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Meltwater and ice discharge from a retreating Antarctic Ice Sheet could have important impacts on future global climate. Here, we report on multi-century (present–2250) climate simulations performed using a coupled numerical model integrated under future greenhouse-gas emission scenarios IPCC RCP4.5 and RCP8.5, with meltwater and ice discharge provided by a dynamic-thermodynamic ice sheet model. Accounting for Antarctic discharge raises subsurface ocean temperatures by >1°C at the ice margin relative to simulations ignoring discharge. In contrast, expanded sea ice and 2° to 10°C cooler surface air and surface ocean temperatures in the Southern Ocean delay the increase of projected global mean anthropogenic warming through 2250. In addition, the projected loss of Arctic winter sea ice and weakening of the Atlantic Meridional Overturning Circulation are delayed by several decades. Our results demonstrate a need to accurately account for meltwater input from ice sheets in order to make confident climate predictions. 

Abstract. The use of a boundary-layer parameterization ofbuttressing and ice flux across grounding lines in a two-dimensionalice-sheet model is improved by allowing general orientations of thegrounding line. This and another modification to the model's grounding-lineparameterization are assessed in three settings: rectangular fjord-likedomains – the third Marine Ice Sheet Model Intercomparison Project (MISMIP+) and Marine Ice Sheet Model Intercomparison Project for plan view models (MISMIP3d) – and future simulations of West Antarcticice retreat under Representative Concentration Pathway (RCP)8.5-based climates. The new modifications are found tohave significant effects on the fjord-like results, which are now within theenvelopes of other models in the MISMIP+ and MISMIP3d intercomparisons. Incontrast, the modifications have little effect on West Antarctic retreat,presumably because dynamics in the wider major Antarctic basins areadequately represented by the model's previous simpler one-dimensionalformulation. As future grounding lines retreat across very deep bedrocktopography in the West Antarctic simulations, buttressing is weak anddeviatoric stress measures exceed the ice yield stress, implying thatstructural failure at these grounding lines would occur. We suggest thatthese grounding-line quantities should be examined in similar projections byother ice models to better assess the potential for future structuralfailure. 

Abstract. It is widely accepted that orbital variations areresponsible for the generation of glacial cycles during the latePleistocene. However, the relative contributions of the orbital forcingcompared to CO2 variations and other feedback mechanisms causing thewaxing and waning of ice sheets have not been fully understood. Testingtheories of ice ages beyond statistical inferences, requires numericalmodeling experiments that capture key features of glacial transitions. Here,we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6covering the period from 240 to 170 ka (ka: thousand years before present). Thistransition from interglacial to glacial conditions includes one of thefastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupledatmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate thetransient evolution of Northern Hemisphere and Southern Hemisphere ice sheets duringthe MIS 7–6 period in response to orbital and greenhouse gas forcing. For arange of model parameters, the simulations capture the evolution of globalice volume well within the range of reconstructions. Over the MIS 7–6period, it is demonstrated that glacial inceptions are more sensitive toorbital variations, whereas terminations from deep glacial conditions needboth orbital and greenhouse gas forcings to work in unison. For someparameter values, the coupled model also exhibits a critical North Americanice sheet configuration, beyond which a stationary-wave–ice-sheettopography feedback can trigger an unabated and unrealistic ice sheetgrowth. The strong parameter sensitivity found in this study originates fromthe fact that delicate mass imbalances, as well as errors, are integratedduring a transient simulation for thousands of years. This poses a generalchallenge for transient coupled climate–ice sheet modeling, with suchcoupled paleo-simulations providing opportunities to constrain suchparameters. 

Retreat or advance of an ice sheet perturbs the Earth's solid surface, rotational vector, and the gravitational field, which in turn feeds back onto the evolution of the ice sheet over a range of timescales. Throughout the last glacial cycle, ice sheets over the Northern Hemisphere have gone through multiple growth and retreat phases, but the dynamics during these phases are not well understood. In this study, we apply a coupled ice sheet‐glacial isostatic adjustment model to simulate the Northern Hemisphere Ice Sheets over the last glacial cycle. We focus on understanding the influence of solid Earth deformation and gravitational field perturbations associated with surface (ice and water) loading changes on the dynamics of terrestrial and marine‐based ice sheets during different phases of the glacial cycle. Our results show that solid Earth deformation enhances glaciation during growth phases and melting during retreat phases in terrestrial regions through ice‐elevation feedback, and gravitational field perturbations have a stabilizing influence on marine‐based ice sheets in regions such as Hudson Bay in North America and Barents and Kara Seas in Eurasia during retreat phases through sea‐level feedback. Our results also indicate that solid Earth deformation influences the relative sensitivity of the North American and Eurasian ice sheets to climate and thus the timing and magnitude of their fluctuations throughout the last glacial cycle.

 

Abstract. Rapidly retreating thick ice fronts can generate large amounts of mélange(floating ice debris), which may affect episodes of rapid retreat ofAntarctic marine ice. In modern Greenland fjords, mélange providessubstantial back pressure on calving ice faces, which slows ice front calvingrates. On the much larger scales of West Antarctica, it is unknown ifmélange could clog seaways and provide enough back pressure to act as anegative feedback slowing retreat. Here we describe a new mélange model,using a continuum-mechanical formulation that is computationally feasible forlong-term continental Antarctic applications. It is tested in an idealizedrectangular channel and calibrated very basically using observed modernconditions in Jakobshavn fjord, West Greenland. The model is then applied todrastic retreat of Antarctic ice in response to warm mid-Pliocene climate.With mélange parameter values that yield reasonable modern Jakobshavnresults, Antarctic marine ice still retreats drastically in the Pliocenesimulations, with little slowdown despite the huge amounts of mélangegenerated. This holds both for the rapid early collapse of West Antarcticaand for later retreat into major East Antarctic basins. If parameter valuesare changed to make the mélange much more resistive to flow, far outsidethe range for reasonable Jakobshavn results, West Antarctica still collapsesand retreat is slowed or prevented only in a few East Antarctic basins.

 

Rapidly retreating thick ice fronts can generate large amounts of mélange (floating ice debris), which may affect episodes of rapid retreat of Antarctic marine ice. In modern Greenland fjords, mélange provides substantial back pressure on calving ice faces, which slows ice-front velocities and calving rates. On the much larger scales of West Antarctica, it is unknown if mélange could clog seaways and provide enough back pressure to act as a negative feedback slowing retreat. Here we describe a new mélange model, using a continuum mechanical formulation that is computationally feasible for long-term continental Antarctic applications. It is tested in an idealized rectangular channel, and calibrated very basically using observed modern conditions in Jakobshavn fjord, West Greenland. The model is then applied to drastic retreat of Antarctic ice in response to warm mid-Pliocene climate. With mélange parameter values that yield reasonable modern Jakobshavn results, Antarctic marine ice still retreats drastically in the Pliocene simulations, with little slowdown despite the huge amounts of mélange generated. This holds both for the rapid early collapse of West Antarctica, and later retreat into major East Antarctic basins. If parameter values are changed to make the mélange much more resistive to flow, far outside the range for reasonable Jakobshavn results, West Antarctica still collapses and retreat is slowed or prevented only in a few East Antarctic basins.