Reinhard Calov

Results from the EISMINT model intercomparison: the effects of thermomechanical coupling

This paper discusses results from the second phase of the European Ice sheet Modelling Initiative (EISMINT). It reports the intercompartison of ten operational ice-sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour. A schematic, circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate. The major finding is that radial symmetry implied in the experimental design can, under certain circumstances, break down with the formation of distinct, regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt. These features also manifest themselves in the thickness and velocity distributions predicted by the models. They appear to be a common feature to all of the models which took part in the intercomparison, and may stem from interactions between ice temperature, flow and surface form. The exact nature of these features varies between models, and their existence appears to be controlled by the overall thermal regimne of the ice sheet. A second result is that there is considerable agreement between the models in their predictions of global-scale response to imposed climate change.

Large-scale instabilities of the Laurentide ice sheet simulated in a fully coupled climate-system model

[1] Heinrich events, related to large-scale surges of the Laurentide ice sheet, represent one of the most dramatic types of abrupt climate change occurring during the last glacial. Here, using a coupled atmosphere-ocean-biosphereice sheet model, we simulate quasi-periodic large-scale surges from the Laurentide ice sheet. The average time between simulated events is about 7,000 yrs, while the surging phase of each event lasts only several hundred years, with a total ice volume discharge corresponding to 5–10 m of sea level rise. In our model the simulated ice surges represent internal oscillations of the ice sheet. At the same time, our results suggest the possibility of a synchronization between instabilities of different ice sheets, as indicated in paleoclimate records. INDEX TERMS: 1827 Hydrology: Glaciology (1863); 1620 Global Change: Climate dynamics (3309); 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 5416 Planetology: Solid Surface Planets: Glaciation. Citation: Calov, R., A. Ganopolski, V. Petoukhov, M. Claussen, and R. Greve, Largescale instabilities of the Laurentide ice sheet simulated in a fully coupled climate-system model, Geophys. Res. Lett., 29(24), 2216, doi:10.1029/2002GL016078, 2002.

A semi-analytical solution for the positive degree-day model with stochastic temperature variations

Title A semi-analytical solution for the positive degree-day model with stochastic temperature variations Author(s) Calov, Reinhard; Greve, Ralf Citation Journal of Glaciology, 51(172), 173-175 https://doi.org/10.3189/172756505781829601 Issue Date 2005-01 Doc URL http://hdl.handle.net/2115/34536 Rights

Results From the Ice-Sheet Model Intercomparison Project-Heinrich Event INtercOmparison (ISMIP HEINO)

Results from the Heinrich Event INtercOmparison (HEINO) topic of the Ice-Sheet Model Intercomparison Project (ISMIP) are presented. ISMIP HEINO was designed to explore internal large- scale ice-sheet instabilities in different contemporary ice-sheet models. These instabilities are of interest because they are a possible cause of Heinrich events. A simplified geometry experiment reproduces the main characteristics of the Laurentide ice sheet, including the sedimented region over Hudson Bay and Hudson Strait. The model experiments include a standard run plus seven variations. Nine dynamic/thermodynamic ice-sheet models were investigated; one of these models contains a combination of the shallow-shelf (SSA) and shallow-ice approximation (SIA), while the remaining eight models are of SIA type only. Seven models, including the SIA-SSA model, exhibit oscillatory surges with a period of ∼1000 years for a broad range of parameters, while two models remain in a permanent state of streaming for most parameter settings. In a number of models, the oscillations disappear for high surface temperatures, strong snowfall and small sediment sliding parameters. In turn, low surface temperatures and low snowfall are favourable for the ice-surge cycles. We conclude that further improvement of ice-sheet models is crucial for adequate, robust simulations of cyclic large-scale instabilities.

