B. B. Cael, Philip Goodwin, David Stainforth

Limiting global warming to 1.5$^\circ$C will very likely require, or to 2$^\circ$C may require, large-scale removal of carbon dioxide (CO$_2$) from the atmosphere. Many CO$_2$ removal strategies (CDRSs), or negative emissions technologies, have been proposed, which vary widely in both price per ton of CO$_2$ removed and storage timescale of this removed CO$_2$, as well as mechanism, maturity, scalability, and other factors. It has not yet been assessed whether the benefits, in terms of climate change-related damages avoided, of CDRSs' deployment exceed their costs at current reported prices and storage timescales, nor what cost is required for a CDRS with a given storage timescale to provide net benefits, nor how these depend on socioeconomic assumptions. For a long-storage-timescale CDRS, these questions reduce to whether its price is lower than the social cost of carbon, but for CDRSs with shorter storage timescales, they may also depend on its storage timescale. We show that for CDRSs with reported storage timescales from decades to centuries, the benefits of their deployment outweigh their reported costs under middle-of-the-road socioeconomic assumptions. For some, their benefits still outweigh their costs under optimistic socioeconomic assumptions. These CDRSs' associated benefit-cost ratios vary by more than an order of magnitude, and are strongly influenced by both price and storage timescale. The price threshold where a CDRS yields net benefits depends strongly on storage timescale, particularly for storage timescales $\leq$50 years. Our results provide a framework to assess and compare different CDRSs quantitatively for future CDRSs research, development, and policy.