Search for: All records

Rapid, millennial-scale changes in sea level have been proposed for the beginning, middle, and/or end of the Last Interglacial (LIG) [~129 to 116 thousand years ago (ka)]. Each of these scenarios has different implications for polar ice sheet behavior in a warming world. Here, we present a suite of²³⁰Th ages for fossil corals in the Seychelles within a detailed sedimentary and stratigraphic context to evaluate the evolution of sea level during this past warm period. The rise to peak sea level at ~122 to 123 ka was punctuated by two abrupt stratigraphic discontinuities, defining three distinct generations of reef growth. We attribute the evidence of episodic reef growth and ephemeral sea-level fall to the competing influence of Northern Hemisphere ice melt and Antarctic ice regrowth. Asynchronous ice sheet contributions would mask the full extent of retreat for individual ice sheets during the LIG and imply greater temperature sensitivity of ice sheets than previously inferred. 

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. 

Abstract. The contribution of the Greenland Ice Sheet (GIS) to sea level rise (SLR) is accelerating and there is an urgent need to improve predictions of when and from what parts of the ice sheet Greenland will contribute its first meter. Estimating the volume of Greenland ice that was lost during past warm periods offers a way to constrain the ice sheet’s response to future warming. Sub-ice sediment and bedrock, retrieved from deep ice core campaigns or targeted drilling efforts, yield critical and direct information about past ice-free conditions. However, it is challenging to scale the few available sub-ice point measurements to the geometry of the entire ice sheet. Here, we provide a framework for assessing sea-level potential, which we define as the amount the GIS has contributed to sea level when a particular location in Greenland is ice-free, from an ensemble of ice-sheet model simulations representing a wide range of plausible deglaciation scenarios. An assessment of dominant sources of uncertainty in our paleo ice sheet modelling, including climate forcing, ice-sheet initialization, and solid-Earth properties, reveals spatial patterns in the sensitivity of the ice sheet to these processes and related feedbacks. We find that the sea-level potential of central Greenland is most sensitive to lithospheric feedbacks and ice-sheet initialization, whereas the ice-sheet margins are most sensitive to climate forcing parameters. Our framework allows us to quantify the local and regional uncertainty in sea-level potential, which we use to evaluate the GIS bedrock according to the usefulness of information sub-ice sediments and bedrock provide about past ice-sheet geometry. Through our ensemble approach, we can assign a plausible range of GIS contributions to global sea level for deglaciated conditions at any site. Our results identify primarily areas in southwest Greenland, and secondarily north Greenland, as best-suited for subglacial access drilling that seeks to constrain the response of the ice sheet to past and future warming. 

Abstract In the Ross Sea sector of Antarctica, periodic large-scale marine ice-sheet fluctuations since the mid-Miocene are recorded by drill core and seismic data, revealing a dynamic ice-sheet response to past increases in temperature and atmospheric CO2. In the adjacent, predominantly ice-free McMurdo Dry Valleys (MDVs), preserved terrestrial landscapes reflect persistent cold conditions and have been interpreted as indicators of a stable polar ice sheet, implying that the Antarctic Ice Sheet was largely insensitive during past warm periods. These disparate data-based perspectives highlight a long-standing debate around the past stability of the Antarctic Ice Sheet, with direct implications for the future ice-sheet response to ongoing climate warming. We reconcile marine records of dynamic ice-sheet behavior and episodic open-marine conditions with nearby ancient terrestrial landscapes recording consistent cold-polar conditions. Coupled ice-sheet and regional climate models nested at a high resolution are used to investigate surface temperatures in the MDVs during past warm periods. We find that high-elevation regions of the MDVs remain below freezing even when ice-free conditions prevail in the nearby Ross Sea. We compare observed landscapes with the spatial extent of modeled persistent cold conditions required for preservation of these ancient features, demonstrating that frozen MDVs landscapes could have coexisted with receded or collapsed ice sheets during past warm periods. 

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 The Greenland ice sheet (GrIS) covers a complex network of canyons thought to be preglacial and fluvial in origin, implying that these features have influenced the ice sheet since its inception. The largest of these canyons terminates in northwest Greenland at the outlet of the Petermann Glacier. Yet, the genesis of this canyon, and similar features in northern Greenland, remains unknown. Here, we present numerical model simulations of early GrIS history and show that interactions among climate, the growing ice sheet, and preexisting topography may have contributed to the excavation of the canyon via repeated catastrophic outburst floods. Our results have implications for interpreting sedimentary and geomorphic features beneath the GrIS and around its marine margins, and they document a novel mechanism for landscape erosion in Greenland.