6. Land-Use Issues

Overview

Land-use and forestry issues are important to national and global inventories of greenhouse gases in three 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 vegetation may make significant contributions to reducing net greenhouse gas emissions by serving as carbon “sinks.”
• Harvested wood put into wood products, or eventually into landfills, can potentially sequester carbon dioxide from the atmosphere for decades before the carbon stored in the wood products decays and is released to the atmosphere.
• Human-induced land-use changes and forest management practices can alter the quantities of atmospheric and terrestrial carbon stocks, as well as the natural carbon flux among biomass, soils, and the atmosphere.¹⁰⁸

Land-use issues are of particular interest to U.S. policymakers, 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 before 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 carbon sequestration. Moreover, in addition to technical uncertainties, there are also policy and accounting questions about the aspects of the carbon cycle that should be included in national inventories as anthropogenic emissions and removals. Further, recent studies have indicated the possibility that vegetation may also be a source of methane (see discussion on "Methane Emissions From Vegetation: New Findings").

The 1996 revised guidelines for national emissions inventories, published in 1997 by the Intergovernmental Panel on Climate Change (IPCC), include methods for calculating carbon sequestration and net carbon dioxide flux to the atmosphere resulting from land-use changes and land-use activities, such as forestry.¹⁰⁹ The IPCC Good Practice Guidance for Land Use, Land-Use Change and Forestry¹¹⁰ (LULUCF GPG), published in 2003, complements the 1996 IPCC guidelines. The U.S. Environmental Protection Agency (EPA) estimates annual U.S. carbon sequestration in 2004, based on data generated by the U.S. Department of Agriculture (USDA), at 780.1 million metric tons carbon dioxide equivalent (MMTCO₂e), a decline of approximately 14 percent from the 910.4 MMTCO₂e sequestered in 1990¹¹¹ (Table 33). Land use, land-use change, and forestry (LULUCF) practices offset 11 percent of total U.S. greenhouse gas emissions in 2004 and 15 percent in 1990.¹¹² In terms of anthropogenic carbon dioxide emissions, U.S. LULUCF practices offset 13 percent of U.S. carbon dioxide emissions in 2004, as compared with 18 percent in 1990.

Land-Use Change and Forestry Categories

The EPA, following LULUCF GPG, reported 2004 data on carbon fluxes according to the following categories: forest land remaining forest land, cropland remaining cropland, land converted to cropland, grassland remaining grassland, land converted to grassland, and settlements remaining settlements. Data constraints prevented the EPA from reporting on all the LULUCF GPG categories for land use and land-use change.

Forest Land Remaining Forest Land

The values for forest carbon dioxide fluxes reported for this category are based on estimates of changes in forest carbon stocks. The components analyzed are above-ground biomass, below-ground biomass, dead wood, litter, soil organic carbon, harvested wood products in use, and harvested wood products in landfills. The estimated carbon flux (including all carbon-based greenhouse gases) from each of these components— except for soil organic carbon—was calculated using the USDA Forest Inventory and Analysis (FIA) database (FIADB) and methodologies consistent with the LULUCF GPG and the Revised 1996 IPCC Guidelines.¹¹³ The FIADB is based on State surveys carried out at intervals of 5 to 14 years; accordingly, adjustments were made for temporal and spatial gaps, using FIA’s recently introduced national plot design and annualized sampling.¹¹⁴ Estimation of the average density of soil organic carbon (carbon per unit area) was based on USDA’s State Soil Geographic (STATSGO) data and FIA survey data (areas of broad forest type).¹¹⁵

Nitrous oxide emissions from fertilized forest soils were calculated by using a default methodology consistent with the LULUCF GPG. Pine trees, being the dominant species planted for timber in the southeastern United States, were taken as representative of fertilized forests in the country, and the average reported fertilization rate of 150 pounds of nitrogen per acre was multiplied by the area of pine forest receiving fertilizer.

