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
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