Reply on RC1

Eveline Van Der Linden

doi:10.5194/tc-2021-348-ac1

Abstract

Antarctic mass loss is the largest contributor to uncertainties in sea level projections on centennial timescales. In this study the contribution of Antarctica’s ice discharge to future sea level changes is computed with ocean thermal forcing from 14 earth system models and linear response functions from 16 ice sheet models for three greenhouse gas emission scenarios. Different than in previous studies, basal melt was calibrated on observed Antarctic ice discharge rather than on basal melt itself with an iterative approach. For each model combination, a linear and quadratic melt dependency were calibrated both regionally (in five Antarctic sectors) and at the continental scale. Projections using all model combinations show that the variation in basal melt computation methods affect the projected sea level more than the scenario variations (SSP1-2.6 to SSP5-8.5). After calibration, a high number of model pairs still underestimated ice discharge in hindcasts over 1979–2017. Therefore top 10 % best-performing model combinations were selected for each method. A comparison between these model selections shows that the quadratic melt parameterisation with Antarctic-wide calibration performs best in reproducing past ice discharge. We conclude that calibration of basal melt on past ice discharge combined with model selection makes projections of Antarctic ice discharge (more) consistent with observations over the past four decades. Moreover, calibration of basal melt on past ice discharge results in lower basal melt sensitivities and thus lower projections of Antarctica’s sea level contribution than estimates of previous multi-model studies.

Highlights

Sea level rise poses an increasing threat to densely populated coasts and deltas worldwide (Hinkel et al, 2014)
These percentages show that for the Weddell, 200 Ross and Peninsula sectors less than half of the earth system models (ESMs)-response functions (RF) pairs have greater-than-zero γ values
Ocean thermal forcing is based on regional subsurface ocean temperature from 14 Coupled Model Intercomparison Project Phase 6 (CMIP6) ESMs and 3 Shared Socioeconomic Pathways (SSP) scenarios and bias420 adjusted with GREP ocean reanalysis data

Summary

Introduction

Sea level rise poses an increasing threat to densely populated coasts and deltas worldwide (Hinkel et al, 2014). Even if the 1.5 degree target of the Paris Agreement is met, global mean sea level will rise several meters in the longer term (Clark et al, 2016; Fox-Kemper et al, 2021). Mass loss from land ice is currently accelerating and is (over the period 2006–2018) the largest contributor to the global mean sea level rise (Fox-Kemper et al, 2021). Antarctic ice sheet mass loss has tripled over the last decade (Shepherd et al, 2018), which can be mainly attributed to increased ice discharge in the Amundsen Sea (Rignot et al, 2019). Models and geological data indicate that the Antarctic ice sheet will cause most of the sea level rise in the long term

Methods

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Discussion

Conclusion