This archive provides the GRISLI ice sheet model outputs as part of the manuscript "Deglacial ice sheet instabilities induced by proglacial lakes". Contact: aurelien.quiquet@lsce.ipsl.fr
Abstract. Understanding the ocean circulation changes associated with last glacial abrupt climate events is key to better assess climate variability and understand its different natural modes. Sedimentary Pa / Th, benthic δ13C and Δ14C are common proxies used to reconstruct past circulation flow rate and ventilation. To overcome the limitations of each proxy taken separately, a better approach is to produce multi-proxy measurements on a single sediment core. Yet, different proxies can provide conflicting information about past ocean circulation. Thus, modelling them in a consistent physical framework has become necessary to assess the geographical pattern, the timing and sequence of the multi-proxy response to abrupt circulation changes. We have implemented a representation of the 231Pa and 230Th tracers into the model of intermediate complexity iLOVECLIM, which already included δ13C and Δ14C. We have further evaluated the response of these three ocean circulation proxies to a classical abrupt circulation reduction obtained by freshwater addition in the Nordic seas under preindustrial boundary conditions. Without a priori guess, the proxy response is shown to cluster in modes that resemble the modern Atlantic water masses. The clearest and most coherent response is obtained in the deep (> 2,000 m) Northwest Atlantic, where δ13C and Δ14C significantly decrease while Pa / Th increases. This is consistent with observational data across millennial scale events of the last glacial. Interestingly, while in marine records, except in rare instances, the phase relationship between these proxies remains unclear due to large dating uncertainties, in the model the bottom water carbon isotopes (δ13C and Δ14C) response lags the sedimentary Pa / Th response by a few hundred years.
Abstract. Predicting the climate for the future and how it will impact ice sheet evolution requires coupling ice sheet models with climate models. However, before we attempt to develop a realistic coupled setup, we propose, in this study, to first analyse the impact of a model simulated climate on an ice sheet. We undertake this exercise for a set of regional and global climate models. Modelled near surface air temperature and precipitation are provided as upper boundary conditions to the GRISLI (GRenoble Ice Shelf and Land Ice model) hybrid ice sheet model (ISM) in its Greenland configuration. After 20 kyrs of simulation, the resulting ice sheets highlight the differences between the climate models. While modelled ice sheet sizes are generally comparable to the observed one, there are considerable deviations among the ice sheets on regional scales. These deviations can be explained by biases in temperature and precipitation near the coast. This is especially true in the case of global models. But the deviations between the climate models are also due to the differences in the atmospheric general circulation. To account for these differences in the context of coupling ice sheet models with climate models, we conclude that appropriate downscaling methods will be needed. In some cases, systematic corrections of the climatic variables at the interface may be required to obtain realistic results for the Greenland ice sheet (GIS).
Abstract. In this paper, we present the inclusion of an online dynamical downscaling of heat and moisture within the model of intermediate complexity iLOVECLIM v1.1. We describe the followed methodology to generate temperature and precipitation fields on a 40 km × 40 km Cartesian grid of the Northern Hemisphere from the T21 native atmospheric model grid. Our scheme is non grid-specific and conserves energy and moisture. We show that we are able to generate a high resolution field which presents a spatial variability in better agreement with the observations compared to the standard model. Whilst the large-scale model biases are not corrected, for selected model parameters, the downscaling can induce a better overall performance compared to the standard version on both the high-resolution grid and on the native grid. Foreseen applications of this new model feature includes ice sheet model coupling and high-resolution land surface model.
Ice cores are unique archives capturing records of past temperature (through the ice isotopic composition, e.g. δD) and past atmosphere composition over the last 800 kyr. In particular, their analysis revealed that glacial-interglacial transitions, altering the Earth's climate since the beginning of the Quaternary, are associated with significant variations in the atmospheric levels of CO2 and CH4. However, comparison of past temperatures imprinted in ice-phase and atmospheric composition records imprinted in the air-phase is difficult. Indeed, the air is trapped at a depth of 50-100 m, at the bottom of the firn, where snow transforms into ice. Therefore, at a given depth, the air is always younger than the ice and firn densification modeling is needed to estimate the age difference between the air and the ice at each level. Firn densification modeling is associated with large uncertainties when it is applied to low accumulation and low temperature drilling sites of the East Antarctic plateau. An alternative approach to reconstruct air temperature directly in the air bubbles involves analyzing the isotopic composition of N2 (δ15N). Indeed, local temperature and accumulation rate evolutions affect firn thickness and hence modulated the δ15N in air bubbles trapped at the bottom of the firn via gravitational enrichment of δ15N over large glacial-interglacial transition on the East Antarctic plateau. The observation of a robust correlation between ice core records of δ15N and δD (Dreyfus et al., 2010) confirms the strong influence of local climate on the δ15N. δ15N measurements have already been applied to determine the phasing between CO2 and temperature increases over Antarctic temperature increase associated with glacial terminations. However, this strong relationship between δ15N and δD is not necessarily valid outside of glacial terminations. Here, we address the question to what extent the δ15N can be used to infer past temperatures and to study the CO2-temperature relationship, hence circumventing age uncertainties that arise when comparing ice and gas phase measurements. We first examine the δ15N record from EPICA Dome C with respect to East Antarctic climate over the last eight glacial-interglacial cycles. We use the good agreement between δD and δ15N over Termination II as a satisfactory criterion to discern when the δ15N is a reliable proxy of past temperature. Using this criterion, we assert that the correlation between δ15N and δD is robust over the past eight terminations. Focusing on the 100-300 ka BP period, we note also three intervals characterized by a weak correlation: the glacial inceptions from MIS 7e to 7d and 7a to 6e, and the MIS 6 glacial period. To explain why δ15N and δD evolutions contrast over these periods, we connect water stable isotopes with new δ15N measurements from EDC ice core and explore various snow densification scenarios yielded by a firn model under different climate conditions at the ice sheet surface. Our study permits to identify a criterion to safely use δ15N as an indicator of the past temperature in the air bubbles of the EDC ice core to study the CO2-local temperature relationship.
