The δ13C value of the dissolved inorganic carbon in the surface waters of the Pacific Ocean has decreased by about 0.4 per mil between 1970 and 1990. This decrease has resulted from the uptake of atmospheric CO_2 derived from fossil fuel combustion and deforestation. The net amounts of CO_2 taken up by the oceans and released from the biosphere between 1970 and 1990 have been determined from the changes in three measured values: the concentration of atmospheric CO_2, the δ13C of atmospheric CO_2 and the δ13C value of dissolved inorganic carbon in the ocean. The calculated average net oceanic CO_2 uptake is 2.1 gigatons of carbon per year. This amount implies that the ocean is the dominant net sink for anthropogenically produced CO_2 and that there has been no significant net CO_2 released from the biosphere during the last 20 years.