vegetation (shrubs and bushes, roots, stems, branches, and foliage)
• Forest floor (fine woody debris, tree litter, and humus)
• Down dead wood (logging residue and other dead wood on the ground, stumps, and roots of stumps)
• Organic material in soil
• Wood products carbon pools:
• Harvested wood products in use
• Harvested wood products in landfills.

As a result of natural biological processes occurring in forests, as well as anthropogenic activities, carbon is constantly cycling through these components and between the forest and the atmosphere. The net change in overall forest carbon may not always be equal to the net flux between forests and the atmosphere, because timber harvests may not necessarily result in an instant return of carbon to the atmosphere. Timber harvesting transfers carbon from one of the five “forest carbon pools” to one of the two “wood products carbon pools.” Once carbon is transferred to a product pool, it is emitted over time as carbon dioxide as the product combusts or decays. Emission rates vary significantly, depending on the type of product pool that houses the carbon.¹²⁵

In the United States, enhanced forest management, regeneration of formerly cleared forest areas, and timber harvesting have resulted in the annual sequestration of carbon throughout the past decade. Since the 1920s, deforestation for agricultural purposes has become a nearly defunct practice. More recently, managed growth practices have become common in eastern forests, greatly increasing their biomass density over the past 50 years. In the 1970s and 1980s, federally sponsored tree planting and soil conservation programs were embraced. These programs resulted in the reforestation of formerly harvested lands, improvement in timber management activities, soil erosion abatement, and the conversion of cropland to forests. Forest harvests have also affected carbon sequestration. The majority of harvested timber in the United States is used in wood products. The bulk of the discarded wood products are landfilled, thus large quantities of the harvested carbon are relocated to long-term storage pools rather than to the atmosphere. The size of wood product landfills has increased over the past century.¹²⁶

According to the EPA (Table 33), carbon sequestration by forests and harvested wood products totaled 600.8 million metric tons carbon dioxide equivalent in 2002. Between 1990 and 2002, U.S. forest and harvested wood components accounted for an average annual net sequestration of 736 million metric tons carbon dioxide equivalent, resulting from domestic forest growth and increases in forested land area; however, there was a decrease of approximately 28 percent in annual sequestration over the same period.¹²⁷

The overall decline in forest carbon sequestration was driven largely by a 39.3-percent reduction in the rate of sequestration in the forest carbon pool (636.6 million metric tons carbon dioxide equivalent in 1990 versus 386.4 million metric tons in 2002). The reduction in the forest carbon pool sequestration rate can be attributed primarily to a 74.1-percent decline in the estimated rate of sequestration in forest soils. Forest soil carbon sequestration fell from an annual average of 212.7 million metric tons carbon dioxide equivalent during the period 1990-1996 to an annual average of 55 million metric tons during the period 1997-2002.

The net forest carbon flux has varied significantly from year to year, most notably from 1996 to 1997 and from 1997 to 1998 (Table 33). The U.S. Forest Service reports there are different reasons for these shifts, and those reasons encompass both substantive differences in the source of carbon stocks and the methodology utilized to determine the levels of sequestration for each year. The shift downward from 1996 to 1997 resulted primarily from changes in soil carbon stocks. However, the EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2002 utilized databases containing data on changes in forest area—and thus soil carbon—only for the years 1987, 1992, and 1997. Data for the missing years were interpolated. There was a large shift, however, in forest area—and soil carbon—between 1987 and 1992 and again between 1992 and 1997. Although forest area increased between 1992 and 1997, it increased at a lower rate than between 1987 and 1992, resulting in a shift downward in soil carbon stocks and total sequestration quantities. The shift downward between 1997 and 1998 is attributed primarily to changes in the carbon sequestration level of trees.¹²⁸

Wood products carbon stocks experienced a slight increase in overall annual sequestration levels between 1990 and 2002, reflecting accumulation of carbon in harvested wood pools.¹²⁹ There were small variations from year to year, but the trend in net sequestration amounts has generally been upward, from 210.1 million metric tons carbon dioxide equivalent in 1990 to 214.4 million metric tons carbon dioxide equivalent in 2002 (Table 33). This reflects an increase in both harvesting for wood products and in the amount of wood contained in wood product landfills.

