The East/Japan Sea in the western temperate North Pacific is ventilated from the surface to the bottom over decades. Such short overturning circulation indicates that the anthropogenic CO2 content of the East/Japan Sea is intimately tied to changing surface conditions over similarly short periods. Three surveys in the East/Japan Sea (1992, 1999 and 2007, respectively) have provided a rare opportunity to measure changes in the accumulation rate of anthropogenic CO2 in the East/Japan Sea over the past 15 years in response to changes in surface conditions. We found that the mean uptake rate of anthropogenic CO2 by the East/Japan Sea was 0.3 ± 0.2 mol C m−2 yr−1 for the period 1999–2007, in marked contrast to the rate of 0.6 ± 0.4 mol C m−2 yr−1 for the period 1992–1999. The striking feature is that nearly all anthropogenic CO2 taken up in the more recent period was confined to waters less than 300 m in depth (mean winter mixed layer depth). The rapid and substantial reduction in accumulation in the more recent period is surprising, and is attributed to considerable weakening of overturning circulation, which is responsible for transporting anthropogenic CO2 from the surface to the interior of the East/Japan Sea. This previously undocumented finding may be an indicator of future changes in the global ocean during the coming period of global warming.