Overshoot and recover? On the problem of substitution between negative emissions and emissions reductions

The remaining global carbon budget is so small that carbon dioxide removal (CDR) measures are very likely to be required to avoid dangerous climate change. Multiple scenarios consistent with a high probability of limiting global warming to well below 2 °C include removing hundreds of gigatons of carbon dioxide. At the same time, deep decarbonization pathways show that rapid and drastic emissions reductions can substantially reduce or even avoid the need for CDR. This article discusses one major problem raised by pathways relying on large-scale CDR: By potentially discouraging or at least delaying the implementation of deep decarbonization measures, such pathways might cause a substantial overshoot of the global carbon budget that would lead to severe injustices. More specifically, it highlights the problem of substitution between large-scale CDR and deep emissions reductions by explaining the structure of this problem, stressing the ethical issues it raises, and investigating three conditions under which it is likely to occur.