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.
