Climate variability and ice-sheet dynamics during the last three glaciations

Antarctic ice streams are associated with pressurized subglacial meltwater but the role this water plays in the dynamics of the streams is not known. To address this, we present a model of subglacial water flow below ice sheets, and particularly below ice streams. The base-level flow is fed by subglacial melting and is presumed to take the form of a rough-bedded film, in which the ice is supported by larger clasts, but there is a millimetric water film which submerges the smaller particles. A model for the film is given by two coupled partial differential equations, representing mass conservation of water and ice closure. We assume that there is no sediment transport and solve for water film depth and effective pressure. This is coupled to a vertically integrated, higher order model for ice-sheet dynamics. If there is a sufficiently small amount of meltwater produced (e.g. if ice flux is low), the distributed film and ice sheet are stable, whereas for larger amounts of melt the ice–water system can become unstable, and ice streams form spontaneously as a consequence. We show that this can be explained in terms of a multi-valued sliding law, which arises from a simplified, one-dimensional analysis of the coupled model.

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A century of climate variability and climate gradients from coast to ice sheet in west greenland

Geografiska Annaler Series A Physical Geography ◽

10.1111/j.0435-3676.2004.00226.x ◽

2004 ◽

Vol 86 (2) ◽

pp. 217-224 ◽

Cited By ~ 14

Author(s):

Andrea Taurisano ◽

Carl Egede Billionøggild ◽

Håkon Gjessing Karlsen

Keyword(s):

Climate Variability ◽

Ice Sheet ◽

West Greenland ◽

Climate Gradients

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Greenland land-terminating glaciers velocity trends during the last two decades

10.5194/egusphere-egu21-2084 ◽

2021 ◽

Author(s):

Paul Halas ◽

Jeremie Mouginot ◽

Basile de Fleurian ◽

Petra Langebroek

Keyword(s):

Water Pressure ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Melting ◽

Limited Area ◽

Ice Dynamics ◽

Basal Sliding ◽

Surface Melt ◽

Ice Sheet Dynamics ◽

The Impact

<div> <p>Ice losses from the Greenland Ice Sheet have been increasing in the last two decades, leading to a larger contribution to the global sea level rise.&#160;Roughly 40% of the contribution comes from ice-sheet dynamics, mainly regulated by basal sliding.&#160;The sliding component&#160;of glaciers has been observed to be strongly related to surface melting, as water&#160;can eventually&#160;reach the bed and impact&#160;the subglacial water pressure, affecting the basal sliding.&#160;&#160;</p> </div><div> <p>The link between ice velocities and surface melt on multi-annual time scale is still not totally understood&#160;even though it&#160;is of major importance with expected increasing surface melting.&#160;Several studies showed&#160;some&#160;correlation between an increase in surface melt and a slowdown in&#160;velocities, but&#160;there is no&#160;consensus&#160;on those trends.&#160;Moreover&#160;those&#160;investigations&#160;only&#160;presented results&#160;in a limited area over&#160;Southwest&#160;Greenland.&#160;&#160;</p> </div><div> <p>Here we present the ice motion over many land-terminating glaciers on the Greenland Ice Sheet for the period 2000 - 2020. This type of glacier is ideal for studying processes at the interface between the bed and the ice since they are exempted from interactions with the sea while still being relevant for all glaciers since they share the same basal friction laws. The velocity data was obtained using optical Landsat 7 & 8 imagery and feature-tracking algorithm. We attached importance keeping the starting date of our image pairs similar, and avoided stacking pairs starting before and after melt seasons, resulting in multiple velocity products for each year.&#160;&#160;</p> </div><div> <p>Our results show similar velocity trends for previously studied areas with a slowdown until 2012 followed by an acceleration.&#160;This trend however does not seem to be observed on the whole ice sheet and is probably&#160;specific&#160;to&#160;this region&#8217;s&#160;climate forcing.&#160;</p> </div><div> <p>Moreover comparison between ice velocities from different parts of Greenland allows us to observe the impact of different climatic trends on ice dynamics.</p> </div>

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Implications for the lithospheric structure of Greenland by applying different heat flow models

10.5194/egusphere-egu21-10689 ◽

2021 ◽

Author(s):

Agnes Wansing ◽

Jörg Ebbing ◽

Mareen Lösing ◽

Sergei Lebedev ◽

Nicolas Celli ◽

...

