7. Land Use Issues
7. Land Use IssuesEmissions of carbon dioxide from the combustion of fossil fuels are only a small fraction of the carbon dioxide emissions and absorption from natural processes. As noted in Chapter 1, the oceans and biomass are responsible for most carbon dioxide absorption and emissions. The same is true for methane and nitrous oxide . Most methane and nitrous oxide are created by bacteria in soils and wetlands . Thus, it is possible that the human race's most profound influences on greenhouse gas concentrations in the atmosphere come from modifying land use patterns.
Unfortunately, this is a difficult proposition to prove or disprove. 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 (dangerously) 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.
This chapter describes some of the ways in which land use changes can affect emissions and sequestration of greenhouse gases, and provides "order of magnitude" estimates of the scale of such emissions and sequestration.
Every green plant is, in effect, a solar-powered factory that extracts carbon dioxide from the atmosphere, separates the carbon atom from the oxygen atoms, returns oxygen to the atmosphere, and uses the carbon to make biomass in the form of roots, stems, and leaves. Thus, the most important natural carbon sequestration process is photosynthesis . Dead vegetation may accumulate as soil, sequestering additional mineral carbon in the ground.
Even without the gradual sequestration of carbon in the soil, the current inventory of biomass in the form of forests and natural vegetation contains enormous amounts of carbon. A recent study, conducted by Richard Birdsey of the U.S. Forest Service, estimated that U.S. forest ecosystems contained 52.5 billion metric tons of carbon in 1987 (153) -the equivalent of nearly 40 years of U.S. carbon emissions from fossil fuel consumption.
In the tropics, the destruction of tropical forests releases large volumes of carbon dioxide into the atmosphere. In the United States, however, the process is moving in reverse. Much of the North American continent was cleared for farming during the 19th century. Today, as agriculture grows in productivity, marginal farmland, particularly in the eastern and southern United States, is being abandoned and is reverting to timberland. Forests that were cleared for timber over the past hundred years are also regenerating.
As forests grow, they absorb carbon from the atmosphere and incorporate it into biomass and soils. Birdsey estimated that U.S. forests absorbed 460 million metric tons of carbon between 1982 and 1987- equivalent to an annual growth rate in sequestered carbon of 0.8 percent. When carbon lost through logging, burning, and land clearing is subtracted (355 million metric tons), forests absorbed a net 106 million metric tons of carbon, which is equivalent to about 8 percent of energy-related carbon emissions. A later estimate, made in a report recently completed by the EPA, estimated net absorption from forested areas in the coterminous United States at 114 million metric tons for the year 1990. (154) That report also estimated total carbon storage in U.S. forestland at 38.5 billion metric tons, (155) which illustrates some of the uncertainties associated with calculations of this sort.
A third, and most recent, estimate by Richard Birdsey used information from the latest (1992) survey of forest areas in the United States, conducted by the U.S. Forest Service. Using this information, and incorporating it into his model of carbon sequestration, Birdsey estimated a somewhat higher figure for 1990 than the 1987 figure: a net 130 million metric tons. Further work based on the 1992 estimate continues, and revised estimates are expected.
While trees on existing acreage continue to absorb carbon, the acreage planted in forests can also expand or contract. Farm and pastureland can be converted to forestland, and forestland can be cleared and converted to other uses. Clearing forestland sets the stage for large-scale losses of carbon to the atmosphere, while allowing crop or pastureland to grow trees sets the stage for large-scale absorption of carbon.
Other types of land use conversion (156) produce more ambiguous results. In general, converting pasturelands or grasslands into cropland probably produces emissions of carbon through the destruction of biomass and loss of soil carbon. Typical estimates of the amount of soil carbon lost are approximately 30 percent of the amount in place at the time of conversion. These losses can be expected to take place over a period of 20 years, or longer, following conversion. Similarly, abandoning cropland or converting it to pasture or forest should lead to net carbon sequestration through the creation of biomass and gradual additions to soil carbon.
