scholarly journals Simulating Marine Isotope Stage 7 with a coupled climate–ice sheet model

2020 ◽  
Vol 16 (6) ◽  
pp. 2183-2201
Author(s):  
Dipayan Choudhury ◽  
Axel Timmermann ◽  
Fabian Schloesser ◽  
Malte Heinemann ◽  
David Pollard
Keyword(s):  
Greenhouse Gas ◽  
Three Dimensional ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Coupled Model ◽  
Stationary Wave ◽  
Marine Isotope Stage ◽  
Model Parameters ◽  
Glacial Cycles ◽  
Ice Sheet Modeling

Abstract. It is widely accepted that orbital variations are responsible for the generation of glacial cycles during the late Pleistocene. However, the relative contributions of the orbital forcing compared to CO2 variations and other feedback mechanisms causing the waxing and waning of ice sheets have not been fully understood. Testing theories of ice ages beyond statistical inferences, requires numerical modeling experiments that capture key features of glacial transitions. Here, we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6 covering the period from 240 to 170 ka (ka: thousand years before present). This transition from interglacial to glacial conditions includes one of the fastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupled atmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate the transient evolution of Northern Hemisphere and Southern Hemisphere ice sheets during the MIS 7–6 period in response to orbital and greenhouse gas forcing. For a range of model parameters, the simulations capture the evolution of global ice volume well within the range of reconstructions. Over the MIS 7–6 period, it is demonstrated that glacial inceptions are more sensitive to orbital variations, whereas terminations from deep glacial conditions need both orbital and greenhouse gas forcings to work in unison. For some parameter values, the coupled model also exhibits a critical North American ice sheet configuration, beyond which a stationary-wave–ice-sheet topography feedback can trigger an unabated and unrealistic ice sheet growth. The strong parameter sensitivity found in this study originates from the fact that delicate mass imbalances, as well as errors, are integrated during a transient simulation for thousands of years. This poses a general challenge for transient coupled climate–ice sheet modeling, with such coupled paleo-simulations providing opportunities to constrain such parameters.

scholarly journals Simulating Marine Isotope Stage 7 with a coupled climate-ice sheet model

2020 ◽  
Author(s):  
Dipayan Choudhury ◽  
Axel Timmermann ◽  
Fabian Schloesser ◽  
Malte Heinemann ◽  
David Pollard
Keyword(s):  
Greenhouse Gas ◽  
Three Dimensional ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Coupled Model ◽  
Stationary Wave ◽  
Marine Isotope Stage ◽  
Model Parameters ◽  
Glacial Cycles ◽  
Ice Sheet Modeling

Abstract. It is widely accepted that orbital variations are responsible for the generation of glacial cycles during the late Pleistocene. However, the relative contributions of the orbital forcing compared to CO2 variations and other feedback mechanisms causing the waxing and waning of ice-sheets have not been fully understood. Testing theories of ice-ages beyond statistical inferences, requires numerical modeling experiments that capture key features of glacial transitions. Here, we focus on the glacial build-up from Marine Isotope Stage (MIS) 7 to 6 covering the period from 240–170 ka (thousand years before present). This transition from interglacial to glacial conditions includes one of the fastest Pleistocene glaciation/deglaciation events which occurred during MIS 7e-7d-7c (236–218 ka). Using a newly developed three-dimensional coupled atmosphere-ocean-vegetation-ice-sheet model (LOVECLIP), we simulate the transient evolution of northern and southern hemisphere ice-sheets during the MIS 7-6 period in response to orbital and greenhouse-gas forcing. For a range of model parameters, the simulations capture the reconstructed evolution of global ice volume reasonably well. It is demonstrated that glacial inceptions are more sensitive to orbital variations, whereas terminations from deep glacial conditions need both orbital and greenhouse gas forcings to work in unison. For some parameter values, the coupled model also exhibits a critical North American ice sheet configuration, beyond which a stationary wave – ice-sheet topography feedback can trigger an unabated and unrealistic ice-sheet growth. The strong parameter sensitivity found in this study originates from the fact that delicate mass imbalances, as well as errors, are integrated during a transient simulation for thousands of years. This poses a general challenge for transient coupled climate-ice sheet modeling.

