Environmental Engineering Reference
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airborne fraction and how it evolves with time. At present, the ocean and
the land biosphere contribute about equally to the drawdown of excess
carbon dioxide from the atmosphere. Theoretical arguments and numerical
models suggest that the efficiency of both the land and ocean carbon sinks
may decline in the future under warmer climate conditions, which would
act to amplify climate warming (Fung et al., 2005; Friedlingstein et al.,
2006). The magnitude of the climate-carbon cycle feedback, however, var-
ies substantially across model simulations and is a substantial uncertainty
in future climate projections.
Excess atmospheric carbon dioxide dissolves in surface seawater as
inorganic carbon through well-known physical-chemical reactions. The dis-
tribution and global inventory of anthropogenic carbon dioxide in the ocean
are well characterized based on global ship-based observations collected
during the late 1980s and 1990s (Sabine et al., 2004). Ongoing measure-
ments at time-series sites and along ocean sections constrain uptake over
decadal time periods (IOC, 2009). Oceanic anthropogenic CO 2 uptake up
to present has been governed primarily by atmospheric CO 2 concentrations
and the rate of ocean circulation that exchanges surface waters equilibrated
with elevated CO 2 levels with subsurface waters. In particular, key pathways
include the ventilation of the wind-driven thermocline and deep and inter-
mediate water formation. Ocean models constrained by field data provide
estimates of the oceanic transport and air-sea flux of anthropogenic CO 2 as
well as reconstructions of past ocean uptake and projections for the future
(Matsumoto et al., 2004; Gruber et al., 2009; Khatiwala et al., 2009).
Future ocean uptake of anthropogenic carbon is expected to decrease
in efficiency (i.e., absorb a smaller fraction of the emissions); the ocean
CO 2 sink is expected to continue to increase, but more slowly than the
emissions. Under elevated CO 2 , the chemical buffer capacity of seawater
decreases, lowering the amount of inorganic carbon absorbed when surface
waters are equilibrated with the atmosphere. Upper-ocean warming reduces
the solubility of carbon dioxide in seawater. Anthropogenic CO 2 uptake
will be further reduced because of increased vertical stratification, reduced
ocean ventilation rates, and reduced deep and intermediate water formation
rates, which are expected due to warming in the tropics and subtropics and
increased freshwater input in temperate and polar regions due to elevated
precipitation and sea-ice melt (Sarmiento et al., 1998). In contrast, an in-
crease in the strength of Southern Ocean winds, associated with a more
positive phase of the Southern Annular Mode, may increase future uptake
of anthropogenic CO 2 (Russell et al., 2006).
Ocean biogeochemistry plays an important role in ocean carbon stor-
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