Although these facts demonstrate that 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 amounts that are still significant. The magnitude of this influence is difficult to measure. Natural processes, particularly below-ground growth and soil dynamics, are often highly variable, unpredictable, and therefore difficult to measure. A related problem arises in 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 U.S. share of the carbon budget are those that increase or reduce forest land. Each year, a very large amount of carbon dioxide, on the order of 100 billion metric tons, is removed from the atmosphere and sequestered into biomass and soil worldwide. At the same time, carbon is released to the atmosphere from vegetative respiration, combustion of wood in natural or purpose-ly set forest fires or as fuel, degradation of manufactured wood products, and the natural decay of rotting vegetative detritus. The net numerical difference, or flux, between carbon sequestration and carbon release due to natural factors can be viewed as a measure of the relative contribution of biomass to the carbon cycle. A further 127 million metric tons of carbon were believed to be sequestered in forest soils and the forest floor in 1992.