Transient Pleistocene simulations with a new coupled climate-ice-sheet model

2020 ◽  
Author(s):  
Dipayan Choudhury ◽  
Axel Timmermann ◽  
Fabian Schloesser ◽  
David Pollard
Keyword(s):  
Regional Scale ◽  
Three Dimensional ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Coupled Model ◽  
Multiple Equilibrium ◽  
Ice Age ◽  
Physical Factors ◽  
Glacial Cycles ◽  
Abrupt Changes

<p>Orbital and CO<sub>2 </sub>variations over glacial timescales are widely held responsible as drivers of the ice-age cycles of the Pleistocene. Alongside these glacial cycles, our paleoclimate history is marked with abrupt changes and millennium scale variabilities. However, the relative contributions of these forcings over glacial transitions and mechanisms of abrupt changes are not very well understood. Here, using the recently developed three-dimensional coupled climate – ice-sheet model (LOVECLIM – Penn State University ice-sheet model), we simulate the glacial inception over the period of MIS7 to MIS6 (240-170ka). This period is the coldest interglacial post the Mid-Brunhes Event and includes one of the fastest glaciation/deglaciation events of the Late Pleistocene, over MIS7e-7d-7c (236-218ka); which we use here to benchmark our transient coupled model runs. Our results suggest that glacial inceptions are more sensitive to orbital variations, whereas terminations need both forcings to work in tandem over a tiny ablation zone at the southern margins of ice sheets. And abrupt changes may result from a critical interplay between the climate and the cryosphere systems. Using multiple ensembles in combination with conceptual dynamical systems’ models, we test the sensitivity of ice-sheets to various physical factors and discuss the presence of multiple equilibrium states and runaway effects. Additionally, our simulations show that regional scale variations at the southern end of Laurentide can lead to a bifurcation of the system and play a role even in orbital-scale ice-sheet growth/decay.</p>

Antarctic ice-sheet hysteresis in a three-dimensional hybrid ice-sheet model

2020 ◽  
Author(s):  
Marisa Montoya ◽  
Jorge Alvarez-Solas ◽  
Alexander Robinson ◽  
Javier Blasco ◽  
Ilaria Tabone ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Future Climate Change ◽  
Model Parameters ◽  
Antarctic Ice Sheet ◽  
Positive Feedbacks ◽  
Surface Mass ◽  
Ice Sheet Dynamics ◽  
Basal Melting

<p>Ice sheets, in particular the Antarctic Ice Sheet (AIS), are considered as potential tipping elements (TEs) of the Earth system. The mechanism underlying tipping is the existence of positive feedbacks leading to self-amplification processes that, once triggered, dominate the dynamics of the system. Positive feedbacks can also lead to hysteresis, with implications for reversibility in the context of long-term future climate change. The main mechanism underlying ice-sheet hysteresis is the positive surface mass balance-elevation feedback.  Marine-based ice sheets, such as the western sector of the AIS, are furthermore subject to specific instability mechanisms that can potentially also lead to hysteresis. Simulations with ice-sheet models have robustly confirmed the presence of different degrees of hysteresis in the evolution of the AIS volume with respect to model parameters and/or climate forcing, suggesting that ice-sheet changes are potentially irreversible on long timescales. Nevertheless, AIS hysteresis is only now becoming a focus of more intensive modeling efforts, including active oceanic forcing in particular. Here, we investigate the hysteresis of the AIS in a three-dimensional hybrid ice-sheet--ice-shelf model with respect to individual atmospheric forcing, ocean forcing and both. The aim is to obtain a probabilistic assessment of the AIS hysteresis and of its critical temperature thresholds by investigating the effect of structural uncertainty, including the representation of ice-sheet dynamics, basal melting and internal feedbacks.</p>

scholarly journals Experimental protocol for sealevel projections from ISMIP6 standalone ice sheet models

2020 ◽  
Author(s):  
Sophie Nowicki ◽  
Antony J. Payne ◽  
Heiko Goelzer ◽  
Helene Seroussi ◽  
William H. Lipscomb ◽  
...  
Keyword(s):  
Sea Level ◽  
Climate Models ◽  
Ice Sheet ◽  
Coupled Model ◽  
Sea Level Change ◽  
Model Intercomparison ◽  
Level Change ◽  
Ice Sheet Modeling ◽  
Emissions Scenarios ◽  
Intercomparison Project

Abstract. Projection of the contribution of ice sheets to sea-level change as part of the Coupled Model Intercomparison Project – phase 6 (CMIP6) takes the form of simulations from coupled ice-sheet-climate models and standalone ice sheet models, overseen by the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). This paper describes the experimental setup for process-based sea-level change projections to be performed with standalone Greenland and Antarctic ice sheet models in the context of ISMIP6. The ISMIP6 protocol relies on a suite of polar atmospheric and oceanic CMIP-based forcing for ice sheet models, in order to explore the uncertainty in projected sea-level change due to future emissions scenarios, CMIP models, ice sheet models, and parameterizations for ice-ocean interactions. We describe here the approach taken for defining the suite of ISMIP6 standalone ice sheet simulations, document the experimental framework and implementation, as well as present an overview of the ISMIP6 forcing to be used by participating ice sheet modeling groups.

