7. Land Use Issues

Emissions of carbon dioxide from the combustion of fossil fuels are dwarfed by carbon dioxide emissions and absorption from natural processes. As noted in Chapter 1, natural processes in the oceans and biomass are responsible for most carbon dioxide absorption and emissions. This is also true of methane and nitrous oxide. Most methane and nitrous oxide are created by bacteria in soils and wetlands. Although the most important natural processes affecting greenhouse gas concentrations in the atmosphere are not subject to human control, modifications in land use can influence their concentrations in significant ways.

The magnitude of the influence is difficult to measure. Unlike service station pumps, trees and swamps do not come equipped with meters. Thus, analysts can only estimate emissions or sequestration on the basis of small sample surveys and extrapolate (with associated uncertainty) to much larger regions. A second, related problem is distinguishing between human-caused and natural phenomena. An electric power plant is clearly a human artifact. Trees growing back over abandoned farmland are a more ambiguous case.

The most important changes in land use affecting the carbon budget are those that increase or reduce forest land. Every year in the United States and throughout the world a very large amount of carbon dioxide, on the order of 100 billion metric tons, is removed from the atmosphere and sequestered into biomass [96]. At the same time, carbon is released to the atmosphere from vegetative respiration, combustion of wood as fuel, degradation of manufactured wood products, and the natural decay of expired vegetation. The net numerical difference, or flux, between carbon sequestration and release can be viewed as a measure of the relative contribution of biomass to the carbon cycle. World flux is difficult to measure but is thought to be close to zero; in other words, sequestration and respiration are roughly in balance worldwide [97]. In the United States, however, forests and the wood products produced from them sequestered a net of approximately 111 million metric tons of carbon (407 million metric tons of carbon dioxide) in 1992, including 12 million metric tons of carbon sequestered in wood products and 15 million metric tons of carbon sequestered in landfilled wood product waste [98]. A further 127 million metric tons of carbon is believed to be sequestered in forest soils. Sequestered carbon offset 8 to 17 percent of the 1,381 million metric tons of carbon (or 5,068 million metric tons of carbon dioxide) emitted in the United States in 1992 from the burning of fossil fuels (see Chapter 2).

Estimates of carbon sequestration for years after 1992 are not currently available, but the U.S. Forest Service has made projections for 1993-1995 based on the assumption that historical trends before 1992 continued unchanged. These projections, which are 2 to 3 percent lower than the 1992 estimate, result in projected carbon sequestration in 1994 equivalent to approximately 12 percent of U.S. carbon dioxide emissions for that year [99].

The current inventory of carbon in U.S. forests is enormous. A study by researchers Birdsey and Heath of the U.S. Forest Service estimated that U.S. forest ecosystems contained 54.6 billion metric tons of carbon in 1992 [100]—the equivalent of nearly 40 years of U.S. carbon emissions from fossil fuel consumption.

Table 37 shows U.S. Department of Agriculture Economic Research Service estimates of the major uses of land in the United States, developed through a periodic sample survey that is updated every 5 years (the last update was 1992). The USDA publication that gives the most comprehensive view of land use, Major Uses of Land in the United States, indicates that between 1987 and 1992 there was a net increase of 6 million acres of forest land [101]. Table 37 indicates that “Forest-Use Land” remained stable at 648 million acres between 1987 and 1992. However, “Special-Use Areas” include extensive forest land in the form of National Parks and wilderness areas. In 1992, forested “Special-Use Areas” accounted for nearly 89 million acres. As a result, forested land in the U.S. totals about 737 million acres.

Table 37. Major Uses of Land in the United States (Million Acres)
Land Use	1978	1982	1987	1992
Cropland	471	469	464	460
Used for Crops	369	383	331	338
Idle Cropland	26	21	68	56
Pasture	76	65	65	67
Grassland Pasture and Range	587	597	591	591
Forest-Use Land	703	655	648	648
Grazed Land	172	158	155	145
Other Use	531	497	493	503
Special Use Areas	158	270	279	281
Miscellaneous Other Land	345	274	283	283
Total Land Area	2,264	2,265	2,265	2,263
Sources: A. Daugherty, Major Uses of Land in the United States: 1987, Economic Research Service Report 643 (Washington, DC: U.S. Department of Agriculture, January 1991), p. 4; and A. Daugherty, Major Uses of Land in the United States: 1992, Agricultural Economic Report Number 723 (Washington, DC, September 1995), p. 4.