Mechanisms and time scales of glacial inception simulated with an earth system model of intermediate complexity

Abstract. We investigate glacial inception and glacial thresholds in the climate-cryosphere system utilising the Earth system model of intermediate complexity CLIMBER-2, which includes modules for atmosphere, terrestrial vegetation, ocean and interactive ice sheets. The latter are described by the three-dimensional polythermal ice-sheet model SICOPOLIS. A bifurcation which represents glacial inception is analysed with two different model setups: one setup with dynamical ice-sheet model and another setup without it. The respective glacial thresholds differ in terms of maximum boreal summer insolation at 65° N (hereafter referred as Milankovitch forcing (MF)). The glacial threshold of the configuration without ice-sheet dynamics corresponds to a much lower value of MF compared to the full model. If MF attains values only slightly below the aforementioned threshold there is fast transient response. Depending on the value of MF relative to the glacial threshold, the transient response time of inland-ice volume in the model configuration with ice-sheet dynamics ranges from 10 000 to 100 000 years. Due to these long response times, a glacial threshold obtained in an equilibrium simulation is not directly applicable to the transient response of the climate-cryosphere system to time-dependent orbital forcing. It is demonstrated that in transient simulations just crossing of the glacial threshold does not imply large-scale glaciation of the Northern Hemisphere. We found that in transient simulations MF has to drop well below the glacial threshold determined in an equilibrium simulation to initiate glacial inception. Finally, we show that the asynchronous coupling between climate and inland-ice components allows one sufficient realistic simulation of glacial inception and, at the same time, a considerable reduction of computational costs.

Simulation of the Antarctic ice sheet with a three-dimensional polythermal ice-sheet model, in support of the EPICA project. II. Nested high-resolution treatment of Dronning Maud Land, Antarctica

Abstract The modern dynamic and thermodynamic state of the entire Antarctic ice sheet is computed for a 242 200 year paleoclimatic simulation with the three-dimensional polythermal ice-sheet model SICOPOLIS. The simulation is driven by a climate history derived from the Vostok ice core and the SPECMAP sea-level record. In a 872 km × 436 km region in western Dronning Maud Land (DML), where a deep ice core is planned for EPICA, new high-resolution ice-thickness data are used to compute an improved bedrock topography and a locally refined numerical grid is applied which extends earlier work (Calov and others, 1998). The computed fields of basal temperature, age and shear deformation, together with the measured accumulation rates, give valuable information for the selection of a drill site suitable for obtaining a high-resolution climate record for the last glacial cycle. Based on these results, a possible drill site at 73°59′ S, 00°00′ E is discussed, for which the computed depth profiles of temperature, age, velocity and shear deformation are presented. The geographic origin of the ice column at this position extends 320 km upstream and therefore does not leave the DML region.

Simulation of Glacial Cycles with an Earth System Model

It is generally accepted that, as postulated by the Milankovitch theory, variations of the Earth’s orbital parameters play a fundamental role in driving glacial cycles. However, many aspects of glacial climate variability, such as strongly nonlinear response of the ice sheets to orbital forcing and the role of carbon-dioxide climate ice-sheet feedback, still remain poorly understood. In recent years, it became increasingly clear that solving of the glacial cycle problem requires application of comprehensive Earth system models. Here we use the Earth system model of intermediate complexity CLIMBER-2 to simulate the last eight glacial cycles. The model was forced by variations of the Earth’s orbital parameters and atmospheric concentration of the major greenhouse gases. Simulated temporal dynamics of ice volume and other climate characteristics agree favorably with the paleoclimate reconstructions. Additional experiments performed with fixed concentrations of the greenhouse gases demonstrate that the 100-kiloyear cyclicity appears even in model simulations with constant greenhouse forcing as a direct and strongly nonlinear response to orbital variations. However, the simulated 100-kiloyear cyclicity is much weaker with constant CO2, which suggests that the carbon-dioxide climate ice-sheet feedback strongly amplifies the 100-kiloyear cycles. Our experiments also reveal the important role of eolian dust in shaping of glacial cycles and, especially, glacial terminations. Simulations with fully interactive carbon and dust cycle models are required for a better understanding of Quaternary climate dynamics.

