Agricultural practices, fossil fuel burning and cement manufacturing have resulted in a more than 35% increase in atmospheric carbon dioxide (CO2) concentrations over the last 200 years. However, this increase would have been much more dramatic if the ocean had not been absorbing a significant amount of the CO2 resulting from human activity (anthropogenic CO2). There are three major reservoirs with exchange rates fast enough to vary significantly on the timescale of decades to centuries: the atmosphere, terrestrial biosphere and the ocean. Of this three-component system, ~93% of the carbon is located in the ocean (Fig. 6.2). Although this means that the ocean has a tremendous capacity to influence future atmospheric CO2 concentrations, it also means that it is very difficult to accurately constrain the oceanic sink for anthropogenic CO2.

The anthropogenic signal that has accumulated in the ocean over the last 200 years is generally less than 3% of the natural carbon in surface sea water, making it difficult to distinguish the anthropogenic CO2 from the observed natural variability. Unfortunately, ocean carbon measurements cannot directly distinguish between anthropogenic CO2 and natural inorganic carbon; moreover, the historical measurements of ocean carbon are inadequate to document directly the increase in ocean inventories. Until recently, almost all estimates of oceanic CO2 uptake have been based on modelling studies, which must be evaluated and improved based on geochemi-cal tracers other than anthropogenic CO2 (e.g. bomb radiocarbon, chlorofluorocarbons).

Concerns over the greenhouse effect and global climate change have inspired scientists to focus their attention on improving our understanding of the ocean's role in the global carbon cycle. Recently, oceanographers have made great advances in both ocean carbon observations and modelling, leading to an improved understanding of the cycling of carbon in the ocean and exchanges with other carbon reservoirs (e.g. Sabine et al., 2004a). This chapter will focus primarily on the oceanic sink for CO2 and the seasonal, interan-nual and decadal-scale variability in sea-air fluxes. In particular, it will discuss how the sea-air exchange of CO2 relates to the oceanic uptake of anthropogenic CO2, and potential feedbacks within the carbon cycle as well as the carbon-climate system that may change the oceanic uptake of CO2 in the future.

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