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Changes in global mean sea level (GMSL) during the late Cenozoic remain uncertain. We use a reconstruction of changes in δ¹⁸O of seawater to reconstruct GMSL since 4.5 million years ago (Ma) that accounts for temperature-driven changes in the δ¹⁸O of global ice sheets. Between 4.5 and 3 Ma, sea level highstands remained up to 20 m above present whereas the first lowstands below present suggest onset of Northern Hemisphere glaciation at 4 Ma. Intensification of global glaciation occurred from 3 Ma to 2.5 Ma, culminating in lowstands similar to the Last Glacial Maximum lowstand at 21,000 years ago and that reoccurred throughout much of the Pleistocene. We attribute the middle Pleistocene transition in ice sheet variability (1.2 Ma to 0.62 Ma) to modulation of 41-thousand-year (kyr) obliquity forcing by an increase in ~100-kyr CO₂variability. 

Abstract Pliocene global temperatures periodically exceeded modern levels, offering insights into ice sheet sensitivity to warm climates. Ice-proximal geologic records from this period provide crucial but limited glimpses of Antarctic Ice Sheet behavior. We use an ice sheet model driven by climate model snapshots to simulate transient glacial cyclicity from 4.5 to 2.6 Ma, providing spatial and temporal context for geologic records. By evaluating model simulations against a comprehensive synthesis of geologic data, we translate the intermittent geologic record into a continuous reconstruction of Antarctic sea level contributions, revealing a dynamic ice sheet that contributed up to 25 m of glacial-interglacial sea level change. Model grounding line behavior across all major Antarctic catchments exhibits an extended period of receded ice during the mid-Pliocene, coincident with proximal geologic data around Antarctica but earlier than peak warmth in the Northern Hemisphere. Marine ice sheet collapse is triggered with 1.5 °C model subsurface ocean warming. 

Abstract Freshwater discharge from ice sheets induces surface atmospheric cooling and subsurface ocean warming, which are associated with negative and positive feedbacks respectively. However, uncertainties persist regarding these feedbacks’ relative strength and combined effect. Here we assess associated feedbacks in a coupled ice sheet-climate model, and show that for the Antarctic Ice Sheet the positive feedback dominates in moderate future warming scenarios and in the early stage of ice sheet retreat, but is overwhelmed by the negative feedback in intensive warming scenarios when the West Antarctic Ice Sheet undergoes catastrophic collapse. The Atlantic Meridional Overturning Circulation is affected by freshwater discharge from both the Greenland and the Antarctic ice sheets and, as an interhemispheric teleconnection bridge, exacerbates the opposing ice sheet’s retreat via the Bipolar Seesaw. These results highlight the crucial role of ice sheet-climate interactions via freshwater flux in future ice sheet retreat and associated sea-level rise. 

The response of the Antarctic Ice Sheet (AIS) to climate change is the largest uncertainty in projecting future sea level. The impact of three-dimensional (3D) Earth structure on the AIS and future global sea levels is assessed here by coupling a global glacial isostatic adjustment model incorporating 3D Earth structure to a dynamic ice-sheet model. We show that including 3D viscous effects produces rapid uplift in marine sectors and reduces projected ice loss for low greenhouse gas emission scenarios, lowering Antarctica’s contribution to global sea level in the coming centuries by up to ~40%. Under high-emission scenarios, ice retreat outpaces uplift, and sea-level rise is amplified by water expulsion from Antarctic marine areas. 

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.