Exploring the impact of atmospheric forcing and basal drag on the Antarctic Ice Sheet under Last Glacial Maximum conditions

Little is known about the distribution of ice in the Antarctic Ice Sheet (AIS) during the Last Glacial Maximum (LGM). Whereas marine and terrestrial geological data indicate that the grounded ice advanced to a position close to the continental-shelf break, the total ice volume is unclear. Glacial boundary conditions are potentially important sources of uncertainty, in particular basal friction and climatic boundary conditions. Basal friction exerts a strong control on the large-scale dynamics of the ice sheet and thus affects its size and is not well constrained. Glacial climatic boundary conditions determine the net accumulation and ice temperature and are also poorly known. Here we explore the effect of the uncertainty in both features on the total simulated ice storage of the AIS at the LGM. For this purpose we use a hybrid ice sheet shelf model that is forced with different basal drag choices and glacial background climatic conditions obtained from the LGM ensemble climate simulations of the third phase of the Paleoclimate Modelling Intercomparison Project (PMIP3). Overall, we find that the spread in the simulated ice volume for the tested basal drag parameterizations is about the same range as for the different general circulation model (GCM) forcings (4 to 6 m sea level equivalent). For a wide range of plausible basal friction configurations, the simulated ice dynamics vary widely but all simulations produce fully extended ice sheets towards the continental-shelf break. More dynamically active ice sheets correspond to lower ice volumes, while they remain consistent with the available constraints on ice extent. Thus, this work points to the possibility of an AIS with very active ice streams during the LGM. In addition, we find that the surface boundary temperature field plays a crucial role in determining the ice extent through its effect on viscosity. For ice sheets of a similar extent and comparable dynamics, we find that the precipitation field determines the total AIS volume. However, precipitation is highly uncertain. Climatic fields simulated by climate models show more precipitation in coastal regions than a spatially uniform anomaly, which can lead to larger ice volumes. Our results strongly support using these paleoclimatic fields to simulate and study the LGM and potentially other time periods like the last interglacial. However, their accuracy must be assessed as well, as differences between climate model forcing lead to a large spread in the simulated ice volume and extension

Time-scale synchronisation of oscillatory responses can lead to non-monotonous R-tipping

Rate-induced tipping (R-tipping) describes the fact that, for multistable dynamic systems, an abrupt transition can take place not only because of the forcing magnitude, but also because of the forcing rate. In the present work, we demonstrate through the case study of a piecewise-linear oscillator (PLO), that increasing the rate of forcing can make the system tip in some cases but might also prevent it from tipping in others. This counterintuitive effect is further called non-monotonous R-tipping (NMRT) and has already been observed in recent studies. We show that, in the present case, the reason for NMRT is the peak synchronisation of oscillatory responses operating on different time scales. We further illustrate that NMRT can be observed even in the presence of additive white noise of intermediate amplitude. Finally, NMRT is also observed on a van-der-Pol oscillator with an unstable limit cycle, suggesting that this effect is not limited to systems with a discontinuous right-hand side such as the PLO. This insight might be highly valuable, as the current research on tipping elements is shifting from an equilibrium to a dynamic perspective while using models of increasing complexity, in which NMRT might be observed but hard to understand

Description and validation of the ice-sheet model Yelmo (version 1.0)

We describe the physics and features of the ice-sheet model Yelmo, an open-source project intended for collaborative development. Yelmo is a thermomechanical model, solving for the coupled velocity and temperature solutions of an ice sheet simultaneously. The ice dynamics are currently treated via a “hybrid” approach combining the shallow-ice and shallow-shelf/shelfy-stream approximations, which makes Yelmo an apt choice for studying a wide variety of problems. Yelmo's main innovations lie in its flexible and user-friendly infrastructure, which promotes portability and facilitates long-term development. In particular, all physics subroutines have been designed to be self-contained, so that they can be easily ported from Yelmo to other models, or easily replaced by improved or alternate methods in the future. Furthermore, hard-coded model choices are eschewed, replaced instead with convenient parameter options that allow the model to be adapted easily to different contexts. We show results for different ice-sheet benchmark tests, and we illustrate Yelmo's performance for the Antarctic ice sheet

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

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 and oceanic consequences of transient ice-sheet evolution throughout the LGP and as a source of climate input to other ice-sheet models

