Is the marine ice cliff hypothesis collapsing?

doi:10.1126/science.abj3266

GSTDTAP > 气候变化

DOI	10.1126/science.abj3266
	Is the marine ice cliff hypothesis collapsing?
	Nicholas R. Golledge; Daniel P. Lowry
	2021-06-18
发表期刊	Science
出版年	2021
英文摘要	Marine margins, where ice sheets flow from land into the ocean, become exposed to environmental conditions and internally generated forces markedly different from those governing the flow of ice further inland. Prior research has suggested that if the retreat of such margins formed tall ice cliffs, they may become structurally unstable and collapse, leading to further retreat ([ 1 ][1]). Although the veracity and importance of the “marine ice-cliff instability” are still uncertain ([ 2 ][2]), increasing the accuracy of simulated ice cliff processes is a key challenge because the Greenland and Antarctic ice sheets could raise global sea level by 65 m. On page 1342 of this issue, Bassis et al. ([ 3 ][3]) developed a model that reliably captures the complex behavior of ice cliffs as they deform and fracture. In doing so, they find that marine-terminating parts of Antarctica may be less vulnerable than previously suggested to rapid and irreversible collapse ([ 4 ][4], [ 5 ][5]). Ice sheets terminating in the ocean lose mass through melting either at the surface or on the underside of floating ice, and through calving of icebergs. For the calving fronts of thick glaciers to remain stable, they need to be grounded in deep water ([ 6 ][6]). Thicker ice or shallower water increases the height of the ice cliff exposed above water, and where this exceeds the threshold for structural stability (∼100 m), the cliff may collapse through slumping ([ 7 ][7]) (see the figure). The rate at which such calving proceeds is governed by the yield strength of ice ([ 8 ][8]), as well as the geometric configuration of the glacier front and the environmental forcing. Experimentation has also revealed that the critical height required for collapse of a marine ice cliff depends on how rapidly it becomes exposed ([ 9 ][9]). The sudden break-up of buttressing ice shelves, such as witnessed in the Antarctic Peninsula over recent decades, could expose a new ice cliff within days or weeks. This “preconditioning” means that cliffs could fail at lower heights than if ice shelf collapse proceeded more slowly. A “speed limit” for how quickly an ice shelf can collapse has been recently computed ([ 10 ][10]), but one of the key difficulties in accurately modeling or predicting any ice cliff failure that then follows is that the calving process itself comprises two distinct modes of failure. Thick and undamaged ice has a high tensile strength and tends to fail through shearing (deformation) ([ 11 ][11]), whereas rapidly sliding glaciers are prone to tensile failure (fracturing) because of their extensional flow regime and the intersection of crevasses that develop as the glacier is “stretched.” This dual mode of failure is where Bassis et al. make such an important advance. The authors' “m-ice” model simulates ice as a viscous material whose deformation rate is described by a power law only until its yield strength is reached, at which point it then deforms rapidly. Accumulation of plastic strain in the failed ice reduces its strength, allowing it to deform even more rapidly, progressively localizing failure within discrete bands. Thus, although not explicitly modeling the brittle processes of ice fracturing, the m-ice model uses a pioneering composite rheology that allows the flow and failure modes of ice cliff calving to be captured in a single continuum framework. Bassis et al. use their model to explore the environmental and geometric controls on how a range of marine-terminating glacier margins might evolve. The authors find that resistive forces at the ice front, caused either by sea ice or the calved debris that typically chokes narrow fjords during winter months, can slow or even completely prevent the retreat of an ice cliff. But even without this kind of buttressing, Bassis et al. conclude that ice cliffs may be inherently stable if the speed of ice flow or slope of the bed underlying the ice are within certain bounds. Specifically, the authors find that the upstream gradient of ice thickness exerts a first-order control on the tendency of an ice cliff to reach a critical (collapse-prone) height, and that the rate at which ice flows toward the calving front exerts a secondary control. For the majority of thickness gradients considered, faster-flowing ice allows ice margins to advance, whereas slower-flowing ice produces cliff retreat. Where ice thickness gradients are greater than about 30 m per kilometer, ice cliffs tend to collapse regardless of the inflowing ice velocity. ![Figure][12] A refined mechanism of marine ice cliff retreat The manner in which ice cliffs fail is important for projecting glacier retreat in a warming environment. GRAPHIC: C. BICKEL/ SCIENCE BASED ON N. GOLLEDGE/ANTARCTIC RESEARCH CENTRE The idea that brittle failure of vertical ice cliffs could lead to catastrophic collapse of a large part of the West Antarctic Ice Sheet has recently been put forward and is used to project sea level contributions from Antarctica of as much as 1 m by 2100 ([ 4 ][4]), under a future greenhouse gas emissions scenario that leads to an increase in radiative forcing of 8.5 W/m2. These studies have done much to catalyze and focus research efforts within the glaciological community, and to highlight to policy-makers the deep uncertainty associated with Antarctic ice sheet retreat in particular. But parameterized models that use extrapolations from sparse observations of a poorly understood process mean that resulting predictions of retreat rates vary considerably between model versions ([ 1 ][1], [ 4 ][4], [ 5 ][5]), so the impact of ice cliff collapse on future sea level rise remains uncertain ([ 2 ][2], [ 12 ][13]). What is certain, however, is that if the processes involved in ice cliff collapse are to be included in models used for future sea level projections, they need to incorporate robust physics and be well-validated by comparison to observations. Bassis et al. have made a substantial leap forward in both of these areas with the m-ice model that offers exciting avenues for better understanding ice cliff behavior. 1. [↵][14]1. D. Pollard, 2. R. M. DeConto, 3. R. B. Alley , Earth Planet. Sci. Lett. 412, 112 (2015). [OpenUrl][15] 2. [↵][16]1. H.-O. Pörtner et al. 1. M. Meredith et al ., IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, H.-O. Pörtner et al., Eds. (IPCC, 2019), pp. 203–320. 3. [↵][17]1. J. N. Bassis et al ., Science 372, 1342 (2021). [OpenUrl][18][Abstract/FREE Full Text][19] 4. [↵][20]1. R. M. DeConto, 2. D. Pollard , Nature 531, 591 (2016). [OpenUrl][21][CrossRef][22][GeoRef][23][PubMed][24] 5. [↵][25]1. R. M. DeConto et al ., Nature 593, 83 (2021). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计	被引频次：3[WOS] [WOS记录] [WOS相关记录]
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/330802
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Nicholas R. Golledge,Daniel P. Lowry. Is the marine ice cliff hypothesis collapsing?[J]. Science,2021.
APA	Nicholas R. Golledge,&Daniel P. Lowry.(2021).Is the marine ice cliff hypothesis collapsing?.Science.
MLA	Nicholas R. Golledge,et al."Is the marine ice cliff hypothesis collapsing?".Science (2021).