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WIREs Clim Change
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Long‐term projections of sea‐level rise from ice sheets

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Abstract Under future climate change scenarios it is virtually certain that global mean sea level will continue to rise. But the rate at which this occurs, and the height and time at which it might stabilize, are uncertain. The largest potential contributors to sea level are the Greenland and Antarctic ice sheets, but these may take thousands of years to fully adjust to environmental changes. Modeled projections of how these ice masses will evolve in the future are numerous, but vary both in complexity and projection timescale. Typically, there is greater agreement between models in the present century than over the next millennium. This reflects uncertainty in the physical processes that dominate ice‐sheet change and also feedbacks in the ice–atmosphere–ocean system, and how these might lead to nonlinear behavior. Satellite observations help constrain short‐term projections of ice‐sheet change but these records are still too short to capture the full ice‐sheet response. Conversely, geological records can be used to inform long‐term ice‐sheet simulations but are prone to large uncertainties, meaning that they are often unable to adequately confirm or refute the operation of particular processes. Because of these limitations there is a clear need to more accurately reconstruct sea level changes during periods of the past, to improve the spatial and temporal extent of current ice sheet observations, and to robustly attribute observed changes to driving mechanisms. Improved future projections will require models that capture a more extensive suite of physical processes than are presently incorporated, and which better quantify the associated uncertainties. This article is categorized under: Climate Models and Modeling > Knowledge Generation with Models
Subglacial bed topography for (a) the Greenland and (b) the Antarctic ice sheets. Black lines delimit major subglacial basins. Note the very different physiographies of the two ice sheet beds, with central Greenland only slightly below sea level due to present‐day ice loading, compared to the much more extensive and deeper basins of East and West Antarctica that extend in some areas to depths of approximately 2 km. The impact that these differences in topographic situation exert on each ice sheet is summarized in Boxes and
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Long‐term response of the Greenland (a–c) and Antarctic (d–f) ice sheets to short‐term environmental perturbations. For the Greenland ice sheet, forcing under an RCP 8.5 scenario from 2000 to 2100 leads to (a) steady volume loss (solid lines) and increasing sea‐level contribution (dashed lines, central axes) that continue for many centuries following climatic stabilization, primarily because the surface mass balance rate continues to become more negative (b). The rate of melting of marine‐terminating glacier margins peaks when the environmental forcing is greatest but then rapidly reduces as these margins retreat above sea level (c). Stabilizing the climate at 2050 (green line), 2020 (pale blue line), or at year 2000 levels (dark blue line) reduces but does not prevent the continued loss of grounded ice, suggesting that the tipping point for some level of irreversible loss may have already been passed. In Antarctica, volume loss is slower to respond to the applied forcing but accelerates in the latter half of the forcing period (2050–2100; d). The rate of surface accumulation initially increases under the prescribed warming climate, but decreases subsequently because dynamic thinning and retreat of parts of the ice sheet reduce the total ice surface area (e). The rate of ice shelf melting follows a similar pattern, but is complicated by changes in ice shelf area as grounded ice goes afloat, or calves (f). Simulations from Golledge et al. ()
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Response curves of the Antarctic (blue squares) and Greenland (green circles) ice sheets to century‐long rise in (a) global mean annual air temperature and (b) global mean annual ocean temperature, based on data from Golledge et al. (). Although the environmental forcings for both ice sheets are more or less linear over the century, it is clear that the Greenland ice sheet responds more rapidly and more significantly to small changes in external conditions, especially the atmospheric forcing. However, although the Antarctic ice sheet response is initially slower, the magnitude of its sea‐level contribution exceeds that from Greenland once air temperatures are more than approximately 2°C above year‐2000 levels, with its contribution then also increasing more rapidly. This reflects the dominance of marine‐ice sheet dynamic losses in Antarctica, compared to the somewhat slower surface melting that ultimately governs the long‐term response of the Greenland ice sheet
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