scholarly journals The sensitivity of the Greenland Ice Sheet to glacial–interglacial oceanic forcing

2018 ◽  
Vol 14 (4) ◽  
pp. 455-472 ◽  
Author(s):  
Ilaria Tabone ◽  
Javier Blasco ◽  
Alexander Robinson ◽  
Jorge Alvarez-Solas ◽  
Marisa Montoya
Keyword(s):  
Sea Level ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
High Sensitivity ◽  
Glacial Cycles ◽  
The Past ◽  
Primary Driver ◽  
Oceanic Forcing ◽  
Atmospheric Variations

Abstract. Observations suggest that during the last decades the Greenland Ice Sheet (GrIS) has experienced a gradually accelerating mass loss, in part due to the observed speed-up of several of Greenland's marine-terminating glaciers. Recent studies directly attribute this to warming North Atlantic temperatures, which have triggered melting of the outlet glaciers of the GrIS, grounding-line retreat and enhanced ice discharge into the ocean, contributing to an acceleration of sea-level rise. Reconstructions suggest that the influence of the ocean has been of primary importance in the past as well. This was the case not only in interglacial periods, when warmer climates led to a rapid retreat of the GrIS to land above sea level, but also in glacial periods, when the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes. However, the GrIS response to palaeo-oceanic variations has yet to be investigated in detail from a mechanistic modelling perspective. In this work, the evolution of the GrIS over the past two glacial cycles is studied using a three-dimensional hybrid ice-sheet–shelf model. We assess the effect of the variation of oceanic temperatures on the GrIS evolution on glacial–interglacial timescales through changes in submarine melting. The results show a very high sensitivity of the GrIS to changing oceanic conditions. Oceanic forcing is found to be a primary driver of GrIS expansion in glacial times and of retreat in interglacial periods. If switched off, palaeo-atmospheric variations alone are not able to yield a reliable glacial configuration of the GrIS. This work therefore suggests that considering the ocean as an active forcing should become standard practice in palaeo-ice-sheet modelling.

Bayesian analysis of sea-level sensitivity to CO2 forcing across the mid-Pleistocene transition: possible implications for early-Pleistocene ice-sheet extent

2021 ◽  
Author(s):  
Parker Liautaud ◽  
Peter Huybers
Keyword(s):  
Energy Balance ◽  
Bayesian Analysis ◽  
Sea Level ◽  
Late Pleistocene ◽  
Radiative Forcing ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Early Pleistocene ◽  
Middle Pleistocene ◽  
Model Parameters

<p><span>Foregoing studies have found that sea-level transitioned to becoming approximately twice as sensitive to CO</span><span><sub>2</sub></span><span> radiative forcing between the early and late Pleistocene (Chalk et al., 2017; Dyez et al., 2018). In this study we analyze the relationships among sea-level, orbital variations, and CO</span><span><sub>2</sub></span><span> observations in a time-dependent, zonally-averaged energy balance model having a simple ice sheet. Probability distributions for model parameters are inferred using a hierarchical Bayesian method representing model and data uncertainties, including those arising from uncertain geological age models. We find that well-established nonlinearities in the climate system can explain sea-level becoming 2.5x (2.1x - 4.5x) more sensitive to radiative forcing between 2 and 0 Ma. Denial-of-mechanism experiments show that the increase in sensitivity is diminished by 36% (31% - 39%) if omitting geometric effects associated with thickening of a larger ice sheet, by 81% (73% - 92%) if omitting the ice-albedo feedback, and by more than 96% (93% - 98%) if omitting both. We also show that prescribing a fixed sea-level age model leads to different inferences of ice-sheet dimension, planetary albedo, and lags in the response to radiative forcing than if using a more complete approach in which sea-level ages are jointly inferred with model physics. Consistency of the model ice-sheet with geologic constraints on the southern terminus of the Laurentide ice sheet can be obtained by prescribing lower basal shear stress during the early Pleistocene, but such more-expansive ice sheets imply lower CO</span><span><sub>2</sub></span><span> levels than would an ice-sheet having the same aspect ratio as in the late Pleistocene, exacerbating disagreements with </span><span>𝛿</span><span><sup>11</sup></span><span>B-derived CO</span><span><sub>2</sub></span><span> estimates. These results raise a number of possibilities, including that (1) geologic evidence for expansive early-Pleistocene ice sheets represents only intermittent and spatially-limited ice-margin advances, (2) </span><span>𝛿</span><span><sup>11</sup></span><span>B-derived CO</span><span><sub>2</sub></span><span> reconstructions are biased high, or (3) that another component of the global energy balance system, such as the average ice albedo or a process not included in our model, also changed through the middle Pleistocene. Future work will seek to better constrain early-Pleistocene CO</span><span><sub>2</sub></span><span> levels by way of a more complete incorporation of proxy uncertainties and biases into the Bayesian analysis.</span></p>

