Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning

The ocean is the largest sink for anthropogenic carbon dioxide (CO₂), having absorbed roughly 40 per cent of CO₂ emissions since the beginning of the industrial era. Recent data show that oceanic CO₂ uptake rates have been growing over the past decade, reversing a trend of stagnant or declining carbon uptake during the 1990s. Here we show that ocean circulation variability is the primary driver of these changes in oceanic CO₂ uptake over the past several decades. We use a global inverse model to quantify the mean ocean circulation during the 1980s, 1990s and 2000s, and then estimate the impact of decadal circulation changes on the oceanic CO₂ sink using a carbon cycling model. We find that during the 1990s an enhanced upper-ocean overturning circulation drove increased outgassing of natural CO₂, thus weakening the global CO₂ sink. This trend reversed during the 2000s as the overturning circulation weakened. Continued weakening of the upper-ocean overturning is likely to strengthen the CO₂ sink in the near future by trapping natural CO₂ in the deep ocean, but ultimately may limit oceanic uptake of anthropogenic CO₂.