Cropland Remaining Cropland

Estimates of carbon stock changes from this category include changes in agricultural soil carbon stocks involving both mineral and organic soils on cropland remaining cropland. Also included in this category are carbon stock changes in organic soils on land converted to cropland and emissions of carbon dioxide from the application of crushed limestone and dolomite to all managed lands. The estimation methods used for these estimates are consistent with the Revised 1996 IPCC Guidelines and the LULUCF GPG.

Land Converted to Cropland

Carbon stock changes for this category include only carbon stock changes in mineral soils. Carbon stock changes in organic soils and emissions of carbon dioxide from the application of crushed limestone and dolomite that occur on land converted to cropland, as indicated above, are reported in the category of cropland remaining cropland. This adjustment is made because of the difficulty in separating the land-use components (cropland remaining cropland) from the land-use change components (land converted to cropland) of the carbon stock changes.

Grassland Remaining Grassland

This category includes carbon stock changes in both organic and mineral soils. It also includes changes in organic soils on land converted to grassland, because it is not possible to separate them from carbon stock changes in organic soils on existing grassland. Emissions of carbon dioxide from the application of crushed limestone and dolomite to grassland remaining grassland are included in the category of cropland remaining cropland because of the difficulty in separating the land-use and land-use change components of the carbon stock changes.

Land Converted to Grassland

This category includes carbon stock changes in mineral soils on land recently converted to grassland. Changes in organic soil carbon stocks and carbon dioxide emissions from the application of crushed limestone and dolomite to land converted to grassland are reported in the category of cropland remaining cropland because of the difficulty in separating the land-use and land-use change components of the carbon stock changes.

Settlements Remaining Settlements

This category includes carbon stock changes from settlements remaining settlements and from land converted to settlements. Carbon stock changes from settled lands include stock changes in urban trees as well as landfilled yard trimmings and food scraps. Stock changes in urban trees were estimated on the basis of field measurements and data on national urban tree cover, using a methodology consistent with the LULUCF GPG to estimate carbon flux. Carbon stocks in landfilled yard trimmings and food scraps were estimated by determining the fraction of carbon stocks from earlier years that had decayed by 2004. Emissions of carbon dioxide emissions from the application of crushed limestone and dolomite to settled lands were reported in the category of cropland remaining cropland. Nitrous oxide emissions from nitrogen applied to turf grass were estimated by assuming that such applications represented 10 percent of all synthetic fertilizer used in the United States.

Land-Use Change and Forestry Carbon Sequestration

The EPA’s estimates for carbon sequestration from land-use change and forestry in 2004 include the following categories: (1) changes in forest carbon stocks for forest land remaining forest land (637.2 MMTCO₂e or 82 percent of the total); (2) changes in agricultural soil carbon stocks for cropland remaining cropland (28.9 MMTCO₂e or 3.7 percent of the total); (3) changes in agricultural soil carbon stocks for land converted to cropland (2.8 MMTCO₂e or less than 0.5 percent of the total); (4) changes in agricultural soil carbon stocks for grassland remaining grassland (-7.3 MMTCO₂e or -0.9 percent of the total¹¹⁶); (5) changes in agricultural soil carbon stocks for land converted to grassland (21.1 MMTCO₂e or 2.7 percent of the total); and (6) changes in settlements remaining settlements (97.3 MMTCO₂e or 12 percent of the total, including 88.0 MMTCO₂e from urban trees and 9.3 MMTCO₂e from landfilled yard trimmings and food scraps).¹¹⁷

Forest Land Remaining Forest Land: Changes in Forest Carbon Stocks

In the United States, the most significant pressures on the amount of carbon sequestered through forest land 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.¹¹⁸ 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 fluxes of carbon between land-based sinks and the atmosphere (see text box below for the most recent global assessment of the world’s forests).