The 737 million acres of forest land in the United States represents approximately two-thirds of the area that was forested in the year 1600 (1.04 billion acres) [102]. Figure 12 illustrates the extent of forest land in the coterminous United States in 1620, 1850, 1920, and 1992. Three important facts are highlighted by these maps. First, slightly less than 50 percent of the land area of the United States has historically contained forest, primarily because large areas of U.S. land are inhospitable to trees, limiting maximum potential carbon sequestration. Areas suitable for forest are concentrated primarily east of the Mississippi, in the Pacific Northwest, and in western Colorado. Second, very little of the extensive forest present in 1620 remained by 1920, most having been cleared for agriculture and to produce timber; by 1920, the clearing of forests for agriculture had largely come to an end. Third, large areas returned to forest between 1920 and 1992, especially in the East where abandonment of agriculture set the stage for large-scale forest regrowth.

Aggregate annual carbon sequestration generally follows trends in forest cover, except when biomass density on forest lands changes dramatically, such as from either extensive timber harvesting (which results in forest land that sequesters relatively little carbon) or from a long-term lull in harvesting (which results in forest land that sequesters relatively large amounts of carbon).

The regrowth of U.S. forests has had important impacts on net U.S. carbon dioxide emissions. U.S. forests have been a net carbon sink since 1952. According to Forest Service researchers Richard Birdsey and Linda Heath, between 1952 and 1992, carbon stored on U.S. forest land increased by 11.3 billion metric tons, an average of 281 million metric tons per year, and an amount that offset approximately 25 percent of U.S. emissions of carbon for the period [103]. In addition to reforestation associated with the abandonment of agriculture in the East, more than 4 million acres of marginal cropland have been reforested since 1974 under such Federal programs as the Conservation Reserve Program, Agricultural Conservation Program, and Forestry Incentives Program [104]. Birdsey and Heath estimated that U.S. forests will continue to be net carbon sinks well into the future, sequestering carbon at an average net annual rate of 177 million metric tons between 1992 and 2040 (not including sequestration into wood products and landfills), for a total increase in stored carbon of 8.5 billion metric tons [105].

It is difficult to be specific about how much carbon might be gained or lost through transformations of grasslands, pasturelands, or croplands. Although the amount of carbon in a square meter of forest might be on the order of 9 to 26 kilograms, depending on the condition of the forest and the age and type of trees growing, typical estimates of carbon storage in cultivated lands range from 1 to 8 kilograms per square meter, and estimates for uncultivated (but cultivatable) lands range from 2 to 10 kilograms per square meter [106]. Thus, there is less carbon to be gained or lost, and the range of possible outcomes per unit of land is consequently smaller.

Between 1987 and 1992, the extent of cropland in use in the United States increased by 9 million acres, while the amount of idle cropland declined by 12 million acres. The shift from idled to cultivated and grazed cropland should, in principle, lead to small decreases in net carbon storage. Shifts from any of the above to urban land, the fastest single growth category, should lead to stable or slightly reduced storage. Land in urban areas, as measured by the Bureau of the Census, totaled 55.9 million acres in 1990, up from 47.3 million acres in 1980 [107].

The range of observed methane fluxes from U.S. wetlands is enormous. One survey of experiments conducted in the United States found estimates ranging from a negative flux (methane absorption) to a flux of 213 grams of methane per square meter per year, largely dependent on habitat type [108]. Thus, it is difficult to extrapolate from experimental data to large-scale emissions estimates.

Estimates of global methane fluxes from wetlands tend to indicate that methane emissions from temperate-zone wetlands are minimal—typically between 5 and 10 million metric tons of methane per year for worldwide temperate-zone wetlands (which include U.S. wetlands)—when compared with estimated global wetlands emissions of 110 million metric tons [109]. The U.S. share of all temperate-zone wetlands is about 57 percent, and U.S. wetlands lost during the 1980s accounted for about 0.5 percent of the extent of wetlands at the beginning of the period. Consequently, the reduction in natural methane emissions from U.S. wetlands lost might be on the order of 10,000 to 20,000 metric tons annually over the decade.

The scientific literature suggests that grass and forest lands are both weak natural sinks for methane and weak natural sources for nitrous oxide, although adequate research to establish accurate estimates of aggregate methane and nitrous oxide emissions and sequestration is lacking. Natural soils apparently serve as methane sinks: well-aerated soils contain a class of bacteria called “methanotrophs” which use methane as food and oxidize it into carbon dioxide. Experiments indicate that cultivation reduces methane uptake by soils and increases nitrous oxide emissions [110]. Exactly how much methane is absorbed by natural soils, and how much nitrous oxide is emitted, is difficult to estimate, although total amounts are very small.

It is known that conversion of forests and grasslands to cropland accelerates nitrogen cycling and increases nitrous oxide emissions from the soil. It is not known with certainty by how much (see Appendix A) [111].

TO:
Appendix A. Estimation Methods

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