Glacial Fennoscandian GRISLI-UCM simulations

Animations from GRISLI-UCM simulation

Changements abrupts et variabilité rapide dans différents contextes climatiques (une étude basée sur une stratégie de plusieurs modèles)

La période du Quaternaire est caractérisée par une alternance de périodes froides, les phases glaciaires, dans lesquelles deux calottes additionnelles se développent sur l'hémisphère Nord (en Amérique du Nord et en Eurasie), et des périodes relativement chaudes (similaires à l'Holocène) où les seules calottes de glace présentes à la surface du globe sont le Groenland et l'Antarctique. L'étude des carottes marines et continentales ainsi que celles de glace ont révélé l'existence d'une variabilité millénaire dans le système climatique du Quaternaire. Cette variabilité semble beaucoup plus importante lors des périodes glaciaires que lors des périodes interglaciaires, mais à l'heure actuelle il n'existe toujours pas un consensus total sur son origine. Premièrement, le Dansgaard-Oeschger sont caractérisés par de transitions abruptes (quelques décennies), mais d'un caractère pseudo-périodique d'une fréquence trop élevée pour être associée aux seules variations de l'insolation dues à des changements orbitaux. Deux types d'explications concernant le mécanisme déclencheur ont été proposées: un forçage périodique externe et des oscillations internes au système climatique dans lesquelles la circulation océanique est vraisemblablement impliquée. Quels sont exactement ces mécanismes ? Comment les différents sous-systemes climatiques interagissent entre eux ? Pourquoi cette variabilité se manifeste-t-elle plus fortement en période glaciaire ? Ces questions intrigantes, qui restent toujours d'actualité au sein de la communauté scientfiique, constituent l'objet principal de cette thèse. J'ai donc essayé au cours de ce travail d'apporter de nouvelles réponses plus cohérentes avec nos connaissances actuelles des différents sous-systèmes. En particulier, les épisodes de dépôts de matériel détritique qui ont marqué les enregistrements sédimentaires de l'Océan Nord-Atlantique glaciaire (connus comme les événements de Heinrich) sont toujours particulièrement durs à comprendre. Ces événements sont attribués à un relargage massif d'icebergs originairesdes calottes polaires de l'hémisphère Nord, mais le mécanisme permettant le développement de ces immenses flottes d'icebergs fait encore l'objet d'une controverse passionnante.Nous utilisons ici un modéle numérique conceptuel pour simuler les effets des changements de la circulation océanique sur la géometrie des plates-formes de glace flottantes. Nous analisons l'impacte de ces changements sur la dynamique des fleuves de glace et de la calotte posée. Nos résultats demontrent que des oscillations de la température océanique impactent la fusion basale sous les plates-formes et génèrent des débâcles périodiques d'icebergs vers la mer. Ce travail est ensuite focalisé sur l'événement de Heinrich 1. Grâce à des simulations effectués avec un modèle tri-dimensionelle des calottes polaires et un modèle climatique couplé océan-atmosphère, nous proposons un nouveau mécanisme déclencheur basé sur les effets d'un réchauffement océanique de subsurface sur la dynamique des fleuves de glace.Finalement, dans le contexte du climat actuel qui risque de se modifer considérablement du fait de la pression anthropique, une autre question surgit : est-il possible que la transition vers un climat plus chaud dans le futur provoque l'apparition d'une nouvelle variabilité rapide ? La dernière partie de cette thèse est dediée à cette thematique.Ice core data as well as marine and continental records reveal the existence of pronounced millennial time-scale variability in the Quaternary climate system. Such rapid climate variability appears to be stronger in glacial periods than during interglacials, but there is not yet a full consensus about its origin. Firstly, the Dansgaard-Oeschger events are characterized by abrupt transitions occurring in a few decades, and by a period of a few thousand years. Two types of explanation have been suggested concerning its triggering mechanism: periodic external forcing and internal oscillations in the climate system, for which ocean circulation is a likely candidate. On the other hand, six periods of extreme cooling in the Northern Hemisphere were marked by an enhanced discharge of icebergs into the North Atlantic Ocean, increasing the deposition of ice-rafted debris (known as Heinrich events). Increased sliding at the base of ice sheets as a result of basal warming has been proposed to explain the iceberg pulses, but recent observations suggest that iceberg discharge is related to a strong coupling between ice sheets, ice shelves and ocean conditions. In this work, I tried to bring new insights about the mechanisms responsible for the millennial glacial variability, more consistent with the present knowledge of the different Earth's components. This work is based on the use of a hierarchy of climate and ice-sheet models of different complexities. We used a conceptual numerical model to simulate the effect of ocean temperature on ice-shelf width, as well as the impact of the resulting changes in ice-shelf geometry on ice-stream velocities. Our results demonstrate that ocean temperature oscillations affect the basal melting of the ice shelf and will generate periodic pulses of iceberg discharge in an ice sheet with a fringing shelf. Using a state-of-the-art tri-dimensionnal ice-sheet model we also explore the conditions leading to internal oscillations of geometrically idealised ice sheets. We describe in detail the thermomechanical feedback responsible of the so-called binge-purge'' oscillations and we analyse the effects of ocean circulation changes on ice shelves and the dynamic implications resulting from a break-up of these ice shelves. Our studies are then focalised on the Heinrich event 1, showing a new mechanism based on the effects of a subsurface warming on the ice shelves stability. We demonstrate that such ice-shelf break-up and the subsequent ice-stream acceleration should be considered as a likely candidate to generate the icebergs surge implicated in Heinrich event 1. Leaving glacial period we finally focus on the present-day anthropically perturbed interglacial. We analyse with a fully coupled climate ice sheet model whether the shift into a warmer climate in the future could favor the occurrence of a new millennial-scale climate variability.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Rapid Climate Variability: Description and Mechanisms