The components or “pools” of forest carbon analyzed by the EPA for its most recent inventory include above-ground biomass, below-ground biomass, dead wood, litter, and soil organic carbon. The EPA also assessed harvested wood products in use, and harvested wood products in landfills. As a result of natural biogeochemical 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 or methane as the product decays or is combusted. Emission rates vary significantly, depending on the type of product pool that houses the carbon.¹¹⁹

In the United States, enhanced forest management, regeneration of formerly cleared forest areas, and timber harvesting have resulted in net annual sequestration of carbon throughout the past decade. Since the 1920s, deforestation for agricultural purposes has become a nearly defunct practice. Managed growth practices have become common in eastern forests since the early 1950s, almost doubling their biomass density.¹²⁰ In the 1970s and 1980s, federally sponsored tree planting and soil conservation programs were embraced. These programs led to 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 is landfilled, and thus large quantities of the harvested carbon are relocated to long-term storage pools rather than to the atmosphere. The combined size of the long-term storage pools has increased over the past century.¹²¹

According to the EPA, carbon sequestration in U.S. forests and harvested wood pools totaled 637.2 MMTCO₂e in 2004 (Table 34). From 1990 to 2004, U.S. forests and harvested wood pools accounted for an average annual net sequestration of 627.0 MMTCO₂e, resulting from domestic forest growth and increases in forested land area; however, there was a decrease of approximately 18 percent in annual sequestration over the same period.¹²²

The overall decline of carbon sequestration in forests and harvested wood pools resulted from a 25-percent reduction in the level of sequestration in the forest carbon pool (420.2 MMTCO₂e in 2004 versus 563.3 MMTCO₂e in 1990). The reduction in the sequestration rate for forest carbon pools can be attributed primarily to a reduction in sequestration levels in litter and soil organic carbon. Sequestration in litter declined by 68 percent, from 82.9 MMTCO₂e in 1990 to 26.6 MMTCO₂e in 2004, and sequestration in soil organic carbon declined by 130 percent—that is, soil organic carbon went from being a carbon sink of 33.6 MMTCO₂e in 1990 to an emissions source of 10.1 MMTCO₂e in 2004.

The EPA explains that, because its soil carbon estimates currently assume that soil carbon density depends only on broad forest type, the estimated decrease in annual carbon sequestration depends only on changes in total forest area or changes in forest type.¹²³ Net forest growth and increasing forest area, particularly before 1997, contributed to rising sequestration; but since 1997, forest land area has remained relatively constant, and the increase in carbon density (per area) has resulted in net forest carbon sequestration. National estimates of forest land are obtained by summing State surveys for the conterminous United States. Because the State surveys are not completed each year, interpolation between data points is used to provide estimates for years without surveys.

Overall annual sequestration levels in harvested wood carbon stocks increased slightly from 1990 to 2004. The trend in net sequestration amounts has been generally upward, from 210.1 MMTCO₂e in 1990 to 217.0 MMTCO₂e in 2004 (Table 34). Annual sequestration levels in landfilled wood declined from 162.4 MMTCO₂e in 1990 to 156.2 MMTCO₂e in 2004, but that decline was offset by an increase in carbon sequestration in harvested wood products, from 47.6 MMTCO₂e in 1990 to 60.8 MMTCO₂e in 2004.

The EPA has estimated carbon stocks in wood products in use and in landfills from 1910 onward, based on USDA Forest Service historical data and analyses using the North American Pulp and Paper (NAPAP) model,¹²⁴ the Timber Assessment Market Model (TAMM),¹²⁵ and the Aggregate Timberland Assessment System (ATLAS) model.¹²⁶ Carbon decay in harvested wood was analyzed by the EPA for the period 1910 through 2004, using data on annual wood and paper production. The analysis included changes in carbon stocks in wood products, changes in carbon in landfills, and the amount of carbon (carbon dioxide and methane) emitted to the atmosphere both with and without energy recovery. The EPA also followed the “production approach”; that is, carbon stored in imported wood products was not counted, but carbon stored in exports was counted, including logs processed in other countries.¹²⁷