Keyword(s):

Heat Flow ◽

Heat Dissipation ◽

Ice Sheet ◽

Magnetic Data ◽

Similarity Function ◽

Lithospheric Structure ◽

Temperature Structure ◽

Geothermal Heat ◽

Flow Models ◽

Ice Sheet Dynamics

<p>The lithospheric structure of Greenland is still poorly known due to its thick ice sheet, the sparseness of seismological stations, and the limitation of geological outcrops near coastal areas. As only a few geothermal measurements are available for Greenland, one must rely on geophysical models. Such models of Moho and LAB depths and sub-ice geothermal heat-flow vary largely.</p><p>Our approach is to model the lithospheric architecture by geophysical-petrological modelling with LitMod3D. The model is built to reproduce gravity observations, the observed elevation with isostasy assumptions and the velocities from a tomography model. Furthermore, we adjust the thermal parameters and the temperature structure of the model to agree with different geothermal heat flow models. We use three different heat flow models, one from machine learning, one from a spectral analysis of magnetic data and another one which is compiled from a similarity study with tomography data.</p><p>For the latter, a new shear wave tomography model of Greenland is used. Vs-depth profiles from Greenland are compared with velocity profiles from the US Array, where a statistical link between Vs profiles and surface heat flow has been established. A similarity function determines the most similar areas in the U.S. and assigns the mean heat-flow from these areas to the corresponding area in Greenland.</p><p>The geothermal heat flow models will be further used to discuss the influence on ice sheet dynamics by comparison to friction heat and viscous heat dissipation from surface meltwater.</p>

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Natural variability and recent warming in central Greenland ice cores

10.5194/egusphere-egu21-12914 ◽

2021 ◽

Author(s):

Maria Hoerhold ◽

Thomas Münch ◽

Stefanie Weißbach ◽

Sepp Kipfstuhl ◽

Bo Vinther ◽

...

Keyword(s):

Climate Variability ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ice Cores ◽

Natural Variability ◽

The Arctic ◽

Last Millennium ◽

Recent Warming ◽

Temperature Reconstructions ◽

Firn Core

<p>Climate variability of the Arctic region has been investigated by means of temperature reconstructions based on proxies from various climate archives around the Arctic, compiled over the last 2000a in the so called Arctic2k record. However, the representativeness of the Arctic2k reconstruction for central Greenland remains unclear, since only a few ice cores have been included in the reconstruction, and observations from the Greenland Ice Sheet (GIC) report ambiguous warming trends for the end of the 20th and the beginning of the 21st century which are not displayed by Arctic2k. Today, the GIC experiences periods with temperatures close to or above the freezing point at high elevations, area-wide melting and mass loss. In order to assess the recent warming as signature of global climate change, records of past climate changes with appropriate temporal and spatial coverage can serve as a benchmark for naturally driven climate variability. Instrumental records for Greenland are short and geographically sparse, and existing temperature reconstructions from single ice cores are noisy, leading to an inconclusive assessment of the recent warming for Greenland.</p><p>Here, we provide a Greenland firn-core stack covering the time span of the last millennium until the first decade of the 21<sup>st </sup>century in unprecedented quality by re-drilling as well as analyzing 16 existing firn core sites. We find a strong decadal to bi-decadal natural variability in the record, and, while the record exhibits several warming events with trends that show a similar amplitude as the recent one, we find that the recent absolute values of stable oxygen isotope composition are unprecedented for the last 1000 years.</p><p>&#160;</p><p>Comparing our Greenland record with the Arctic 2k temperature reconstruction shows that the correlation between the two records changes throughout the last millennium. While in the periods of 1200-1300 and 1400-1650 CE the records correlate positively, between 1300 and 1400 and 1650-1700 CE shorter periods with negative correlation are found. Since then the correlation is characterized by alternation between positive and zero correlation, with a drop towards negative values at the end of the 20<sup>th</sup> century. Including re-analysis data, we hypothesize that the climate on top of the GIC was decoupled from the surrounding Arctic for the last decades, leading to the observed mismatch in observations of warming trends.</p><p>We suggest that the recently observed Greenland temperatures are a superposition of a strong natural variability with an anthropogenic long-term trend. Our findings illustrate that global warming has reached the interior of the Greenland ice sheet, which will have implications for its surface mass balance and Greenland&#8217;s future contribution to sea level rise.</p><p>Our record complements the Arctic 2k record to a profound view on the Arctic climate variability, where regional compilations may not be representative for specific areas.</p>

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SHaKTI: Subglacial Hydrology and Kinetic Transient Interactions v1.0

10.5194/gmd-2018-58 ◽

2018 ◽

Author(s):

Aleah Sommers ◽

Harihar Rajaram ◽

Mathieu Morlighem

Keyword(s):