International audienc

Holocene thinning in central Greenland controlled by the Northeast Greenland Ice Stream

Abstract Ice-core records from the interior of the Greenland ice sheet suggest widespread thinning during the Holocene. However, the recurring underestimation of this thinning in numerical models raises concerns about both the veracity of such reconstructions and the reliability of glaciological models. Recent work suggests the 8000-year-old Northeast Greenland Ice Stream (NEGIS), including a now-extinct northern tributary, may have been an early influence on Greenland ice-sheet dynamics. Yet, the inaccurate reproduction of NEGIS-like dynamics in most models hampers investigation of whether this feature played a role in Holocene ice-sheet thinning. Here we show that grounding-line retreat in northeast Greenland triggers elevation changes at the northern summit via ice-dynamic effects modulated by the paleo NEGIS system. In our simulations, fast ice-stream flow caused by transiently imposed reduced basal shear stress following the northeast retreat explains 55% ( ± 18%) of the estimated ice thinning, showing that ice-stream dynamics is one of the main drivers of the NGRIP Holocene surface elevation drop. Our findings show that the ice-flow in northeast Greenland plays a large role in ice-surface elevation changes in central Greenland

Time-scale synchronisation of oscillatory responses can lead to non-monotonous R-tipping

Rate-induced tipping (R-tipping) describes the fact that, for multistable dynamic systems, an abrupt transition can take place not only because of the forcing magnitude, but also because of the forcing rate. In the present work, we demonstrate through the case study of a piecewise-linear oscillator (PLO), that increasing the rate of forcing can make the system tip in some cases but might also prevent it from tipping in others. This counterintuitive effect is further called non-monotonous R-tipping (NMRT) and has already been observed in recent studies. We show that, in the present case, the reason for NMRT is the peak synchronisation of oscillatory responses operating on different time scales. We further illustrate that NMRT can be observed even in the presence of additive white noise of intermediate amplitude. Finally, NMRT is also observed on a van-der-Pol oscillator with an unstable limit cycle, suggesting that this effect is not limited to systems with a discontinuous right-hand side such as the PLO. This insight might be highly valuable, as the current research on tipping elements is shifting from an equilibrium to a dynamic perspective while using models of increasing complexity, in which NMRT might be observed but hard to understand.Te authors would like to thank Peter Ashwin for his valuable comments and his expertise on rate-induced tipping, as well as Johannes Lohmann for the constructive exchange of ideas and his helpful recommendations of articles. J.S.-J. and J.B. have received funding from the European Union’s Horizon 2020 research and innovation Programme, respectively under the Marie Skłodowska-Curie grant agreement no. 956170 and no. 820970. A. R. was supported by the Ramón y Cajal Programme of the Spanish Ministry for Science, Innovation and Universities (grant no. RYC-2016-20587). A. R. and J. A-S. are also supported by the Spanish Ministry of Science and Innovation project ICEAGE (grant no. PID2019-110714RA-I00).Peer reviewe