Cropland Remaining Cropland: Changes in Agricultural Soil Carbon Stocks

The amount of organic carbon in soils depends on the balance between the addition of organic material and the 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 (cropland) soils, is divided into three categories of land use and land management activities (Table 35): (1) agricultural land use and land management activities on mineral soils;¹²⁸ (2) agricultural land use and land management activities on organic soils;¹²⁹ 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 carbon sequestration from 1990 through 2004. Sequestration in mineral soils in 2004 was estimated to be 63.2 MMTCO₂e, and emissions from organic soils and liming were estimated at 30.3 and 4.0 MMTCO₂e, respectively. Together, these three activities resulted in a net 28.9 MMTCO₂e sequestered through agricultural soils in 2004, or 12 percent below the 1990 carbon sequestration level of 33.0 MMTCO₂e.¹³⁰

Land Converted to Cropland

The EPA for the first time provided an estimate of carbon stock changes for land converted to cropland in its 2004 data release. The estimate covers only mineral soils, with estimates for organic soil and liming on land converted to cropland being included in the category of cropland remaining cropland, because it was not possible to subdivide those estimates by land use. Land use and management of land converted to cropland led to carbon losses (emissions) in the early 1990s. In 1990, for example, land converted to cropland led to net emissions of 1.5 MMTCO₂e (Table 33). The trend has since been reversed, and in 2004 land converted to cropland resulted in net carbon sequestration equivalent to 2.8 MMTCO₂e, primarily in the intermountain west and central areas of the country.¹³¹

Grassland Remaining Grassland

Carbon stock changes for this category—also provided for the first time by the EPA in its 2004 data release— include changes in soil carbon storage resulting from agricultural land-use and management activities on mineral and organic soils. Carbon dioxide emissions due to the liming of soils on grassland remaining grassland are not included in this category but instead are placed in the category of cropland remaining cropland, because it is not possible to separate the emissions by land-use categories. In 2004, this category accounted for emissions of 7.3 MMTCO₂e, including 4.6 MMTCO₂e from organic soils and 2.7 MMTCO₂e from mineral soils (Table 36). In 1990, this category sequestered 4.5 MMTCO₂e, based on net sequestration of 8.8 MMTCO₂e in mineral soils and emissions of 4.3 MMTCO₂e from organic soils. The change in this category to a source of emissions is the result of reduced rates of carbon sequestration in mineral soils in the southern United States and increased emissions from the drainage of organic soils in other regions.¹³²

Land Converted to Grassland

Estimates of carbon stock changes for land converted to grassland were also provided for the first time by the EPA in its 2004 data release. The estimates cover only mineral soils. Estimates of changes in organic soil carbon stocks for this category are included in the estimates for the category of grassland remaining grassland, and emissions from liming of soils for this category are included in those reported for the category of cropland remaining cropland, because it was not possible to subdivide the estimates by land use. Net soil carbon storage for this category increased from 17.6 MMTCO₂e in 1990 to 21.1 MMTCO₂e in 2004 (Table 33). The upswing was the result of increased acreage of cropland converted to pasture, primarily in the Southeast and Northwest.¹³³

Settlements Remaining Settlements

Carbon stock changes for this category include carbon stock changes for urban trees and for landfilled yard trimmings and food scraps. Carbon sequestration for this category increased by 17 percent, from 83.2 MMTCO₂e in 1990 to 97.3 MMTCO₂e in 2004 (Table 33), with significant increases in carbon storage by urban trees more than offsetting declines in net carbon storage in landfilled yard trimmings and food scraps.

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 4.4 percent of the continental United States, account 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 from 1989 through 1999 in 10 U.S. cities. Currently, annual changes in sequestration estimates are based solely on changes in total U.S. urban area. Net carbon dioxide sequestration by urban trees increased by 50 percent, to 88.0 MMTCO₂e in 2004 from 58.7 MMTCO₂e in 1990 (Table 33), primarily as a result of increases in urban land area.¹³⁵

Changes in Landfilled Yard Trimming and Food Scrap Carbon Stocks

Carbon stored in landfilled yard trimmings and food scraps 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 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, to 9.3 MMTCO₂e in 2004 from 24.5 MMTCO₂e in 1990—a reduction of 62 percent (Table 37).

