No compromise in efficiency from the co-application of a marine and a terrestrial CDR method

Modelled pathways consistent with the Paris Agreement goals to mitigate warming typically include the large-scale application of Carbon Dioxide Removal (CDR), which can include both land- and marine-based CDR methods. However, the Earth system responses and feedbacks to scaling up and/or combining different CDR methods remain understudied. Here, these are assessed by employing two Earth System Models, with a multifactorial setup of 42 emission-driven simulations covering the whole spectrum of Afforestation/Reforestation (0-927 Mha) and of Ocean Alkalinity Enhancement (0-18 Pmol) over the 21^st century. We show that global carbon flux responses scale linearly when different CDR methods are scaled up and/or combined, which suggests that the efficiency of CDR is insensitive to both the amount of CDR and the CDR portfolio composition. Therefore, combining CDR methods, which seems beneficial for diversifying risks and remaining below sustainability thresholds, does not compromise the efficiency of individual applications.