Table 38 shows U.S. Department of Agriculture estimates of the major uses of land in the United States, which were developed through a periodic sample survey. These surveys are conducted every 5 years, most recently for the year 1992. The Agriculture Department has not yet completed publication of all the 1992 survey results: in particular, the publication that gives the comprehensive view of land use, Major Uses of Land in the United States, has not yet been published. However, material that has been published indicates that between 1987 and 1992 there was a net increase of 6 million acres of forestland. (157) Between 1982 and 1987 there was a large net reduction in the amount of active cropland. The amount of land used for crops declined by 52 million acres, while the amount of idle cropland increased by 47 million acres. Similar national-level statistics are not yet available for 1992, but available data indicate that the total amount of non- Federal rural cropland (excluding Alaska) declined by 24 million acres. (158) While the total amount of cropland declined, the amount of idle cropland increased by 1 million acres. There were much smaller declines in private pastureland and rangeland (1.6 million acres and 3.7 million acres, respectively).
The shift from cultivated to idle cropland-or shifts from cropland to range or pasture-should, in principle, lead to small increases in net carbon storage. Shifts from any of the above to urban land should lead to reductions in storage.
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. (159) Thus, there is less carbon to be gained or lost, and the range of possible outcomes per unit of land is consequently smaller.
A recent study commissioned by the EPA estimated a current average soil carbon content for an area of 110 million hectares (272 million acres) of farmland in the United States at 4.8 to 7.9 kilograms per square meter. (160) The study estimated that 1.0 billion to 1.6 billion metric tons of soil carbon had been lost from the farmland since it had been placed in cultivation, equivalent to 16 percent of the estimated original carbon content of the soil. The study also noted, however, that land with a soil carbon content of less than 4 kilograms per square meter was generally not being cultivated at the time of the study. It would not be surprising if the least fertile farmland were the most likely to be removed from cultivation. Therefore, the carbon gains from idling cropland are likely to be small: if the gains are commensurate with the original losses, they would be on the order of 0.6 kilograms per square meter (2.4 metric tons per acre), accrued over 20 to 50 years. However, the small gain would be distributed over a large acreage, since, as noted above, some 47 million acres were withdrawn from cultivation between 1982 and 1987, implying eventual carbon storage of nearly 114 million metric tons, accrued at a rate of 2 million to 5 million metric tons per year. An estimate made for the year 1980 suggested that carbon sequestration from the abandonment of croplands in the United States and Canada was 3 million metric tons. (161)
Similarly, converting land to forest should produce carbon gains, both through the addition of biomass (i.e., carbon stored in trees) and through the accretion of carbon into the soil, as dead limbs and trees pile up and gradually decay. On average, the amount of carbon stored in U.S. forests is 17.7 kilograms per square meter of forestland (using the Birdsey/USDA estimate for storage), or 13 kilograms per square meter (using the EPA estimate). The range in forest storage across States is very large: from 9 kilograms per square meter in Nevada to 26 kilograms per square meter in Alaska, according to Birdsey. As a crude numerical example, converting former cropland to forest might sequester approximately 10 kilograms per square meter over a 70-year period. (The actual amount would depend on the soil fertility and original carbon content of the land, the type of trees planted, and other factors specific to a particular plot of land.) This would produce 40.5 metric tons per acre over a 70-year period, or an average of 0.58 metric tons per year per acre. If the actual characteristics of the 6 million acres converted over the past 5 years matched this example, the conversion would ultimately store about 243 million tons of carbon over the next 70 years, at an average rate of 3.5 million metric tons per year. The actual annual rate could vary considerably from the average, since some species of trees grow much more rapidly in their early years than do others.
The examples above highlight two methodological problems common to adding carbon sources and sinks derived from land use data to more conventional greenhouse gas emissions:
As discussed in Chapter 1, there are numerous natural sources of methane . Anthropogenic land use changes inevitably affect these natural sources. One such natural source is wetlands. However, the stock of natural wetlands in the United States has diminished considerably over the past 2 centuries, which should, in principle, have reduced biogenic methane emissions. A recent study of wetland losses concluded that the United States had lost approximately 30 percent of its wetlands between colonial times and the mid- 1980s. (162) Remaining wetlands total 274 million acres; wetlands lost total 119 million acres.