The EPA employs methodology consistent with the Revised 1996 IPCC Guidelines to estimate the net sequestration resulting from harvested wood. The IPCC provides two alternative approaches to account for carbon emissions from harvested wood. These are: (1) assume that all harvested wood replaces wood products that decay in the inventory year, thus the amount of wood harvested annually equates to annual emissions from harvests; or (2) account for the variable rate of decay of harvested wood depending on its disposition (e.g., product pool, landfill, and combustion). The estimates used by EPA in its inventory, and reported in this chapter, result from using the second approach, employing estimates of carbon stored in wood products and landfilled wood.¹³⁰ EPA also employs the “production approach”; that is, carbon stored in imported wood products is not counted, but carbon stored in exports is counted, even when logs are processed in other countries.¹³¹

EPA estimates of carbon stocks in wood products and landfilled wood from 1910 onward are based on historical data and data derived from models utilized by the USDA Forest Service. These models (the forest sector modeling system) include an area change model, a timber market model, a pulp and paper model, and an inventory model. Estimates were derived using data on annual wood and paper production, and by tracking the disposition of carbon in harvested wood for each year from 1910 through 2002. Estimates include the change in carbon stocks in wood products and landfilled wood, and carbon emissions to the atmosphere both with and without energy recovery. Carbon in exported wood was counted as if it remained in the United States, and carbon in imported wood was not counted.¹³²

EPA estimates of carbon stored in harvested wood products are currently being revised. Updated estimates will use more detailed wood products production and use data and improved parameters on disposition and decay of products. Estimation methods may also change as a result of discussions to be held by the UNFCCC in August 2004 regarding accounting for changes in harvested wood products (see box on page 77). Preliminary results suggest that estimates of carbon stored in harvested wood may in fact be lower than the estimates included in the EPA Inventory¹³³ and detailed in this chapter.

Changes in Urban Tree Carbon Stocks

Urban forests make up a considerable portion of the total tree canopy cover in the United States. Urban areas, which cover 3.5 percent of the continental United States, are estimated to contain about 3.8 billion trees, accounting for approximately 3 percent of total tree cover in the United States. The EPA’s carbon sequestration estimates for urban trees are derived from estimates by Nowak and Crane,¹³⁴ based on data collected throughout the 1990s and applied to the entire time series in this report. Net carbon dioxide flux from urban trees is estimated at 58.7 million metric tons carbon dioxide equivalent annually from 1990 through 2002 (Table 32).¹³⁵

Changes in Agricultural Soil Carbon Stocks

The amount of organic carbon in soils depends on the balance between addition of organic material and loss of carbon through decomposition. The quantity and quality of organic matter within soils, as well as decomposition rates, are determined by the interaction of climate, soil properties, and land use. Agricultural practices— including clearing, drainage, tillage, planting, grazing, crop residue management, fertilization, and flooding— can alter organic matter inputs and decomposition, causing a net flux of carbon to or from soils. The IPCC methodology, which is used by the EPA to estimate the net flux from agricultural soils (Table 34), is divided into three categories of land use and land management activities: (1) agricultural land use and land management activities on mineral soils;¹³⁶ (2) agricultural land use and land management activities on organic soils;¹³⁷ and (3) liming of soils. Of the three activities, the use and management of mineral soils is estimated to be the most significant contributor to total flux from 1990 through 2002. Sequestration in mineral soils in 2002 was estimated to be 64.7 million metric tons carbon dioxide equivalent, while emissions from organic soils and liming were estimated at 34.7 and 8.8 million metric tons carbon dioxide equivalent, respectively. In net, these activities resulted in 21.2 million metric tons carbon dioxide equivalent sequestered through agricultural soils in 2002.¹³⁸

Changes in Landfilled Yard Trimming and Food Scrap Carbon Stocks

Carbon stored in landfilled yard trimmings can remain sequestered indefinitely. In the United States, yard trimmings (grass clippings, leaves, and branches) and food scraps make up a considerable portion of the municipal waste stream, and significant amounts of the yard trimmings and food scraps collected are discarded in landfills. Both the amount of yard trimmings and food scraps collected annually and the percentage that is landfilled have declined over the past decade. Net carbon dioxide sequestration from landfilled yard trimmings and food scraps has declined accordingly, from 26.0 million metric tons carbon dioxide equivalent in 1990 to 10.1 million metric tons carbon dioxide equivalent in 2002 (Table 35). Since 1990, programs limiting disposal of yard trimmings have led to an increase in backyard composting and the use of mulching mowers. The number of municipal composting facilities has also risen, further reducing the amount of trimmings that are discarded in landfills. The EPA’s methodology for estimating carbon storage relies on a life-cycle analysis of greenhouse gas emissions and sinks associated with solid waste management.¹³⁹

Land Use and International Climate Change Negotiations

In past international negotiations on climate change, the United States and many other countries have maintained that the inclusion of LULUCF activities in a binding agreement that limits greenhouse gas emissions is of the utmost importance; however, issues of whether and how terrestrial carbon sequestration could be accepted for meeting various commitments and targets have remained subjects of complex and difficult international negotiations on climate change.

