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Despite considerable advances in process understanding, numerical modeling, and the observational record of ice sheet contributions to global mean sea-level rise (SLR) since the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change, severe limitations remain in the predictive capability of ice sheet models. As a consequence, the potential contributions of ice sheets remain the largest source of uncertainty in projecting future SLR. Here, we report the findings of a structured expert judgement study, using unique techniques for modeling correlations between inter- and intra-ice sheet processes and their tail dependences. We find that since the AR5, expert uncertainty has grown, in particular because of uncertain ice dynamic effects. For a +2 °C temperature scenario consistent with the Paris Agreement, we obtain a median estimate of a 26 cm SLR contribution by 2100, with a 95th percentile value of 81 cm. For a +5 °C temperature scenario more consistent with unchecked emissions growth, the corresponding values are 51 and 178 cm, respectively. Inclusion of thermal expansion and glacier contributions results in a global total SLR estimate that exceeds 2 m at the 95th percentile. Our findings support the use of scenarios of 21st century global total SLR exceeding 2 m for planning purposes. Beyond 2100, uncertainty and projected SLR increase rapidly. The 95th percentile ice sheet contribution by 2200, for the +5 °C scenario, is 7.5 m as a result of instabilities coming into play in both West and East Antarctica. Introducing process correlations and tail dependences increases estimates by roughly 15%. 

DAVID POPP, FRANCESCO VONA, GIOVANNI MARIN and ZIQIAO CHEN: “The Employment Impact of a Green Fiscal Push: Evidence from the American Recovery and Reinvestment Act “ Comments by Gabriel Chodorow-Reich and Valerie A. Ramey and General Discussion SHELBY R. BUCKMAN, LAURA Y. CHOI, MARY C. DALY and LILY M. SEITELMAN: “The Economic Gains from Equity” Comments by Nicole Fortin and Erik Hurst and General Discussion SUSAN CHERRY, ERICA JIANG, GREGOR MATVOS, TOMASZ PISKORSKI and AMIT SERU: “Government and Private Household Debt Relief during COVID-19” Comments by Pascal Noel and Susan Wachter and General Discussion KEVIN RENNERT, BRIAN C. PREST, WILLIAM A. PIZER, RICHARD G. NEWELL, DAVID ANTHOFF, CORA KINGDON, LISA RENNELS, ROGER COOKE, ADRIAN E. RAFTERY, HANA ŠEVČÍKOVÁ and FRANK ERRICKSON: “The Social Cost of Carbon: Advances in Long-Term Probabilistic Projections of Population, GDP, Emissions, and Discount Rates" RICARDO REIS: “Losing the Inflation Anchor” Comments by Alan S. Blinder and Yuriy Gorodnichenko and General Discussion DAVID AUTOR, DAVID DORN and GORDON HANSON: “On the Persistence of the China Shock “ Comments by Marianne Bertrand and Edward Glaeser and General Discussion 

1. DAVID POPP, FRANCESCO VONA, GIOVANNI MARIN and ZIQIAO CHEN: “The Employment Impact of a Green Fiscal Push: Evidence from the American Recovery and Reinvestment Act “ Comments by Gabriel Chodorow-Reich and Valerie A. Ramey and General Discussion SHELBY R. BUCKMAN, LAURA Y. CHOI, MARY C. DALY and LILY M. SEITELMAN: “The Economic Gains from Equity” Comments by Nicole Fortin and Erik Hurst and General Discussion SUSAN CHERRY, ERICA JIANG, GREGOR MATVOS, TOMASZ PISKORSKI and AMIT SERU: “Government and Private Household Debt Relief during COVID-19” Comments by Pascal Noel and Susan Wachter and General Discussion KEVIN RENNERT, BRIAN C. PREST, WILLIAM A. PIZER, RICHARD G. NEWELL, DAVID ANTHOFF, CORA KINGDON, LISA RENNELS, ROGER COOKE, ADRIAN E. RAFTERY, HANA ŠEVČÍKOVÁ and FRANK ERRICKSON: “The Social Cost of Carbon: Advances in Long-Term Probabilistic Projections of Population, GDP, Emissions, and Discount Rates" RICARDO REIS: “Losing the Inflation Anchor” Comments by Alan S. Blinder and Yuriy Gorodnichenko and General Discussion DAVID AUTOR, DAVID DORN and GORDON HANSON: “On the Persistence of the China Shock “ Comments by Marianne Bertrand and Edward Glaeser and General Discussion 

1. DAVID POPP, FRANCESCO VONA, GIOVANNI MARIN and ZIQIAO CHEN: “The Employment Impact of a Green Fiscal Push: Evidence from the American Recovery and Reinvestment Act “ Comments by Gabriel Chodorow-Reich and Valerie A. Ramey and General Discussion SHELBY R. BUCKMAN, LAURA Y. CHOI, MARY C. DALY and LILY M. SEITELMAN: “The Economic Gains from Equity” Comments by Nicole Fortin and Erik Hurst and General Discussion SUSAN CHERRY, ERICA JIANG, GREGOR MATVOS, TOMASZ PISKORSKI and AMIT SERU: “Government and Private Household Debt Relief during COVID-19” Comments by Pascal Noel and Susan Wachter and General Discussion KEVIN RENNERT, BRIAN C. PREST, WILLIAM A. PIZER, RICHARD G. NEWELL, DAVID ANTHOFF, CORA KINGDON, LISA RENNELS, ROGER COOKE, ADRIAN E. RAFTERY, HANA ŠEVČÍKOVÁ and FRANK ERRICKSON: “The Social Cost of Carbon: Advances in Long-Term Probabilistic Projections of Population, GDP, Emissions, and Discount Rates" RICARDO REIS: “Losing the Inflation Anchor” Comments by Alan S. Blinder and Yuriy Gorodnichenko and General Discussion DAVID AUTOR, DAVID DORN and GORDON HANSON: “On the Persistence of the China Shock “ Comments by Marianne Bertrand and Edward Glaeser and General Discussion 

Abstract The ice sheets covering Antarctica and Greenland present the greatest uncertainty in, and largest potential contribution to, future sea level rise. The uncertainty arises from a paucity of suitable observations covering the full range of ice sheet behaviors, incomplete understanding of the influences of diverse processes, and limitations in defining key boundary conditions for the numerical models. To investigate the impact of these uncertainties on ice sheet projections we undertook a structured expert judgement study. Here, we interrogate the findings of that study to identify the dominant drivers of uncertainty in projections and their relative importance as a function of ice sheet and time. We find that for the 21st century, Greenland surface melting, in particular the role of surface albedo effects, and West Antarctic ice dynamics, specifically the role of ice shelf buttressing, dominate the uncertainty. The importance of these effects holds under both a high‐end 5°C global warming scenario and another that limits global warming to 2°C. During the 22nd century the dominant drivers of uncertainty shift. Under the 5°C scenario, East Antarctic ice dynamics dominate the uncertainty in projections, driven by the possible role of ice flow instabilities. These dynamic effects only become dominant, however, for a temperature scenario above the Paris Agreement 2°C target and beyond 2100. Our findings identify key processes and factors that need to be addressed in future modeling and observational studies in order to reduce uncertainties in ice sheet projections.