Six Feet Of Sea Level Rise From Thwaites Glacier Still Possible By 2100; Thwaites Ice Shelf On Brink Of Collapse

[OKIsItJustMe]

5. In "Scientific reticence and sea level rise" Hansen refers to a 1978 paper by John Mercer

Here is a more recent paper, looking back at that earlier work

https://doi.org/10.1080/14702541.2020.1853870

West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster
Douglas I. Benn & David E. Sugden
Pages 13-23 | Published online: 14 Feb 2021

ABSTRACT
Over 40 years ago, the glaciologist John Mercer warned that parts of the West Antarctic Ice Sheet were at risk of collapse due to the CO2 greenhouse effect. Mercer recognised the unique vulnerability of ice sheets resting on beds far below sea level (marine-based ice sheets), where an initial warming signal can initiate irreversible retreat. In this paper, we review recent work on evidence for ice sheet collapse in warmer periods of the recent geological past, the current behaviour of the ice sheet, and computer models used to predict future ice-sheet response to global warming. Much of this work points in the same direction: warming climates can indeed trigger collapse of marine-based portions of the West Antarctic Ice Sheet, and retreat in response to recent warming has brought parts of the ice sheet to the threshold of instability. Further retreat appears to be inevitable, but the rate of collapse depends critically on future emissions.

Introduction
The title of this paper is identical to that used by John Mercer in Nature over 40 years ago, in which he predicted that anthropogenic climate change could threaten the stability of the West Antarctic Ice Sheet (Mercer, 1978). The introduction to Mercer’s paper has an impressively modern touch and highlights the existence of a marine basin beneath the West Antarctic Ice Sheet. He argues that the stability of the ice sheet covering the basin depends on the presence of shallow topographic thresholds and ice shelves around its periphery. With continued rise in CO2 this stability is threatened, bringing a danger of ice-sheet collapse and eventual global sea-level rise of several metres. Mercer warned that we should keep an eye on ice shelves in the Antarctic Peninsula as an early warning sign. Since then, several ice shelves in the Peninsula have been lost, such as the Larsen B Ice Shelf in 2002 (Cook & Vaughan, 2010; Scambos et al., 2000) and rapid changes have occurred around the margins of the Amundsen Sea (Christie et al., 2016; MacGregor et al., 2012).

The word ‘collapse’ is often used in the context of ice sheets and glaciers, but is seldom defined. Here, we use it to mean an irreversible process of mass loss initiated when some trigger causes the system to cross a threshold into instability. Most land-based glaciers do not exhibit this kind of behaviour. Although warming may cause rapid melting of land-based glaciers, their responses to climatic signals are typically linear and reversible. That is, if the climate signal changes, melting will slow down and the glacier may stabilise or even grow again. Marine-based glaciers, on the other hand, can undergo irreversible retreat in response to warming of the atmosphere and oceans, and ice loss may continue until all is gone even if the initial signal is removed. Another important point about the idea of ‘collapse’ is that it may happen over short or long timescales. In the case of a floating ice shelf such as Larsen B, collapse may occur over a few days once the critical stability threshold is crossed (MacAyeal et al., 2003). On the other hand, collapse of a large marine-based ice sheet may play out over hundreds or a thousand years. The key issue is not the rate of ice loss, but the fact that the system has no stable state after the initial ‘push’.

So, how much of the Antarctic Ice Sheet is at risk of collapse, and how fast could ice be lost to the ocean? There are three main approaches to answering these questions. First, study of the long-term history of an ice sheet reveals what has happened in the past. It is particularly useful to examine what happened during climates warmer than present, for example in the Pliocene 5.3–2.6 million years Ma ago and the last interglacial period some 120 thousand years ka ago. Second, we can study the current behaviour of the ice sheet alongside observations of the oceans and atmosphere, to identify areas of rapid change and understand the key processes at work. Third, computer modelling techniques allow us to perform experiments with ‘virtual glaciers’, and to study system response to changing conditions. Computer models can help us understand what has happened in the past, analyse the controls on ice-sheet behaviour in the present, and address the all-important question of how rapidly the ice sheet may change in response to alternative greenhouse gas futures.

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