Modeling the oxygen isotope composition of the Antarctic ice sheet and its significance to Pliocene sea leve

Recent estimates of global mean sea level based on the oxygen isotope composition of mid-Pliocene benthic foraminifera vary from 9 to 21 m above present, which has differing implications for the past stability of the Antarctic ice sheet during an interval with atmospheric CO2 comparable to present. Here we simulate the oxygen isotope composition of the Antarctic ice sheet for a range of configurations using isotope-enabled climate and ice sheet models. We identify which ice-sheet configurations are consistent with the oxygen isotope record and suggest a maximum contribution from Antarctica to the mid-Pliocene sea level highstand of ~13 m. We also highlight that the relationship between the oxygen isotope record and sea level is not constant when ice is lost from deep marine basins, which has important implications for the use of oxygen isotopes as a sea level proxy

Improvements in one-dimensional grounding-line parameterizations in an ice-sheet model with lateral variations (PSUICE3D v2.1)

The use of a boundary-layer parameterization of buttressing and ice flux across grounding lines in a two-dimensional ice-sheet model is improved by allowing general orientations of the grounding line. This and another modification to the model\u27s grounding-line parameterization are assessed in three settings: rectangular fjord-like domains – 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 Antarctic ice retreat under Representative Concentration Pathway (RCP)8.5-based climates. The new modifications are found to have significant effects on the fjord-like results, which are now within the envelopes of other models in the MISMIP+ and MISMIP3d intercomparisons. In contrast, the modifications have little effect on West Antarctic retreat, presumably because dynamics in the wider major Antarctic basins are adequately represented by the model\u27s previous simpler one-dimensional formulation. As future grounding lines retreat across very deep bedrock topography in the West Antarctic simulations, buttressing is weak and deviatoric stress measures exceed the ice yield stress, implying that structural failure at these grounding lines would occur. We suggest that these grounding-line quantities should be examined in similar projections by other ice models to better assess the potential for future structural failure

Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sadai, S., Condron, A., DeConto, R., & Pollard, D. Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming. Science Advances, 6(39), (2020): eaaz1169, doi:10.1126/sciadv.aaz1169.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.This research was supported by the NSF Office of Polar Programs through NSF grant 1443347, the Biological and Environmental Research (BER) division of the U.S. Department of Energy through grant DE-SC0019263, the NSF through ICER 1664013, and by a grant to the NASA Sea Level Science Team 80NSSC17K0698

Windblown Pliocene Diatoms and East Antarctic Ice Sheet Retreat

Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise

Could the Last Interglacial Constrain Projections of Future Antarctic Ice Mass Loss and Sea‐Level Rise?

Previous studies have interpreted Last Interglacial (LIG; ∼129–116 ka) sea-level estimates in multiple different ways to calibrate projections of future Antarctic ice-sheet (AIS) mass loss and associated sea-level rise. This study systematically explores the extent to which LIG constraints could inform future Antarctic contributions to sea-level rise. We develop a Gaussian process emulator of an ice-sheet model to produce continuous probabilistic projections of Antarctic sea-level contributions over the LIG and a future high-emissions scenario. We use a Bayesian approach conditioning emulator projections on a set of LIG constraints to find associated likelihoods of model parameterizations. LIG estimates inform both the probability of past and future ice-sheet instabilities and projections of future sea-level rise through 2150. Although best-available LIG estimates do not meaningfully constrain Antarctic mass loss projections or physical processes until 2060, they become increasingly informative over the next 130 years. Uncertainties of up to 50 cm remain in future projections even if LIG Antarctic mass loss is precisely known (±5 cm), indicating that there is a limit to how informative the LIG could be for ice-sheet model future projections. The efficacy of LIG constraints on Antarctic mass loss also depends on assumptions about the Greenland ice sheet and LIG sea-level chronology. However, improved field measurements and understanding of LIG sea levels still have potential to improve future sea-level projections, highlighting the importance of continued observational efforts

Could the Last Interglacial Constrain Projections of Future Antartic Ice Mass Loss and Sea-Level Rise?

Previous studies have interpreted Last Interglacial (LIG; ∼129–116 ka) sea‐level estimates in multiple different ways to calibrate projections of future Antarctic ice‐sheet (AIS) mass loss and associated sea‐level rise. This study systematically explores the extent to which LIG constraints could inform future Antarctic contributions to sea‐level rise. We develop a Gaussian process emulator of an ice‐sheet model to produce continuous probabilistic projections of Antarctic sea‐level contributions over the LIG and a future high‐emissions scenario. We use a Bayesian approach conditioning emulator projections on a set of LIG constraints to find associated likelihoods of model parameterizations. LIG estimates inform both the probability of past and future ice‐sheet instabilities and projections of future sea‐level rise through 2150. Although best‐available LIG estimates do not meaningfully constrain Antarctic mass loss projections or physical processes until 2060, they become increasingly informative over the next 130 years. Uncertainties of up to 50 cm remain in future projections even if LIG Antarctic mass loss is precisely known (±5 cm), indicating that there is a limit to how informative the LIG could be for ice‐sheet model future projections. The efficacy of LIG constraints on Antarctic mass loss also depends on assumptions about the Greenland ice sheet and LIG sea‐level chronology. However, improved field measurements and understanding of LIG sea levels still have potential to improve future sea‐level projections, highlighting the importance of continued observational efforts

