The release of fossil fuel CO(2) to the atmosphere by human activity has been implicated as the predominant cause of recent global climate change. The ocean plays a crucial role in mitigating the effects of this perturbation to the climate system, sequestering 20 to 35 per cent of anthropogenic CO(2) emissions. Although much progress has been made in recent years in understanding and quantifying this sink, considerable uncertainties remain as to the distribution of anthropogenic CO(2) in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in anthropogenic CO(2) sequestration. Here we address these questions by presenting an observationally based reconstruction of the spatially resolved, time-dependent history of anthropogenic carbon in the ocean over the industrial era. Our approach is based on the recognition that the transport of tracers in the ocean can be described by a Green's function, which we estimate from tracer data using a maximum entropy deconvolution technique. Our results indicate that ocean uptake of anthropogenic CO(2) has increased sharply since the 1950s, with a small decline in the rate of increase in the last few decades. We estimate the inventory and uptake rate of anthropogenic CO(2) in 2008 at 140 +/- 25 Pg C and 2.3 +/- 0.6 Pg C yr(-1), respectively. We find that the Southern Ocean is the primary conduit by which this CO(2) enters the ocean (contributing over 40 per cent of the anthropogenic CO(2) inventory in the ocean in 2008). Our results also suggest that the terrestrial biosphere was a source of CO(2) until the 1940s, subsequently turning into a sink. Taken over the entire industrial period, and accounting for uncertainties, we estimate that the terrestrial biosphere has been anywhere from neutral to a net source of CO(2), contributing up to half as much CO(2) as has been taken up by the ocean over the same period.

Ocean ventilation is the process that transports water and climatically important trace gases such as carbon dioxide from the surface mixed layer into the ocean interior. Quantifying the dominant source regions and time scales remains a major challenge in oceanography. A mathematically rigorous approach, that accounts for the multiplicity of transport pathways and transit times characteristic of an eddy-diffusive flow such as the ocean, is to quantify ventilation in terms of a probability distribution that partitions fluid parcels according to the time and location of their last surface contact. Here, we use globally gridded radiocarbon data in combination with other transient (CFCs) and hydrographic (temperature, salinity, phosphate, and oxygen) tracer data to estimate the joint distribution of age and surface origin of deep ocean waters. Our results show that ~. 40% and 26% of the global ocean was last in contact with the Southern Ocean and North Atlantic, respectively. Some 80% of the global deep ocean below 1500. m is ventilated from these high latitude regions. However, contrary to the classical description of the deep ocean as a roughly equal mixture of "northern" and "southern" source waters, we find a significantly higher contribution from the Southern Ocean relative to the North Atlantic. We estimate the mean transit time from the surface to the deep North Pacific at 1360±350. y, intermediate between two widely used radiocarbon-based estimates. To reconcile our estimate of the ideal mean age with ventilation age estimates based on radiocarbon, we apply the estimated distribution function to construct a 3-dimensional distribution of the water mass fraction-weighted surface "initial" radiocarbon concentration that can serve as an accurate reservoir age. Radiocarbon ages corrected for this initial reservoir age are found to be in good agreement (within 5%) with our ideal age estimate, demonstrating that it is essential to take into account the spatially variable surface radiocarbon field when computing ventilation ages using radiocarbon. A wide spectrum of ages contributes to the mean age, providing evidence for the fundamentally eddy-diffusive nature of the large-scale general circulation of the ocean. © 2012 Elsevier B.V..

The global ocean has taken up a large fraction of the CO₂ released by human activities since the industrial revolution. Quantifying the oceanic anthropogenic carbon (C_ant) inventory and its variability is important for predicting the future global carbon cycle. The detailed comparison of data-based and model-based estimates is essential for the validation and continued improvement of our prediction capabilities. So far, three global estimates of oceanic C_ant inventory that are "data-based" and independent of global ocean circulation models have been produced: one based on the Δ C* method, and two that are based on constraining surface-to-interior transport of tracers, the TTD method and a maximum entropy inversion method (GF). The GF method, in particular, is capable of reconstructing the history of C_ant inventory through the industrial era. In the present study we use forward model simulations of the Community Climate System Model (CCSM3.1) to estimate the C_ant inventory and compare the results with the data-based estimates. We also use the simulations to test several assumptions of the GF method, including the assumption of constant climate and circulation, which is common to all the data-based estimates. Though the integrated estimates of global C_ant inventories are consistent with each other, the regional estimates show discrepancies up to 50 %. The CCSM3 model underestimates the total C_ant inventory, in part due to weak mixing and ventilation in the North Atlantic and Southern Ocean. Analyses of different simulation results suggest that key assumptions about ocean circulation and air-sea disequilibrium in the GF method are generally valid on the global scale, but may introduce errors in C_ant estimates on regional scales. The GF method should also be used with caution when predicting future oceanic anthropogenic carbon uptake.

The global ocean has taken up a large fraction of the CO₂ released by human activities since the industrial revolution. Quantifying the oceanic anthropogenic carbon (C_ant) inventory and its variability is important for predicting the future global carbon cycle. The detailed comparison of data-based and model-based estimates is essential for the validation and continued improvement of our prediction capabilities. So far, three global estimates of oceanic C_ant inventory that are "data-based" and independent of global ocean circulation models have been produced: one based on the Δ C* method, and two that are based on constraining surface-to-interior transport of tracers, the TTD method and a maximum entropy inversion method (GF). The GF method, in particular, is capable of reconstructing the history of C_ant inventory through the industrial era. In the present study we use forward model simulations of the Community Climate System Model (CCSM3.1) to estimate the C_ant inventory and compare the results with the data-based estimates. We also use the simulations to test several assumptions of the GF method, including the assumption of constant climate and circulation, which is common to all the data-based estimates. Though the integrated estimates of global C_ant inventories are consistent with each other, the regional estimates show discrepancies up to 50 %. The CCSM3 model underestimates the total C_ant inventory, in part due to weak mixing and ventilation in the North Atlantic and Southern Ocean. Analyses of different simulation results suggest that key assumptions about ocean circulation and air-sea disequilibrium in the GF method are generally valid on the global scale, but may introduce errors in C_ant estimates on regional scales. The GF method should also be used with caution when predicting future oceanic anthropogenic carbon uptake.