<p>The ocean’s capacity to absorb anthropogenic CO₂ is predicted to decrease with global warming, reinforcing a climate–carbon cycle feedback. However, the effects of specific mechanisms such as circulation and temperature on marine carbon components and atmospheric CO₂ under future emission scenarios remain poorly quantified, especially on multi-centennial timescales. Here, using a decomposition of dissolved inorganic carbon, we show that under high-emission scenarios, circulation changes dominate the climate–carbon cycle feedback by reducing anthropogenic carbon uptake and redistributing alkalinity, despite compensating increases in biological carbon storage and air–sea disequilibrium. By contrast, under low emissions, temperature changes, amplified by increased physical disequilibrium, dominate the climate–carbon cycle feedback. Previous estimates using the apparent oxygen utilization approximation may have considerably underestimated changes in biological carbon storage. These results improve mechanistic understanding of long-term global carbon cycle dynamics, with implications for efforts to achieving zero net emissions and proposed marine CO₂ removal strategies.</p>