Comment on tc-2021-394

Jeremy Bassis

doi:10.5194/tc-2021-394-rc2

Abstract

Many marine-terminating outlet glaciers have retreated rapidly in recent decades, but these changes have not been formally attributed to anthropogenic climate change. A key challenge for such an attribution assessment is that if glacier termini are sufficiently perturbed from bathymetric highs, ice-dynamic feedbacks can cause rapid retreat even without further climate forcing. In the presence of internal climate variability, attribution thus depends on understanding whether (or how frequently) these rapid retreats could be triggered by climatic noise alone. Our simulations with idealized glaciers show that in a noisy climate, rapid retreat is a stochastic phenomenon. We therefore propose a probabilistic approach to attribution and present a framework for analysis that uses ensembles of many simulations with independent realizations of random climate variability. Synthetic experiments show that century-scale climate trends substantially increase the likelihood of rapid glacier retreat. This effect depends on the timescales over which ice dynamics integrate forcing. For a population of synthetic glaciers with different topographies, we find that external trends increase the number of large retreats triggered within the population, offering a metric for regional attribution. Our analyses suggest that formal attribution studies are tractable and should be further pursued to clarify the human role in recent ice-sheet change. We emphasize that early-industrial-era constraints on glacier and climate state are likely to be crucial for such studies.

Highlights

Many marine-terminating glaciers in Greenland and Antarctica have retreated in recent decades, causing increased discharge to the ocean and accelerating sea-level rise (e.g., Meredith et al, 2019; Mouginot et al, 2019; Rignot et al, 2019)
We identify key physical principles that affect the likelihood of rapid marine-terminating glacier retreats in a noisy climate, and show how model ensembles can 90 be used to clarify the effects of anthropogenic forcing
Our results from large aleatory ensembles show how anthropogenic trends in climate forcing affect the probability of rapid marine-terminating glacier retreat in the presence of bed variations, even when the trend is weak compared to natural climate variability (Fig. 5)

Summary

Introduction

Many marine-terminating glaciers in Greenland and Antarctica have retreated in recent decades, causing increased discharge to the ocean and accelerating sea-level rise (e.g., Meredith et al, 2019; Mouginot et al, 2019; Rignot et al, 2019). In Antarctica, grounding line retreat and dynamic thinning is underway in the Amundsen Sea Embayment (ASE; e.g., Rignot et al, 2014; Joughin et al., 2014), and widespread retreat of glacier termini has been documented on the Antarctic Peninsula (Cook et al, 2014) and parts of East Antarctica (Miles et al, 2013). In both Greenland and Antarctica, glacier retreat has been linked to ocean forcing (Holland et al, 2008; Straneo et al, 2013; Jenkins et al, 2016; Wood et al, 2021), which in many cases is linked to atmospheric changes.

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