Additionality of climate benefits from emission reduction and carbon dioxide removal

Carbon dioxide removal (CDR) has been identified as a necessary complement to industrial decarbonization for climate change mitigation. Marine CDR (mCDR) approaches, such as ocean alkalinity enhancement (OAE) and direct ocean capture (DOC), have received considerable attention for their potential to counteract ocean acidification while simultaneously reducing atmospheric CO 2 . However, most model-based assessments of mCDR efficacy either assume a fixed background atmospheric CO 2 value or adopt an idealized future CO 2 trajectory, thereby excluding impacts of important carbon cycle feedbacks that repartition carbon between the ocean, atmosphere, and biosphere. Here, we use an Earth System Model to address three core questions: whether the net CO 2 effect of CDR depends on background emissions; whether CDR technologies deployed simultaneously yield additive CO 2 reductions; and whether there is synergy between the radiative forcing (climate) benefits of CDR and emission reductions. We evaluate permutations of direct air capture (DAC), OAE, and DOC, quantifying their individual and combined CO 2 effects under various emissions scenarios. We find that the radiative forcing effects of emissions reductions and CDR are largely additive through time, regardless of background emission scenario or CDR type or scale. Furthermore, any given quantity and type of short-term CDR deployment produces a predictable, proportional and durable reduction in atmospheric CO 2 through time (ΔlnCO 2 ), facilitating comparison between CDR strategies. Finally, we show that CDR efficacy is marginally greater at low background CO 2 , supporting the complementary value of decarbonization and CDR in carbon management portfolios. These findings provide a robust physical basis for the design of integrated carbon management strategies and support pursuit of deep emissions cuts and CDR as synergistic climate levers.