Figure 3.1: The global carbon
cycle: storages (PgC) and fluxes (PgC/yr) estimated for the 1980s. (a) Main components
of the natural cycle. The thick arrows denote the most important fluxes from the
point of view of the contemporary CO2 balance of the atmosphere: gross
primary production and respiration by the land biosphere, and physical air-sea
exchange. These fluxes are approximately balanced each year, but imbalances can
affect atmospheric CO2 concentration significantly over years to centuries.
The thin arrows denote additional natural fluxes (dashed lines for fluxes of carbon
as CaCO3), which are important on longer time-scales. The flux of 0.4 PgC/yr from
atmospheric CO2 via plants to inert soil carbon is approximately balanced
on a time-scale of several millenia by export of dissolved organic carbon (DOC)
in rivers (Schlesinger, 1990). A further 0.4 PgC/yr flux of dissolved inorganic
carbon (DIC) is derived from the weathering of CaCO3, which takes up CO2
from the atmosphere in a 1:1 ratio. These fluxes of DOC and DIC together comprise
the river transport of 0.8 PgC/yr. In the ocean, the DOC from rivers is respired
and released to the atmosphere, while CaCO3 production by marine organisms results
in half of the DIC from rivers being returned to the atmosphere and half being
buried in deep-sea sediments - which are the precursor of carbonate rocks. Also
shown are processes with even longer time-scales: burial of organic matter as
fossil organic carbon (including fossil fuels), and outgassing of CO2
through tectonic processes (vulcanism). Emissions due to vulcanism are estimated
as 0.02 to 0.05 PgC/yr (Williams et al., 1992; Bickle, 1994). (b) The human perturbation
(data from Table 3.1). Fossil fuel burning and land-use change are the main anthropogenic
processes that release CO2 to the atmosphere. Only a part of this CO2
stays in the atmosphere; the rest is taken up by the land (plants and soil) or
by the ocean. These uptake components represent imbalances in the large natural
two-way fluxes between atmosphere and ocean and between atmosphere and land. (c)
Carbon cycling in the ocean. CO2 that dissolves in the ocean is found
in three main forms (CO2, CO32-, HCO3-, the sum of which is DIC). DIC
is transported in the ocean by physical and biological processes. Gross primary
production (GPP) is the total amount of organic carbon produced by photosynthesis
(estimate from Bender et al., 1994); net primary production (NPP) is what is what
remains after autotrophic respiration, i.e., respiration by photosynthetic organisms
(estimate from Falkowski et al., 1998). Sinking of DOC and particulate organic
matter (POC) of biological origin results in a downward flux known as export production
(estimate from Schlitzer, 2000). This organic matter is tranported and respired
by non-photosynthetic organisms (heterotrophic respiration) and ultimately upwelled
and returned to the atmosphere. Only a tiny fraction is buried in deep-sea sediments.
Export of CaCO3 to the deep ocean is a smaller flux than total export production
(0.4 PgC/yr) but about half of this carbon is buried as CaCO3 in sediments; the
other half is dissolved at depth, and joins the pool of DIC (Milliman, 1993).
Also shown are approximate fluxes for the shorter-term burial of organic carbon
and CaCO3 in coastal sediments and the re-dissolution of a part of the buried
CaCO3 from these sediments. (d) Carbon cycling on land. By contrast with the ocean,
most carbon cycling through the land takes place locally within ecosystems. About
half of GPP is respired by plants. The remainer (NPP) is approximately balanced
by heterotrophic respiration with a smaller component of direct oxidation in fires
(combustion). Through senescence of plant tissues, most of NPP joins the detritus
pool; some detritus decomposes (i.e., is respired and returned to the atmosphere
as CO2) quickly while some is converted to modified soil carbon, which
decomposes more slowly. The small fraction of modified soil carbon that is further
converted to compounds resistant to decomposition, and the small amount of black
carbon produced in fires, constitute the “inert” carbon pool. It is
likely that biological processes also consume much of the “inert” carbon
as well but little is currently known about these processes. Estimates for soil
carbon amounts are from Batjes (1996) and partitioning from Schimel et al. (1994)
and Falloon et al. (1998). The estimate for the combustion flux is from Scholes
and Andreae (2000). ‘t’ denotes the turnover time for different components
of soil organic matter.
