Human activity is rapidly changing the composition of the earth's atmosphere,
contributing to warming from excess carbon dioxide (CO)
along with other trace gases such as water vapor, chlorofluorocarbons, methane
and nitrous oxide. These anthropogenic "greenhouse gases" play a
critical role in controlling the earth's climate because they increase the
infrared opacity of the atmosphere, causing the surface of the planet to warm.
The release of CO
from
fossil fuel consumption or the burning of forests for farming or pasture contributes
approximately 7 petagrams of carbon (1 Pg C = 1 × 10
g
C) to the atmosphere each year. Approximately 3 Pg C of this "anthropogenic
CO
" accumulates
in the atmosphere annually, and the remaining 4 Pg C is stored in the terrestrial
biosphere and the ocean.
Where and how land and ocean regions vary in their uptake of CO from
year to year is the subject of much scientific research and debate. Future
decisions on regulating emissions of greenhouse gases should be based on more
accurate models of the global cycling of carbon and the regional sources and
sinks for anthropogenic CO
,
models that have been adequately tested against a well-designed system of measurements.
The construction of a believable present-day carbon budget is essential for
the reliable prediction of changes in atmospheric CO
and
global temperatures from available emissions scenarios.
The ocean plays a critical role in the global carbon cycle as a vast reservoir
that exchanges carbon rapidly with the atmosphere, and takes up a substantial
portion of anthropogenically-released carbon from the atmosphere. A significant
impetus for carbon cycle research over the past several decades has been to
achieve a better understanding of the ocean's role as a sink for anthropogenic
CO. There are
only three global reservoirs with exchange rates fast enough to vary significantly
on the scale of decades to centuries: the atmosphere, the terrestrial biosphere
and the ocean. Approximately 93% of the carbon is located in the ocean, which
is able to hold much more carbon than the other reservoirs because most of
the CO
that
diffuses into the oceans reacts with seawater to form carbonic acid and its
dissociation products, bicarbonate and carbonate ions (Figure
1).
Figure 1. Schematic diagram of the carbon dioxide (CO)
system in seawater. The 1 × CO
concentrations
are for a surface ocean in equilibrium with a pre-industrial atmospheric
CO
level of
280 ppm. The 2 × CO
concentrations
are for a surface ocean in equilibrium with an atmospheric CO
level
of 560 ppm. Current model projections indicate that this level could be reached
sometime in the second half of this century. The atmospheric values are in
units of ppm. The oceanic concentrations, which are for the surface mixed
layer, are in units of µmol kg
.
Our present understanding of the temporal and spatial distribution of net
CO flux into
or out of the ocean is derived from a combination of field data, which is limited
by sparse temporal and spatial coverage, and model results, which are validated
by comparisons with the observed distributions of tracers, including natural
carbon-14 (
C),
and anthropogenic chlorofluorocarbons, tritium (
H)
and bomb
C.
The latter two radioactive tracers were introduced into the atmosphere-ocean
system by atomic testing in the mid 20th century. With additional data from
the recent global survey of CO
in
the ocean (19911998), carried out cooperatively as part of the Joint
Global Ocean Flux Study (JGOFS) and the World Ocean Circulation Experiment
(WOCE) Hydrographic Program, it is now possible to characterize in a quantitative
way the regional uptake and release of CO
and
its transport in the ocean. In this paper, we summarize our present understanding
of the exchange of CO
across
the air-sea interface and the storage of natural and anthropogenic CO
in
the ocean's interior.
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