Since 1990, municipal policies limiting pickup and disposal have led to an 18-percent decrease in yard trimmings collected. In addition, composting of yard trimmings in municipal facilities has increased significantly, reducing the percentage of collected yard trimmings discarded in landfills from 72 percent in 1990 to 35 percent in 2004. In contrast, the percentage of food scraps disposed of in landfills has decreased only slightly, from 81 percent in 1990 to 78 percent in 2003.¹³⁶ 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.¹³⁷

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.

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 of “reforestation” and “forest.” Further, implementation may be hindered by the lack of effective accounting rules. According to research published by the Pew Center on Global Climate Change,¹³⁸ implementation of LULUCF provisions in an international climate change agreement raises many issues, 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 of the UNFCCC) was held in Milan, Italy, in December 2003. The parties agreed on some of the rules for carbon sequestration projects under the Clean Development Mechanism (CDM), but the issue of how to treat the non-permanence of carbon sinks projects remained unresolved. Delegates at COP-9 decided to limit the duration of credits generated from carbon sequestration projects and addressed the topics of additionality, leakage, uncertainties, and socioeconomic and environmental impacts.¹³⁹

A year later in Buenos Aires, Argentina, delegates at the Tenth Conference of the Parties (COP-10 of the UNFCCC) did address the issue of small-scale afforestation and reforestation project activities under the CDM. The following decisions were made at COP-10:¹⁴⁰

• Adopt simplified modalities and procedures for small-scale afforestation and reforestation project activities in the first commitment period.
• Limit the designation of small-scale afforestation and reforestation projects to those with net anthropogenic greenhouse gas removals by sinks that are less than 8,000 metric tons carbon dioxide equivalent per year. For projects that result in greenhouse gas removals of more than this quantity, the excess would be ineligible for temporary or long-term certified emissions reductions.
• Exclude funds obtained through small-scale project activities from the share of proceeds to be used to assist developing countries particularly vulnerable to the adverse impacts of climate change. Such countries shall be entitled to a reduced level of the non-reimbursable fee for requesting registration and a reduced rate of the proceeds to cover administrative expenses of the CDM.

In 2005, at the Eleventh Conference of the Parties (COP-11 of the UNFCCC) and the first conference serving as the Meeting of the Parties (MOP) to the Kyoto Protocol, delegates agreed to a set of IPCC Principles, Rules, and Guidelines governing LULUCF activities,¹⁴¹ such as:

• Carbon stocks must be excluded from greenhouse gas accounting.
• Accounting for LULUCF activities does not imply a transfer of commitments to a future commitment period.
• Reversal of any removal due to LULUCF activities must be accounted for at the appropriate time.

Land-Use Data Issues

The EPA’s most recent inventory report discusses the uncertainty inherent in the methodology used to estimate forest carbon stocks.¹⁴² The estimates of forest carbon in live biomass, dead wood, and litter are based on USDA forest survey data for the conterminous United States, because no survey data are available for Alaska, Hawaii, and the U.S. Territories. The survey data are statistical samples designed to represent vast areas of land. The USDA mandates that the survey data be accurate to within 3 percent, at a confidence level of 67 percent.¹⁴³ An analysis of this methodology for the southeastern United States showed that the uncertainty resulted from sampling errors and not from the regression equations used to calculate tree volume (and thus carbon) from survey statistics such as tree height and diameter. The standard errors of 1 to 2 percent for volumes of growing stock in individual States are insignificant; however, those for changes in the volumes of growing stock are much higher, ranging from 12 percent to as much as 139 percent.¹⁴⁴

Additional uncertainty is associated with the estimates of carbon stocks in other carbon pools, which are based on extrapolations of the relationships among variables in site-specific studies to all forest land. Such extrapolation is needed in the absence of survey data on other carbon pools.¹⁴⁵ The extrapolations bring in uncertainty from modeling errors and conversions between different reporting units. The effect of land-use change and forest management activities (such as harvest) on soil stocks is another large source of uncertainty, with little consensus in the literature.

Chapter 6 Notes and Sources

Tables 33-37