Estimating global mean sea-level rise and its uncertainties by 2100 and 2300 from an expert survey

Andrew C. Kemp,Niamh Cahill,Andra J. Garner,Benjamin P. Horton,Nicole S. Khan,Janice S. H. Lee,Simon E. Engelhart,Timothy A. Shaw,Stefan Rahmstorf

doi:10.1038/s41612-020-0121-5

Abstract

Sea-level rise projections and knowledge of their uncertainties are vital to make informed mitigation and adaptation decisions. To elicit projections from members of the scientific community regarding future global mean sea-level (GMSL) rise, we repeated a survey originally conducted five years ago. Under Representative Concentration Pathway (RCP) 2.6, 106 experts projected a likely (central 66% probability) GMSL rise of 0.30–0.65 m by 2100, and 0.54–2.15 m by 2300, relative to 1986–2005. Under RCP 8.5, the same experts projected a likely GMSL rise of 0.63–1.32 m by 2100, and 1.67–5.61 m by 2300. Expert projections for 2100 are similar to those from the original survey, although the projection for 2300 has extended tails and is higher than the original survey. Experts give a likelihood of 42% (original survey) and 45% (current survey) that under the high-emissions scenario GMSL rise will exceed the upper bound (0.98 m) of the likely range estimated by the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, which is considered to have an exceedance likelihood of 17%. Responses to open-ended questions suggest that the increases in upper-end estimates and uncertainties arose from recent influential studies about the impact of marine ice cliff instability on the meltwater contribution to GMSL rise from the Antarctic Ice Sheet.

Highlights

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 public[1,2].The Intergovernmental Panel on Climate Change (IPCC) FifthAssessment Report (AR5) projected GMSL by summing the contributions from physical processes to provide a likely 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
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 incomplete[7]
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 unquantified[4]

Summary

Introduction

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 public[1,2].The Intergovernmental Panel on Climate Change (IPCC) FifthAssessment 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. 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 incomplete[7]. 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 unquantified[4]. The recent IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC)[8] included MISI (but not MICI). The majority of individual studies published since 2013 for high emission scenarios project GMSL rise >1 m by 21001, including several that suggest the possibility of a rise >2 m9,10, in line with projections of semiempirical models[11,12,13,14]

Methods

Results

Conclusion