8. Glaciers of the past

2018 ◽  
pp. 146-152
Author(s):  
David J. A. Evans
Keyword(s):  
Numerical Models ◽  
Ice Core ◽  
Three Dimensional ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Aerial Imagery ◽  
The Past ◽  
Glacial Landforms

To reconstruct the former extent and dynamics of ice sheets and glaciers requires a knowledge of process-form relationships that goes beyond individual landform types. Instead, glacial geomorphologists need to analyse large areas of glaciated terrain in a more holistic way, combining the whole range of glacial landforms and sediments to reconstruct glacier systems of the past, a subject now known as palaeoglaciology. ‘Glaciers of the past’ explains how the combination of aerial imagery and landform analysis is used in palaeoglaciological reconstruction. Increasingly powerful computers are making it possible to compile sophisticated numerical models that use our knowledge of glaciological processes and ice-core-derived palaeoclimate data to create three-dimensional glacier and ice sheet reconstructions.

scholarly journals Basal temperature conditions of the Greenland ice sheet during the glacial cycles

1996 ◽  
Vol 23 ◽  
pp. 226-236 ◽  
Author(s):  
Philippe Huybrechts
Keyword(s):  
Steady State ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Cores ◽  
Ice Thickness ◽  
Glacial Cycle ◽  
Glacial Cycles ◽  
Temperature Conditions ◽  
Strongly Damped

A high-resolution, three-dimensional thermomechanical ice-sheet model, which includes isostasy, the possibility of ice-sheet expansion on the continental shelf and refined climatic parameterizations, was used to investigate the basal thermal regime of the Greenland ice sheet. The thermodynamic calculations take into account the usual terms of heat flow within the ice, a thermally active bedrock layer and all of the effects associated with changes in ice thickness and flow pattern. Basal temperature conditions are documented with respect to glacial–interracial shifts in climatic boundary conditions, both in steady state as during simulations over the last two glacial cycles using the GRIP δ180 record. It is found that the basal temperature field shows a large sensitivity in steady-state experiments but that, during a glacial cycle, basal temperature variations are strongly damped, in particular in central areas. A comparison has been made with measured data from deep ice cores and the implications are discussed.

scholarly journals Submarine melt as a potential trigger of the North East Greenland Ice Stream margin retreat during Marine Isotope Stage 3

2019 ◽  
Vol 13 (7) ◽  
pp. 1911-1923 ◽  
Author(s):  
Ilaria Tabone ◽  
Alexander Robinson ◽  
Jorge Alvarez-Solas ◽  
Marisa Montoya
Keyword(s):  
Three Dimensional ◽  
Ice Sheet ◽  
High Sensitivity ◽  
Marine Isotope Stage ◽  
Atmospheric Conditions ◽  
North East ◽  
Ice Stream ◽  
Grounding Line ◽  
Oceanic Forcing ◽  
Marine Isotope Stage 3

Abstract. The Northeast Greenland Ice Stream (NEGIS) has been suffering a significant ice mass loss during the last decades. This is partly due to increasing oceanic temperatures in the subpolar North Atlantic, which enhance submarine basal melting and mass discharge. This demonstrates the high sensitivity of this region to oceanic changes. In addition, a recent study suggested that the NEGIS grounding line was 20–40 km behind its present-day location for 15 ka during Marine Isotope Stage (MIS) 3. This is in contrast with Greenland temperature records indicating cold atmospheric conditions at that time, expected to favour ice-sheet expansion. To explain this anomalous retreat a combination of atmospheric and external forcings has been invoked. Yet, as the ocean is found to be a primary driver of the ongoing retreat of the NEGIS glaciers, the effect of past oceanic changes in their paleo evolution cannot be ruled out and should be explored in detail. Here we investigate the sensitivity of the NEGIS to the oceanic forcing during the last glacial period using a three-dimensional hybrid ice-sheet–shelf model. We find that a sufficiently high oceanic forcing could account for a NEGIS ice-margin retreat of several tens of kilometres, potentially explaining the recently proposed NEGIS grounding-line retreat during Marine Isotope Stage 3.

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