Water Pressure ◽

Ice Sheet ◽

Governing Equations ◽

Model Formulation ◽

Seasonal Behavior ◽

Wide Transition ◽

Transient Interactions ◽

Ice Sheet Dynamics ◽

Hydrology Modeling ◽

Single Set

Abstract. Subglacial hydrology has a significant influence on ice sheet dynamics, yet remains poorly understood. Complex feedbacks play out between the liquid water and the ice, with constantly changing drainage geometry and flow mechanics. A clear tradition has been established in the subglacial hydrology modeling literature of distinguishing between channelized (efficient) and distributed (inefficient) drainage systems or components. Imposing a distinction that changes the governing physics under different flow regimes, however, may not allow for the full array of drainage characteristics to arise. Here, we present a new subglacial hydrology model: SHaKTI (Subglacial Hydrology and Kinetic Transient Interactions). In this model formulation, a single set of governing equations is applied over the entire domain, with a spatially and temporally varying transmissivity that allows for representation of the wide transition between turbulent and laminar flow, and the geometry of each element is allowed to evolve accordingly to form sheet and channel configurations. The model is implemented as a solution in the Ice Sheet System Model (ISSM). We include steady and transient examples to demonstrate features and capabilities of the model, and we are able to reproduce seasonal behavior of the subglacial water pressure that is consistent with observed seasonal velocity behavior in many Greenland outlet glaciers, supporting the notion that subglacial hydrology may be a key influencer in shaping these patterns.

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A new approach for simulating the paleo-evolution of the Northern Hemisphere ice sheets

Geoscientific Model Development ◽

10.5194/gmd-11-2299-2018 ◽

2018 ◽

Vol 11 (6) ◽

pp. 2299-2314 ◽

Cited By ~ 5

Author(s):

Rubén Banderas ◽

Jorge Alvarez-Solas ◽

Alexander Robinson ◽

Marisa Montoya

Keyword(s):

Climate Variability ◽

Ice Core ◽

Ice Sheets ◽

Ice Sheet ◽

Temperature Reconstruction ◽

Anomaly Field ◽

Temperature Anomalies ◽

Ice Volume ◽

Index Approach ◽

Millennial Scale

Abstract. Offline forcing methods for ice-sheet models often make use of an index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial–interglacial climatic anomaly field, interpolated through an index derived from the Greenland ice-core temperature reconstruction, with present-day climatologies. An important drawback of this approach is that it clearly misrepresents climate variability at millennial timescales. The reason for this is that the spatial glacial–interglacial anomaly field used is associated with orbital climatic variations, while it is scaled following the characteristic time evolution of the index, which includes orbital and millennial-scale climate variability. The spatial patterns of orbital and millennial variability are clearly not the same, as indicated by a wealth of models and data. As a result, this method can be expected to lead to a misrepresentation of climate variability and thus of the past evolution of Northern Hemisphere (NH) ice sheets. Here we illustrate the problems derived from this approach and propose a new offline climate forcing method that attempts to better represent the characteristic pattern of millennial-scale climate variability by including an additional spatial anomaly field associated with this timescale. To this end, three different synthetic transient forcing climatologies are developed for the past 120 kyr following a perturbative approach and are applied to an ice-sheet model. The impact of the climatologies on the paleo-evolution of the NH ice sheets is evaluated. The first method follows the usual index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial–interglacial climatic anomaly field, interpolated through an index derived from ice-core data, with present-day climatologies. In the second approach the representation of millennial-scale climate variability is improved by incorporating a simulated stadial–interstadial anomaly field. The third is a refinement of the second one in which the amplitudes of both orbital and millennial-scale variations are tuned to provide perfect agreement with a recently published absolute temperature reconstruction over Greenland. The comparison of the three climate forcing methods highlights the tendency of the usual index approach to overestimate the temperature variability over North America and Eurasia at millennial timescales. This leads to a relatively high NH ice-volume variability on these timescales. Through enhanced ablation, this results in too low an ice volume throughout the last glacial period (LGP), below or at the lower end of the uncertainty range of estimations. Improving the representation of millennial-scale variability alone yields an important increase in ice volume in all NH ice sheets but especially in the Fennoscandian Ice Sheet (FIS). Optimizing the amplitude of the temperature anomalies to match the Greenland reconstruction results in a further increase in the simulated ice-sheet volume throughout the LGP. Our new method provides a more realistic representation of orbital and millennial-scale climate variability and improves the transient forcing of ice sheets during the LGP. Interestingly, our new approach underestimates ice-volume variations on millennial timescales as indicated by sea-level records. This suggests that either the origin of the latter is not the NH or that processes not represented in our study, notably variations in oceanic conditions, need to be invoked to explain millennial-scale ice-volume fluctuations. We finally provide here both our derived climate evolution of the LGP using the three methods as well as the resulting ice-sheet configurations. These could be of interest for future studies dealing with the atmospheric or/and oceanic consequences of transient ice-sheet evolution throughout the LGP and as a source of climate input to other ice-sheet models.

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