evidence: Variability of global mean sea-level (GMSL) projections among studies, even when using the same emissions scenarios, has led to confusion among decision-making communities and the public1,2. The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) projected GMSL by summing the contributions from physical processes to provide a likely (i.e., central 66% probability; see exegesis by ref. 3) estimate of GMSL rise of 0.52–0.98 m in the case of unmitigated growth of emissions (Representative Concentration Pathway [RCP] 8.5) by 2100, relative to 1986–20054. However, substantial uncertainties surrounding the response of the Greenland and Antarctic Ice Sheets to dynamic processes such as marine ice sheet instability (MISI) and marine ice cliff instability (MICI)5,6 indicate that the physical models used in process-based predictions of GMSL rise are incomplete7. Indeed, AR5 noted that additional GMSL rise up to several decimeters was possible due to MISI, but it was not included in the estimate of Antarctic Ice Sheet rapid dynamics due to likelihood of such a contribution being unquantified4. The recent IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC)8 included MISI (but not MICI). [...] Vermeer, M. & Rahmstorf, S. Global sea level linked to global temperature. Proc. Natl Acad. Sci.106, 21527–21532 (2009).