Climate Simulation and Prediction - Climate Dynamics - page 183

Plant and soil respiration each release about 60 PgC to the atmosphere each
year, and this source is balanced by the carbon capture of 120 PgC/yr dur-
ing photosynthesis. Deforestation and land use are disturbing this balance and
causing the additional release of an estimated 0.9 PgC/yr into the atmosphere.
Approximately 90 PgC/yr is released from the ocean to the atmosphere,
and the ocean uptake is slightly larger, at 92 PgC/yr. The largest single source
of atmospheric carbon (i.e., CO 2 ) is outgassing from the ocean. Note that the
exchange of carbon between the atmosphere and the ocean is unbalanced. The
oceans are currently estimated to be absorbing an extra 2 PgC from the at-
mosphere each year (see section 2.5 on the ocean biological pump). Human
activity is perturbing the natural carbon cycle, leading to the imbalance in the
atmosphere/ocean exchange by the release of 6 PgC/yr in fossil fuel burning
and 0.9 Pg/C/yr in modifying the land surface and vegetation.
Figure 12.1 is highly simplified, and there are many complex processes in-
volved as carbon in various forms is sequestered and flows among the climate
subsystems. The partitioning of CO 2 between the atmosphere and the ocean,
for example, is not only temperature dependent but also affected by the great
variety of inorganic reactions and biological activity in the upper ocean and
rates of exchange with the thermocline and deep ocean. The current challenges
for including the carbon cycle in climate models are to identify the dominant
processes that determine the sizes of carbon reservoirs and their rates of ex-
change, and to express them as accurately as possible in models.
THE METHANE CYCLE
The methane (CH 4 ) cycle is also included in ESMs because of methane's high
GWP ( Table 10.1) , but our current knowledge of global methane cycling is
incomplete. For example, the reason for the hiatus in global CH 4 trends from
2000 to 2006 seen in Figure 10.2 i is not completely understood.
Estimates for components of the global CH 4 budget are listed in Table 12.1.
Methane emissions derive from both biogenic and non-biogenic processes. The
primary natural source of atmospheric methane, estimated at about 79% of
current natural emissions, is wetlands. Other important natural sources include
termites—CH 4 is produced when cellulose is broken down in their digestive
tract—forests, and oceans.
Natural gas is composed of about 75% methane, so its use in human activ-
ity contributes to the observed increases in atmospheric CH 4 . The largest atmo-
spheric CH 4 source associated with the use of natural gas is fugitive emissions,
including leaks from equipment and underground pipes. Vented emissions in
natural gas recovery and combusted emissions also contribute significantly.
Anthropogenic emissions also include rice agriculture, biomass burning, ranch-
ing (CH 4 is released as a result of enteric fermentation, which is the digestive
decomposition of carbohydrates), and landfills and waste treatment facilities
where bacteria release methane as they feed on organic material.
The primary sink for atmospheric CH 4 is its chemical reaction with the hy-
droxyl radical (OH) to form two other greenhouse gases, namely, water vapor
and CO 2 . Because OH is a scavenger species that reacts with and removes pol-
lutants, increases in atmospheric CH 4 can amplify pollution levels by reducing