Quantification of decadal anthropogenic CO2 uptake in the ocean based on dissolved inorganic carbon measurements

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 estim ated spatial and temporal patterns and magnitudes of uptake. Because of its importance climatically, the CO2 flux needs to be verified using field mea surements. Accurate estimates of CO2 uptake have been difficult to obtain, however, as the annual increase of dissolved inorganic carbon (DIC) concent ration in surface water due to anthropogenic input is similar to 0.05% of t he 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 t o two decades(8,9). Here we use recent improvements in DIC measurement tech niques to determine changes in DIC concentrations between 1978 and 1995 in the Indian Ocean. Our method subtracts decadal-scale natural variability, e nabling the ocean anthropogenic CO2 increase in this region over the 17-yea r period to be determined. The calculated uncertainties and known measureme nt capabilities allow us to define the minimum sampling strategies that wil l be required to quantify the regional and global anthropogenic CO2 oceanic uptake over future decades.