EXECUTIVE SUMMARY

The Global Carbon Cycle

• During the period 1850-1998, approximately 405 ± 60 Gt C has been emitted as carbon dioxide (CO2) into the atmosphere as a result of fossil fuel burning and cement production (67 percent), and land use and land-use change (33 percent), predominantly from forested areas. As a result, the atmospheric CO2 concentration has risen from 285 ± 5 ppmv to 366 ppmv (i.e., about a 28 percent increase). This increase in CO2 concentration accounts for about 40 percent of these anthropogenic emissions, the remainder having been absorbed by the oceans and terrestrial ecosystems.
• CO2 that is dissolved into the ocean will be transferred progressively to the deep ocean, and the carbon content of this reservoir is increased. The fate of CO2 that is fixed on land depends on which ecosystem and which carbon pool is the repository (e.g., living biomass or soils). Carbon fixed into a pool with a turnover time of one year or less (leaves, fine roots) is returned to the atmosphere or transferred into pools with a longer turnover time of decades to centuries (stems, trunks, soil organic matter).
• The net global carbon flux between terrestrial ecosystems and the atmosphere is the result of a small imbalance between uptake by photosynthesis and release by various return processes. Plants, soil microbes, biochemical processes, animals, and disturbances contribute to the latter. Variations of climate and human activities have a major impact through land use and land-use changes, as well as indirectly through carbon dioxide fertilization, nutrient deposition, and air pollution.
• This global net carbon exchange has resulted in an uptake of CO2 by the terrestrial biosphere amounting to 0.2 ± 1.0 Gt C yr^-1 (90-percent confidence interval) over the 1980s (1980-89), and 0.7 ± 1.0 Gt C yr^-1 during the most recent decade (1989-98) (see Table 1-2). It is unclear if the increase in the 1990s is a result of natural variability or, to some extent, also a trend induced by human activities.
• The direct effects of land use and land-use change are estimated to have led to a net emission of 1.7 ± 0.8 Gt C yr^-1 during the 1980s, and 1.6 Gt C yr^-1 during the 1990s. The difference between the net global terrestrial uptake and human-induced emissions as a result of land use and land-use change leaves a residual terrestrial uptake of 1.9 ± 1.3 Gt C yr^-1 for the 1980s and 2.3 ± 1.3 Gt C yr^-1 for the 1990s.
• The global net carbon flux varies from one year to another. These variations are on the order ± 1 Gt C yr^-1 and are correlated with variations in climate (e.g., El Niño/La Niña events) and major volcanic eruptions.

Present Knowledge about Global Terrestrial Ecosystems

• Gross Primary Productivity (GPP) is the uptake of carbon from the atmosphere by plants (global total approximately 120 Gt C yr^-1). Carbon losses as a result of plant respiration reduce this uptake to the Net Primary Productivity (NPP; global total approximately 60 Gt C yr^-1). Further losses occur because of decomposition of dead organic matter, resulting in Net Ecosystem Productivity (NEP; global total approximately 10 Gt C yr^-1). Additional losses are caused by disturbances, such as fire, wind-throw, drought, pests, and human activities. The resulting net imbalance of the terrestrial ecosystem can be interpreted as the Net Biome Productivity (NBP; presently approximately 0.7 ± 1.0 Gt C yr^-1, as a decadal average; see Figure 1-2).
• Forests contain a large part of the carbon stored on land, in the form of biomass (trunks, branches, foliage, roots etc.) and in the form of soil organic carbon (Table 1-1). On a time scale of years, most forests accumulate carbon through the growth of trees and an increase in soil carbon, until the next disturbance occurs. The net carbon uptake (NEP) may locally reach 7 t C ha^-1 yr^-1, but losses may also be observed when soil carbon is decreasing or trees are overmature and mortality is occurring.
• In cropland ecosystems, carbon stocks are primarily in the form of below-ground plant organic matter and soil. Most of these ecosystems have large annual carbon uptake rates, but much of the gain is exported in the form of agricultural products and their associated waste materials; this gain is rapidly released to the atmosphere. Although carbon is recaptured during the succeeding cropping season, many agricultural soils are currently net sources of carbon. Shifting to low or no till cultivation is, however, increasingly being used to mitigate such trends.
• By far most of the carbon stocks in grassland and savannas, including rangelands and pasture, are found in the soils. These stocks are stable over long time spans, but losses can occur if grazing pressure exceeds carrying capacity or if the frequency of fires increases.
• Wetland stocks of carbon are found almost entirely in the soil as dead organic matter, which can be released by human activity, such as drainage. Afforestation may effectively compensate for such development. Soil carbon in subarctic wetlands may also be released as a result of reduction of permafrost resulting from climate warming.
• Globally, carbon stocks in the soil exceed carbon stocks in vegetation by a factor of about five (Table 1-1). This ratio ranges from about 1:1 in tropical forests to 5:1 in boreal forests and much larger factors in grasslands and wetlands. Changes in soil carbon stocks are at least as important for carbon budgets as changes in vegetation carbon stocks.

IPCC Homepage