Efficacy of individual and combined terrestrial and marine carbon dioxide removal

Limiting global temperature rise below 2 °C requires significant reduction in greenhouse gas emissions and large-scale carbon dioxide removal (CDR). This study assesses the CO _2 sequestration and efficacy of two CDR approaches, bioenergy with carbon capture and storage (BECCS) and ocean alkalinity enhancement (OAE), applied individually and in combination. Using the Norwegian Earth System Model, simulations were designed to ramp up deployment of BECCS and OAE, to an additional area of 5.2 million km ^2 by 2100 for bioenergy feedstock for BECCS, and a CaO deployment rate of approximately 2.7 Gt yr ^−1 for OAE within the exclusive economic zones of Europe, the United States and China. The combined land–ocean CDR simulation revealed a largely additive carbon removal effect. Over 2030−2100, OAE sequestered 7 ppm of CO _2 with an accumulated 82.3 Gt CaO, achieving a CDR effectiveness of 0.08 ppm (∼0.17 PgC) per Gt CaO, while BECCS reduced 16 ppm of CO _2 , with CDR effectiveness of 3.1 ppm per million km ^2 of bioenergy crops. Together, the carbon removal achieved by BECCS and OAE corresponds to anthropogenic CO _2 emissions of 5.4 Gt CO _2 yr ^−1 by 2100, slightly more than 60% of current global transport sector emissions. Notably, the efficiency of BECCS and OAE alone was unaffected by their concurrent deployment. Nevertheless, simulations revealed distinct non-linear interactions, such as declines in land and soil carbon sinks in the combined scenario. Furthermore, all simulations show negligible effects on the global annual mean temperature. These results highlight near-additive CDR responses even under net-negative emissions, but feedback on land and ocean carbon sinks must be considered when designing CDR portfolios. This study provides new insights into CDR portfolio design and Earth system feedback under an overshoot scenario, highlighting both their potential and the need for continued emissions cuts and supportive policies.