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.
Return to Abstract or go to the next section
PMEL Outstanding Papers
PMEL Publications Search