Initiation of the West Antarctic Ice Sheet and estimates of total Antarctic ice volume in the earliest Oligocene

Reconstructions of Antarctic paleotopography for the late Eocene suggest that glacial erosion and thermal subsidence have lowered West Antarctic elevations considerably since then, with Antarctic land area having decreased ~20%. A new climate-ice sheet model based on these reconstructions shows that the West Antarctic Ice Sheet first formed at the Eocene-Oligocene transition (33.8–33.5 Ma, E-O) in concert with the continental-scale expansion of the East Antarctica Ice Sheet and that the total volume of East and West Antarctic ice (33.4–35.9 × 106 km3) was >1.4 times greater than previously assumed. This larger modeled ice volume is consistent with a modest cooling of 1–2°C in the deep ocean during the E-O transition, lower than other estimates of ~3°C cooling, and suggests the possibility of substantial ice in the Antarctic interior before the Eocene-Oligocene boundary

CO\u3csub\u3e2\u3c/sub\u3e and tectonic controls on Antarctic climate and ice-sheet evolution in the mid-Miocene

Antarctic ice sheet and climate evolution during the mid-Miocene has direct relevance for understanding ice sheet (in)stability and the long-term response to elevated atmospheric CO2 in the future. Geologic records reconstruct major fluctuations in the volume and extent of marine and terrestrial ice during the mid-Miocene, revealing a dynamic Antarctic ice-sheet response to past climatic variations. We use an ensemble of climate – ice sheet – vegetation model simulations spanning a range of CO2 concentrations, Transantarctic Mountain uplift scenarios, and glacial/interglacial climatic conditions to identify climate and ice-sheet conditions consistent with Antarctic mid-Miocene terrestrial and marine geological records. We explore climatic variability at both continental and regional scales, focusing specifically on Victoria Land and Wilkes Land Basin regions using a high-resolution nested climate model over these domains. We find that peak warmth during the Miocene Climate Optimum is characterized by a thick terrestrial ice sheet receded from the coastline under high CO2 concentrations. During the Middle Miocene Climate Transition, CO2 episodically dropped below a threshold value for marine-based ice expansion. Comparison of model results with geologic data support ongoing Transantarctic Mountain uplift throughout the mid-Miocene. Modeled ice sheet dynamics over the Wilkes Land Basin were highly sensitive to CO2 concentrations. This work provides a continental-wide context for localized geologic paleoclimate and vegetation records, integrating multiple datasets to reconstruct snapshots of ice sheet and climatic conditions during a pivotal period in Earth\u27s history

Antarctic Supraglacial Lake Identification Using Landsat-8 Image Classification

Surface meltwater generated on ice shelves fringing the Antarctic Ice Sheet can drive ice-shelf collapse, leading to ice sheet mass loss and contributing to global sea level rise. A quantitative assessment of supraglacial lake evolution is required to understand the influence of Antarctic surface meltwater on ice-sheet and ice-shelf stability. Cloud computing platforms have made the required remote sensing analysis computationally trivial, yet a careful evaluation of image processing techniques for pan-Antarctic lake mapping has yet to be performed. This work paves the way for automating lake identification at a continental scale throughout the satellite observational record via a thorough methodological analysis. We deploy a suite of different trained supervised classifiers to map and quantify supraglacial lake areas from multispectral Landsat-8 scenes, using training data generated via manual interpretation of the results from k-means clustering. Best results are obtained using training datasets that comprise spectrally diverse unsupervised clusters from multiple regions and that include rock and cloud shadow classes. We successfully apply our trained supervised classifiers across two ice shelves with different supraglacial lake characteristics above a threshold sun elevation of 20°, achieving classification accuracies of over 90% when compared to manually generated validation datasets. The application of our trained classifiers produces a seasonal pattern of lake evolution. Cloud shadowed areas hinder large-scale application of our classifiers, as in previous work. Our results show that caution is required before deploying ‘off the shelf’ algorithms for lake mapping in Antarctica, and suggest that careful scrutiny of training data and desired output classes is essential for accurate results. Our supervised classification technique provides an alternative and independent method of lake identification to inform the development of a continent-wide supraglacial lake mapping product

Numerical simulations of a kilometre-thick Arctic ice shelf consistent with ice grounding observations

Recently obtained geophysical data show sets of parallel erosional features on the Lomonosov Ridge in the central Arctic Basin, indicative of ice grounding in water depths up to 1280 m. These features have been interpreted as being formed by an ice shelf—either restricted to the Amerasian Basin (the “minimum model”) or extending across the entire Arctic Basin. Here, we use a numerical ice sheet-shelf model to explore how such an ice shelf could form. We rule out the “minimum model” and suggest that grounding on the Lomonosov Ridge requires complete Arctic ice shelf cover; this places a minimum estimate on its volume, which would have exceeded that of the modern Greenland Ice Sheet. Buttressing provided by an Arctic ice shelf would have increased volumes of the peripheral terrestrial ice sheets. An Arctic ice shelf could have formed even in the absence of a hypothesised East Siberian Ice Sheet