Abstract Carbon dioxide removal (CDR) technologies are increasingly important for meeting climate targets. These can vary widely in terms of their permanence, with some methods temporarily storing carbon for decades and others storing carbon for millennia or longer. Economic equivalency ratios have been developed to understand and compare the value of CDR options in terms of permanence. These ratios enable the conversion of temporary carbon storage (i.e., low permanence) into an equivalent permanent value, using tools like discounting, time horizons, and damage functions. This study comparatively evaluates three key methods-the climate repair value, the social value of an offset, and discounted tonne-year accounting-by combining a Sobol sensitivity analysis with a critical qualitative assessment of their underlying assumptions. These equivalency ratios are found to be highly sensitive to arbitrary input choices, such as the discount rate or policy-defined time horizon, and depart from the physical constraints relevant to long-term temperature stabilisation. The findings demonstrate that physical, or ‘like-for-like’, equivalence offers a considerable improvement in guiding CDR deployment, and distinguishes between Carbon Cycle Matching and Atmospheric Longevity Matching as separate approaches to achieving physical equivalence. Crucially, while these physical equivalence approaches offer closer alignment with climate dynamics, achieving true equivalence between emissions and CDR remains inherently complex. Overall, this study shows that current economic equivalency ratios risk misaligning CDR with long-term temperature goals, and that while physical methods present a more robust framework, their limitations warrant further refinement and consideration before widespread implementation.
Watson et al. (Mon,) studied this question.