MIS-11 duration key to disappearance of the Greenland ice sheet

Palaeo data suggest that Greenland must have been largely ice free during Marine Isotope Stage 11 (MIS-11). However, regional summer insolation anomalies were modest during this time compared to MIS-5e, when the Greenland ice sheet likely lost less volume. Thus it remains unclear how such conditions led to an almost complete disappearance of the ice sheet. Here we use transient climate–ice sheet simulations to simultaneously constrain estimates of regional temperature anomalies and Greenland’s contribution to the MIS-11 sea-level highstand. We find that Greenland contributed 6.1 m (3.9–7.0 m, 95% credible interval) to sea level, ∼7 kyr after the peak in regional summer temperature anomalies of 2.8 °C (2.1–3.4 °C). The moderate warming produced a mean rate of mass loss in sea-level equivalent of only around 0.4 m per kyr, which means the long duration of MIS-11 interglacial conditions around Greenland was a necessary condition for the ice sheet to disappear almost completely.

Modelling the end of an interglacial (MIS 1, 5, 7, 9, 11)

Abstract With the CLIMBER-2 model, the last glacial inception is simulated as a rapid ice-sheet expansion over northern North America, with only little ice sheets in Scandinavia. We present sensitivity studies that more deeply investigate the climatic feedbacks at the end of an interglacial. (i) An ice-free Greenland during the Eemian appears as a second stable state in the model. The initial size of the Greenland ice sheet, however, has only little effect on the subsequent glacial inception. (ii) North American glaciation can be reduced or even suppressed using preindustrial or Eemian vegetation and/or ocean surface conditions. (iii) Timing and amplitude of the last glacial inception cannot be estimated by the use of time-slice simulations. (iv) Changes in precession (perihelion) are crucial for the ice-sheet growth, obliquity and CO 2 merely act as amplifiers. (v) Cold events at the end of the interglacial can be reproduced by the introduction of freshwater disturbances into the North Atlantic. (vi) The model is able to simulate earlier glacial inceptions as well, although difficulties exist with respect to the strength of the glaciation. Future glacial inception in the model happens only in about 50 000 years from now.

Simulation of the future sea level contribution of Greenland with a new glacial system model

We introduce the coupled model of the Greenland glacial system IGLOO 1.0, including the polythermal ice sheet model SICOPOLIS (version 3.3) with hybrid dynamics, the model of basal hydrology HYDRO and a parameterization of submarine melt for marine-terminated outlet glaciers. The aim of this glacial system model is to gain a better understanding of the processes important for the future contribution of the Greenland ice sheet to sea level rise under future climate change scenarios. The ice sheet is initialized via a relaxation towards observed surface elevation, imposing the palaeo-surface temperature over the last glacial cycle. As a present-day reference, we use the 1961–1990 standard climatology derived from simulations of the regional atmosphere model MAR with ERA reanalysis boundary conditions. For the palaeo-part of the spin-up, we add the temperature anomaly derived from the GRIP ice core to the years 1961–1990 average surface temperature field. For our projections, we apply surface temperature and surface mass balance anomalies derived from RCP 4.5 and RCP 8.5 scenarios created by MAR with boundary conditions from simulations with three CMIP5 models. The hybrid ice sheet model is fully coupled with the model of basal hydrology. With this model and the MAR scenarios, we perform simulations to estimate the contribution of the Greenland ice sheet to future sea level rise until the end of the 21st and 23rd centuries. Further on, the impact of elevation–surface mass balance feedback, introduced via the MAR data, on future sea level rise is inspected. In our projections, we found the Greenland ice sheet to contribute between 1.9 and 13.0 cm to global sea level rise until the year 2100 and between 3.5 and 76.4 cm until the year 2300, including our simulated additional sea level rise due to elevation–surface mass balance feedback. Translated into additional sea level rise, the strength of this feedback in the year 2100 varies from 0.4 to 1.7 cm, and in the year 2300 it ranges from 1.7 to 21.8 cm. Additionally, taking the Helheim and Store glaciers as examples, we investigate the role of ocean warming and surface runoff change for the melting of outlet glaciers. It shows that ocean temperature and subglacial discharge are about equally important for the melting of the examined outlet glaciers.