An update of the wetlands study indicates that 654,000 acres were converted from wetlands to other uses between 1982 and 1987, and that an additional 431,000 acres were converted between 1987 and 1991. (163) The report does not, however, provide information on the conversion of other land categories to wetlands during this period, so it is a gross, rather than a net, figure.
The range of observed methane fluxes from U.S. wetlands is enormous. One survey of experiments conducted entirely in the United States, for instance, found estimates ranging from a negative flux (methane absorption) to a flux of 213 grams of methane per square meter per year. (164) Thus, it is difficult to extrapolate from experimental data to large-scale emissions estimates.
Estimates of global methane fluxes from wetlands have tended 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. (165) The U.S. share of all temperate-zone wetlands is about 57 percent, and temperate-zone wetlands lost during the 1980s accounted for about 0.5 percent of U.S. wetlands at the beginning of the period. Consequently, the reduction in natural methane emissions from wetlands lost might be on the order of 0.57 x 0.005 x 5 to 10 million metric tons of methane, or from 10,000 to 20,000 metric tons of methane annually over the decade.
The scientific literature suggests that grasslands and forestlands are both natural sinks for methane and natural sources for nitrous oxide . 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. (166)
Exactly how much methane is absorbed by natural soils, and how much nitrous oxide is emitted, is more difficult to estimate. One report, which was based on a few experiments, indicates that methane uptake in temperate evergreen and deciduous forests in the United States ranged from 0.19 to 3.17 milligrams (measured in carbon units) per square meter per day, which is equivalent to the uptake of 36.8 to 624.4 metric tons of methane per million acres per year. (167) Thus (assuming no methane uptake at all from previous use), adding 6 million acres of forestland from 1987 to 1992 should increase methane absorption by 221 to 3,746 metric tons per year. This figure can be compared with U.S. anthropogenic methane emissions as estimated in this report of about 27 million metric tons per year.
Another report in the scientific literature indicates that some sample plots of pastureland in the United States had methane uptake of 4.1 milligrams (measured in carbon units) per square meter per day (for fertilized pasture) and 6.3 milligrams per square meter per day (for unfertilized pasture), and uptake from fertilized wheat and maize fields ranged from 0.2 to 0.9 milligrams per square meter. (168)
This literature implies that an additional 0.6 to 6.1 milligrams per square meter per day of methane would be absorbed by the abandonment of farmland, equivalent to 118.1 to 1,201.1 grams per acre per year. Applying these figures to the 50 million acres of cropland taken out of production between 1982 and 1987, this implies an increase in methane uptake from 5,870 to 60,055 metric tons per year from this source.
If such estimates are to be applied to emissions inventories, the same problem of crediting such uptakes applies. Removing an acre of farmland from production in a particular year creates a permanent annual methane sink that will absorb small additional amounts of methane each year thereafter, or at least until the use of the land changes. The method that should be used to credit such permanent reductions to a particular year is not obvious.
The same research paper that described methane uptake in fertilized and unfertilized plots also described changes in nitrous oxide emissions from fertilized soils. (169) (The mechanism by which this occurs is discussed in Chapter 4.) That paper indicated that unfertilized soils had emissions of 0.25 to 0.35 milligrams (measured in nitrogen units) per square meter per day, while emissions from fertilized soils ranged from 0.6 to 1.65 milligrams per square meter per day. Thus, abandoning fertilization should reduce nitrous oxide emissions by 0.25 to 1.4 milligrams per square meter per day-the equivalent of 58 to 324 metric tons of nitrous oxide per million acres per year. Applying this figure to the 50 million acres of cropland taken out of production between 1982 and 1987 implies a reduction in nitrous oxide emissions ranging from 2,900 to 16,200 metric tons annually. In principle, however, any reduction in emissions from this source should be captured by reduced application of nitrogen fertilizers; thus, reporting results from this method would result in double counting.
