Of the ocean-atmosphere-land three-component system, ocean contains by far the most natural carbon. There is no realizable physical limit to the uptake capacity of the ocean and it is estimated that on millennial timescales the ocean will ultimately store up to 90% of the CO2 released by human activity. However, on timescales more relevant to human society, the uptake rate of CO2 is controlled by a complicated matrix of physical, chemical and biological processes. Studies suggest that the ocean has been the primary sink for excess CO2 released to the atmosphere over the last 200 years, but the ocean's role may be changing over the next few decades to centuries.
Because the anthropogenic signal in the ocean is relatively small compared to the natural background concentrations and relative to the observed seasonal to interannual variations, it has been difficult to directly quantify the uptake and storage of anthropogenic CO2 in the ocean. This has been further hampered by a paucity of data. The current estimates have been based primarily on indirect approaches or on a number of simplified assumptions, ignoring a number of potential carbon cycle and carbon-climate feedbacks. The potential role of these feedback processes in the ocean carbon cycle is just beginning to be understood and fully appreciated. As we obtain more data on processes and improve their representation in models, we will be better equipped to estimate the long-term role of the ocean in the global carbon cycle and its impact on future climate change.
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