Uncertainty in non-CO2 greenhouse gas mitigation: Make-or-break for global climate policy feasibility

Despite its projected crucial role in stringent, future global climate policy, non-CO2 greenhouse gas (NCGG) mitigation remains a large uncertain factor that has received relatively little scientific attention. A revision of the estimated mitigation potential could have massive implications for the feasibility of global climate policy to reach the Paris Agreement climate goals. Here, we provide a systematic bottom-up estimate of the total uncertainty in NCGG mitigation, by developing “optimistic, default and pessimistic” long-term non-CO2 marginal abatement cost (MAC) curves. The global 1.5-degree climate target is found to be out of reach under pessimistic MAC assumptions, as is the 2-degree target under high emission assumptions. MAC uncertainty translates into a large projected range in (all in a 2-degree scenario) relative NCGG reduction (40–58%), carbon budget (± 120 Gt CO2) and policy costs (± 16%). Partly, the MAC uncertainty signifies a gap that could be bridged by human efforts, but largely it indicates uncertainty in technical limitations.