Complex response of marine carbon pumps to global warming impacts atmospheric CO2 on multi-centennial time scales

The ocean’s capacity to absorb anthropogenic CO₂ is predicted to decrease with global warming, contributing to a positive climate-carbon cycle feedback. However, the precise nature of how climate change will impact the ocean’s various carbon pumps and hence atmospheric CO₂ remains poorly constrained, especially on multi-centennial time scales. Here, we show that under a high emission scenario, reduced carbon uptake and redistribution of alkalinity leads to ~505 ppm (30%) higher atmospheric CO₂ by 2500. Despite compensating changes in biological storage and air-sea disequilibrium, CO₂ is still 16% higher due to climate change. These changes are a net response to slowing circulation and increased stratification, which not only reduces carbon uptake but lengthens by hundreds of years the time anthropogenic and biologically-respired CO₂ are sequestered in the ocean, with long term implications for climate.