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. 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. 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. A further 127 million metric tons of carbon is believed to be sequestered in forest soils. 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.