A two‐dimensional climate model which links the northern hemisphere atmosphere, ocean mixed layer, sea ice, and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock. The coupled model has been used to test the influence of several factors, including snow surface albedo over the ice sheets, in producing plausible ice age simulations using astronomically derived insolation and CO 2 data from the Vostok ice core. The impact of potentially important processes, such as the water vapor transport, clouds, and deep sea circulation, was not investigated in this study. After several sensitivity experiments designed to identify the main mechanisms governing surface temperature and ice accumulation, the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr. Large variations of ice volume are simulated between 122 and 55 kyr B.P., with a rapid latitudinal extension of the North American and Eurasian ice sheets starting at 120 kyr B.P. The simulated last glacial maximum is at 19 kyr B.P. The model is able to simulate deglaciation as well. The simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions. The major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations (linked with an underestimation of the ice sheet extent and an excess in the prescribed CO 2 concentration). Sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations. In the model a key mechanism in the deglaciation after the last glacial maximum appears to be the “aging” of snow, which decreases its albedo. The other factors which play an important role are, in decreasing level of importance, the ice sheet altitude, insolation, taiga cover, and ice sheet extent. A final set of experiments addresses the effects of CO 2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr. This last experiment is made by forcing the model with both insolation and CO 2 variations. This additional forcing improves the temperature and ice volume results. Despite the limitations inherent to the present modeling approach, the sensitivity experiments performed can provide insight into the relative importance of possible mechanisms responsible for the building and melting of huge ice sheets during the last glacial‐interglacial cycle.
Abstract A two-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their underlying bedrock has been used to test the Milankovitch theory over the last glacial—interglacial cycle. It was found that the orbital variations alone can induce, in such a system, feed-backs sufficient to generate the low-frequency part of the climatic variations over the last 122 kyear. These simulated variations at the astronomical time-scale are broadly in agreement with ice volume and sea-level reconstructions independently obtained from geological data. Imperfections in the simulated climate were the insufficient southward extent of the ice sheets and the too small hemispheric cooling during the last glacial maximum. These deficiencies were partly remedied in a further experiment (Gallée and others, in press) by using the time-dependent CO 2 atmospheric concentration given by the Vostok ice core in addition to the astronomical forcing. For this second experiment, the main mechanisms and feedbacks responsible for the glaciation and the deglaciation in the model are discussed here.
The influence of the starting date of model integration on time‐dependent projections of greenhouse‐gas‐induced climatic change is assessed with a coupled atmosphere‐ocean energy‐balance model. It is shown that the starting date effect depends on the rate of forcing change and on the oceanic thermal inertia. On the other hand, the model results indicate that, for the various cases considered here, reliable projections of green‐house‐gas‐induced climatic change over the next century can be obtained by starting the model integration as late as the year 1960.
A two-dimensional zonally averaged model has been developed for simulating the seasonal cycle of the climate of the Northern Hemisphere. The atmospheric component of the model is based on the two-level quasi-geostrophic potential vorticity system of equations. At the surface, the model has land—sea resolution and incorporates detailed snow and sea-ice mass budgets. The upper ocean is represented by an integral mixed-layer model that takes into account the meridional advection and turbulent diffusion of heat. Comparisons between the computed present-day climate and climatological data show that the model does reasonably well in simulating the seasonal cycle of the temperature field. In response to a projected CO 2 trend based on the scenario of Wuebbles et al. (DOE/ NBB-0066 Technical Report 15 (1984)), the modelled annual hemispheric mean surface temperature increases by 2 °C between 1983 and 2063. In the high latitudes, the response undergoes significant seasonal variations. The largest surface warmings occur during autumn and spring. The model is then asynchronously coupled to a model that simulates the dynamics of the Greenland, the Eurasian and the North American ice sheets in order to investigate the transient response of the climate to the long-term insolation anomalies caused by orbital perturbations. Over the last interglacial-glacial cycle, the coupled model produces continental ice-volume changes that are in general agreement with the low-frequency part of palaeoclimatic records.
Abstract A two-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their underlying bedrock has been used to test the Milankovitch theory over the last glacial—interglacial cycle. It was found that the orbital variations alone can induce, in such a system, feed-backs sufficient to generate the low-frequency part of the climatic variations over the last 122 kyear. These simulated variations at the astronomical time-scale are broadly in agreement with ice volume and sea-level reconstructions independently obtained from geological data. Imperfections in the simulated climate were the insufficient southward extent of the ice sheets and the too small hemispheric cooling during the last glacial maximum. These deficiencies were partly remedied in a further experiment (Gallée and others, in press) by using the time-dependent CO 2 atmospheric concentration given by the Vostok ice core in addition to the astronomical forcing. For this second experiment, the main mechanisms and feedbacks responsible for the glaciation and the deglaciation in the model are discussed here.
A two-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their underlying bedrock has been used to test the Milankovitch theory over the last two glacial-interglacial cycles. A series of sensitivity analyses have allowed us to understand better the internal mechanisms which drive the simulated climate system and in particular the feedbacks related to surface albedo and water vapour. It was found that orbital variations alone can induce, in such a system, feedbacks sufficient to generate the low frequency p art of the climatic variations over the last 122 ka. These simulated variations at the astronomical timescale are broadly in agreement with reconstructions of ice-sheet volume and of sea level independently obtained from geological data. Imperfections in the stimulated climate were the insufficient southward extent of the ice sheets and the too small hemispheric cooling at the last glacial maximum . These deficiencies were partly remedied in a further experiment by using the time-dependent atmospheric CO2 concentration given by the Vostok ice core in addition to the astronomical forcing. In this transient simulation, 70% of the Northern Hemisphere ice volume is related to the astronomical forcing and the related changes in the albedo, the rem aining 30% being due to the CO 2 changes. Analysis of the processes involved shows that variations of ablation are more important for the ice-sheet response than are variations of snow precipitation. A key mechanism in the deglaciation after the last glacial maxim um appears to be the ‘ageing’ of snow which significantly decreases its albedo. The other factors which play an important role are ice-sheet altitude, insolation, taiga cover, ice-albedo feedback, ice-sheet configuration (‘continentality’ and ‘desert’ effect), isostatic rebound, CO 2 changes and tem perature-water vapour feedback. Numerical experiments have also been carried out with a one-dimensional radiative-convective model in order to quantify the influence of the CO 2 changes and of the water vapour feedback on the climate evolution of the Northern Hemisphere over the last 122 ka. Results of these experiments indicate that 67% of the simulated cooling at the last glacial maximum can be attributed to the astronomical forcing and the subsequent surface albedo increase, the remaining 33% being associated with the reduced CO 2 concentration. Moreover, the water vapour feedback explains 40% of the simulated cooling in all the experiments done. The transient response of the clim ate system to both the astronomical and CO 2 forcing was also simulated by the LLN (Louvain-la-Neuve) 2.5-dimensional model over the two last glacial-interglacial cycles. It is particularly significant that spectral analysis of the simulated Northern Hemisphere global ice volume variations reproduces correctly the relative intensity of the peaks at the orbital frequencies. Except for variations with timescales shorter than 5 ka, the simulated long-term variations of total ice volume are comparable to that reconstructed from deep sea cores. For example, the model simulates glacial maxima of similar amplitudes at 134 ka BP and 15 ka BP, followed by abrupt deglaciations. The complete deglaciation of the three main Northern Hemisphere ice sheets, which is simulated around 122 ka BP, is in partial disagreement with reconstructions indicating that the Greenland ice sheet survived during the Eemian interglacial. The continental ice volume variations during the last 122 ka of the 200 ka simulation are, however, not significantly affected by this shortcoming.
