Environmental Engineering Reference
In-Depth Information
age, and biogeochemical responses to changing ocean circulation also need
to be considered when assessing future net carbon uptake. The inorganic
carbon concentration in subsurface ocean waters is generally elevated over
surface concentrations because of the downward transport and subsequent
respiration of organic water originally produced in the surface layer. In
coupled carbon-climate models, biogeochemical feedbacks to a warmer
climate tend to partially offset physical-chemical effects and act to reduce
the overall strength of ocean climate-carbon cycle feedbacks. In the South-
ern Ocean, enhanced outgassing of natural CO
2
due to stronger winds and
upwelling may more than compensate for increased anthropogenic CO
2
uptake, leading to a net reduction in ocean uptake (Le Quéré et al., 2008;
Lovenduski et al., 2007, 2008). Recent observations of the air-sea difference
in the partial pressure of carbon dioxide, the driving force for air-sea CO
2
exchange, indicate a weakening of oceanic uptake in a number of regions,
although there remains some debate whether this signal should be attributed
to climate change, ozone depletion, or primarily decadal climate variability
(Le Quéré et al., 2009; Watson et al., 2009).
It is more difficult to directly constrain, on a global scale, the net
fluxes of carbon into and out of the more heterogeneous terrestrial carbon
reservoirs, and terrestrial uptake is often estimated from a combination of
terrestrial biogeochemical models and satellite remote sensing approaches
that have been assessed using process experiments, local CO
2
flux towers,
etc. (Canadell et al., 2007; Raupach et al., 2007; Le Quéré et al., 2009).
Land carbon uptake can be computed in a top-down fashion by difference
from the estimated fluxes to the atmosphere, the ocean sink, and the growth
rate in the atmosphere. Slightly more sophisticated approaches utilize the
spatial and temporal variations in atmospheric CO
2
with transport models
to infer land and ocean surface fluxes (Rödenbeck et al., 2003; Peylin et al.,
2005). Atmospheric carbon isotope and oxygen/nitrogen ratios also provide
critical constraints on the partitioning of carbon uptake between the ocean
and land biosphere (Rayner et al., 1999).
The contemporary land carbon budget is governed by a combination
of interacting natural and anthropogenic processes rather than any single
mechanism (Pacala et al., 2001; Schimel et al., 2001). Deforestation and
biomass burning result in net CO
2
fluxes to the atmosphere as high-carbon
forests are turned into comparatively low-carbon pastures and croplands
(Houghton, 2003). This process is now occurring mainly in the tropics
and is partially countered by temperate regrowth on abandoned farm and
pasture-land (Shevliakova et al., 2009). The impacts of land-use change
can extend for decades after the initial disturbance, and contemporary land
