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reflecting aerosols (Ramanathan et al., 2008). The green curve in Figure 2.8
shows what happens if the aggregate of all aerosols brought under control
sums to a cooling effect before mitigation; the mitigation in this case ac-
celerates the approach to the CO 2 -only curve, as the masking effect of the
aerosols is eliminated. If the long-term situation instead includes a recalci-
trant methane emission rate that is stabilized but not brought to zero, then
the long-term warming is brought above the CO 2 -only case for a period as
long as the methane emissions continue.
2.4 CARBON CYCLE
The evolution of atmospheric carbon dioxide concentrations depends
on the balance of human emissions, natural processes that remove excess
carbon dioxide from the atmosphere, and the sensitivity of land and ocean
carbon reservoirs to climate change and land use (see Box 2.2). The histori-
cal atmospheric growth rate of carbon dioxide is well-constrained for the
past 50 years from direct instrumental measurements and for periods prior to
that from measurements of gases trapped in ice cores. Global average atmo-
spheric CO 2 has risen from a pre-industrial level of about 280 ppm to about
390 ppm by the beginning of 2010. A definitive anthropogenic origin for the
excess carbon dioxide can be assigned based on contemporaneous changes
in carbon isotopes, a parallel decrease in atmospheric oxygen, and by the
fact that the atmospheric carbon dioxide levels for the preceding several
millennia of the Holocene had hovered within plus or minus 5 ppm of the
pre-industrial value. Past fossil fuel combustion rates and carbon emissions
from cement production and land-use change (e.g., deforestation, shifting
land into pasture and agriculture) can be reconstructed, and net land and
ocean carbon sources and sinks can be quantified from a combination of
observations and numerical models. Figure 2.9 presents a recent synthesis
for the global carbon system showing the fluxes between the atmosphere and
various reservoirs versus time (Le Quéré et al., 2009). The terrestrial carbon
fluxes are partitioned with carbon emissions from direct human land-use
change including recovery from earlier human land use, separated from
terrestrial carbon sinks in response to elevated CO 2 and climate.
The response of the global carbon cycle to human perturbations can
be characterized by the airborne CO 2 fraction, the fraction of the cumula-
tive carbon dioxide emitted by fossil fuel combustion and land-use change
that remains in the atmosphere. The contemporary airborne fraction is cur-
rently slightly less than half (~0.45), and for any specified carbon emission
trajectory, future atmospheric carbon dioxide concentrations depend on the
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