Towards Net Zero: Evaluating Combined Terrestrial and Marine CDR Approaches

With the global annual mean temperature in 2024 exceeding 1.5°C above preindustrial levels, there is an urgent need to investigate pathways for returning the Earth system to lower temperature levels. In addition to stringent emission reduction, we need portfolios of Carbon Dioxide Removal (CDR) techniques to achieve the net-zero emission target. Therefore, it is crucial to evaluate various land and ocean-based CDRs for their effectiveness, environmental risks, and additional benefits.

This study evaluates the CO₂ sequestration potential and efficacy of two prominent CDR methods—Bioenergy with Carbon Capture and Storage (BECCS) and Ocean Alkalinity Enhancement (OAE)—applied both individually and in combination. Using the Norwegian Earth System Model (NorESM2-LM), simulations were designed with ramped-up CDR deployment, targeting 5.2 million km² of bioenergy feedstock for BECCS and a CaO deployment rate of 2.7 Gt/year for OAE by 2100 across the exclusive economic zones of Europe, the United States, and China. The results reveal a nearly additive carbon removal effect of BECCS and OAE.   Over the period 2030-2100, OAEsequestered a total of 7 ppm of CO₂ with an accumulated 82.3 Gt CaO, achieving a CDR effectiveness of 0.08 ppm per Gt of CaO, while BECCS removes 23 ppm of CO₂, with CDR effectiveness of 3.1 ppm per million km² of bioenergy crops.  The combined BECCS-OAE simulation offsets anthropogenic CO₂ emissions of 5.4 Gt/year by 2100—equivalent to over 60% of current global transport sector emissions. However, the combined CDR scenario shows negligible effects on the global annual mean temperature, with no clear response detectable against the high internal variability. This underscores the limitations of current CDR approaches in addressing climate warming over the 21^st century and emphasizes the need for substantial emissions reductions, supportive policies and diversified CDR strategies to facilitate a return to lower global temperatures.