About half of the 'anthropogenic' CO2 emitted to the atmosphere is taken up by the oceans and terrestrial biosphere(1), and the amount sequestered by the ocean is generally estimated using numerical ocean carbon-cycle models(2). But these models often differ markedly(3), resulting in different estimated spatial and temporal patterns and magnitudes of uptake. Because of its importance climatically, the CO2 flux needs to be verified using field measurements. Accurate estimates of CO2 uptake have been difficult to obtain, however, as the annual increase of dissolved inorganic carbon (DIC) concentration in surface water due to anthropogenic input is similar to 0.05% of the total DIC, an order of magnitude lower than past measurement precision. Early measurement-based estimates(4,5) of total anthropogenic CO2 inventory in the ocean have recently been improved on(6,7), and new approaches have been proposed for determining changes in ocean DIC concentration over one to two decades(8,9). Here we use recent improvements in DIC measurement techniques to determine changes in DIC concentrations between 1978 and 1995 in the Indian Ocean. Our method subtracts decadal-scale natural variability, enabling the ocean anthropogenic CO2 increase in this region over the 17-year period to be determined. The calculated uncertainties and known measurement capabilities allow us to define the minimum sampling strategies that will be required to quantify the regional and global anthropogenic CO2 oceanic uptake over future decades.
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