Many of the countries involved in climate change negotiations have agreed that implementation of LULUCF activities under an international climate change agreement may be complicated by a lack of clear definitions for words such as “reforestation” and “forest.” Further, implementation may be hindered by the lack of effective accounting rules. According to researchers at the Pew Center on Global Climate Change,¹⁴⁰ implementation of LULUCF provisions in an international climate change agreement raises many issues for such activities and/or projects, such as:

• What is a direct human-induced activity?
• What is a forest and what is reforestation?
• How will the issues of uncertainty and verifiability be addressed?
• How will the issues of (non) permanence and leakage be addressed?
• Which activities beyond afforestation, reforestation and deforestation (ARD), if any, should be included, and what accounting rules should apply?
• Which carbon pools and which greenhouse gases should be considered?

Uncertainties related to data issues have also slowed international negotiations on climate change.

The Ninth Session of the Conference of the Parties to the UN Framework Convention on Climate Change (COP-9) occurred in Milan, Italy, in December 2003. The parties at this meeting agreed on some of the rules for carbon sequestration projects under the Clean Development Mechanism (CDM), but the issue on how to treat the non-permanence of carbon sinks projects is still widely debated. Policymakers at COP-9 decided to limit the duration of credits generated from carbon sequestration projects, and also addressed the topics of additionality, leakage, uncertainties, and socioeconomic and environmental impacts.¹⁴¹

Land Use Data Issues

Uncertainties in the EPA estimates of U.S. carbon sequestration include sampling and measurement errors inherent to forest carbon estimates. The forest surveys engage a statistical sample that represents the expansive variety of growth conditions over large territories. Although more current inventories are conducted annually in each State, much of the existing data may have been collected over more than one year in any given State. Thus, there may be uncertainty about the year associated with the forest survey data. In addition, the existing forest survey data do not include forest stocks in Alaska, Hawaii, and the U.S. territories (although net carbon fluxes from these stocks are anticipated to be insignificant).¹⁴²

Additional uncertainty results from the derivations of carbon sequestration estimates for forest floor, understory vegetation, and soil from models based on forest ecosystem studies. To extrapolate results of these studies to the forest land in question, an assumption was made that the studies effectively described regional or national averages. This assumption may result in bias from applying data from studies that improperly represent average forest conditions, from modeling errors, and/or from errors in converting estimates from one reporting unit to another.¹⁴³

Aside from the land use data issues and uncertainties discussed above, which are specific to the methodologies used for the EPA estimates, there is concern about larger and more general uncertainty surrounding estimates of terrestrial carbon sequestration. It is anticipated to be difficult, as well as expensive, to determine carbon stock changes over shorter time periods, such as the 5-year periods suggested during international climate change negotiations. This concern is especially problematic if the carbon stocks are large and the stock changes are comparatively small.¹⁴⁴ Several countries involved in the negotiations have maintained that the accounting of terrestrial carbon stock changes over a 5-year commitment period fails to account for the differing dynamics of carbon stocks and fluxes over time.

In addition to concerns about uncertainty, permanence, and leakage, a recent scientific study published in the science journal Nature has raised questions about carbon sequestration through terrestrial sinks. The authors of the study, Dr. John Lichter and Dr. William Schlesinger, concluded that while forests do sequester carbon dioxide from the air and store it in the soil, the majority of the sequestered carbon is ultimately released back into the atmosphere as carbon dioxide when organic soil material decomposes. They maintain that their findings highlight the uncertainty of the role of soils as long-term carbon storage pools and assert that considerable long-term net carbon sequestration in forest soils may be unlikely.¹⁴⁵ Many scientists agree that much work remains to be done on the science surrounding terrestrial carbon sequestration; however, a number of the countries involved in international climate change negotiations assert that the potential for terrestrial carbon sequestration should be embraced, or at the very least, not discounted or overlooked.

Research by CarboEurope, a European program that has pioneered research into the carbon budget, reveals that soils in forests release more carbon than their trees will absorb in the first 10 years. Forest soils and the organic matter within them generally contain three to four times as much carbon as does vegetation on the ground. CarboEurope’s researchers contend that, when ground is cleared for forest planting, rotting organic matter in the soil releases a surge of carbon dioxide into the air that will exceed the amount of carbon dioxide absorbed by growing trees for at least the first 10 years of forest growth; only later will the uptake of carbon by the trees begin to offset the release of carbon dioxide from the soil. In fact, their research indicates that some new forests planted on wet, peaty soils may never absorb as much carbon as they release.¹⁴⁶

Thus, while there are methods available for estimating the amount of carbon sequestered through U.S. forests and soils, many uncertainties remain in the accounting methodology and overall conceptual feasibility of carbon sequestration both nationally and globally. For this reason, caution should be employed when accounting for the amount of carbon sequestered through land use and forestry practices, or when making decisions about the amount of sequestered carbon to be treated as an offset to national carbon dioxide emissions.

6.Land Use Issues Tables

Notes and Sources

Released: December 2004