Abstract. We apply a new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR regional climate model (Edwards et al., 2014) to projections of future climate change using five ice sheet models (ISMs). The MAR (Modèle Atmosphérique Régional: Fettweis, 2007) climate projections are for 2000–2199, forced by the ECHAM5 and HadCM3 global climate models (GCMs) under the SRES A1B emissions scenario. The additional sea level contribution due to the SMB–elevation feedback averaged over five ISM projections for ECHAM5 and three for HadCM3 is 4.3% (best estimate; 95% credibility interval 1.8–6.9%) at 2100, and 9.6% (best estimate; 95% credibility interval 3.6–16.0%) at 2200. In all results the elevation feedback is significantly positive, amplifying the GrIS sea level contribution relative to the MAR projections in which the ice sheet topography is fixed: the lower bounds of our 95% credibility intervals (CIs) for sea level contributions are larger than the "no feedback" case for all ISMs and GCMs. Our method is novel in sea level projections because we propagate three types of modelling uncertainty – GCM and ISM structural uncertainties, and elevation feedback parameterisation uncertainty – along the causal chain, from SRES scenario to sea level, within a coherent experimental design and statistical framework. The relative contributions to uncertainty depend on the timescale of interest. At 2100, the GCM uncertainty is largest, but by 2200 both the ISM and parameterisation uncertainties are larger. We also perform a perturbed parameter ensemble with one ISM to estimate the shape of the projected sea level probability distribution; our results indicate that the probability density is slightly skewed towards higher sea level contributions.
Abstract. During the last deglaciation, the climate evolves from a cold state at the Last Glacial Maximum (LGM) at 21 ka (thousand years ago) with large ice sheets to the warm Holocene at ∼9 ka with reduced ice sheets. The deglacial ice sheet melt can impact the climate through multiple ways: changes of topography and albedo, bathymetry and coastlines, and freshwater fluxes (FWFs). In the PMIP4 (Paleoclimate Modelling Intercomparison Project – Phase 4) protocol for deglacial simulations, these changes can be accounted for or not depending on the modelling group choices. In addition, two ice sheet reconstructions are available (ICE-6G_C and GLAC-1D). In this study, we evaluate all these effects related to ice sheet changes on the climate using the iLOVECLIM model of intermediate complexity. We show that the two reconstructions yield the same warming to a first order but with a different amplitude (global mean temperature of 3.9 ∘C with ICE-6G_C and 3.8 ∘C with GLAC-1D) and evolution. We obtain a stalling of temperature rise during the Antarctic Cold Reversal (ACR, from ∼14 to ∼12 ka) similar to proxy data only with the GLAC-1D ice sheet reconstruction. Accounting for changes in bathymetry in the simulations results in a cooling due to a larger sea ice extent and higher surface albedo. Finally, freshwater fluxes result in Atlantic meridional overturning circulation (AMOC) drawdown, but the timing in the simulations disagrees with proxy data of ocean circulation changes. This questions the causal link between reconstructed freshwater fluxes from ice sheet melt and recorded AMOC weakening.
Abstract. Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and inform on the mass loss that would contribute to future sea level rise. However, there is currently no consensus on estimated the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes and the forcings employed. This study presents results from 18 simulations from 15 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015–2100, forced with different scenarios from the Coupled Model Intercomparison Project Phase 5 (CMIP5) representative of the spread in climate model results. The contribution of the Antarctic ice sheet in response to increased warming during this period varies between −7.8 and 30.0 cm of Sea Level Equivalent (SLE). The evolution of the West Antarctic Ice Sheet varies widely among models, with an overall mass loss up to 21.0 cm SLE in response to changes in oceanic conditions. East Antarctica mass change varies between −6.5 and 16.5 cm SLE, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional mass loss of 8 mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 AOGCMs show an overall mass loss of 10 mm SLE compared to simulations done under present-day conditions, with limited mass gain in East Antarctica.
