Modelling the northern hemisphere and Antarctic ice sheet changes through the last glacial cycle

In order to gain a proper understanding of the present state of the Earth's cryosphere in relation to its environment, it is desirable to have a sound knowledge of the glacial history which led to the present conditions. In this study, the ice sheets of the northern hemisphere and the Antarctic are modelled through the last glacial cycle in order to gain some understanding of both present and palaeo environments. The results of the energy balance model of Budd and Rayner (1993), which was driven by orbital variations in radiation as well as internal climate and ice sheet feedbacks, provides the shape of the time series of primary climate forcing for global ice sheet modelling. The amplitude of the climate forcing comes from matching palaeo-reconstructions of ice margins, sea level, temperature and accumulation rate. The ice sheet model is coupled to an ice shelf model and includes isostatic adjustment of the bedrock as well as simple thermodynamics. Results for the northern hemisphere modelling indicated that forcing with a maximum temperature change of about -13°C at the southern margin of the ice sheets leads to ice sheet extents in general agreement with palaeo-observations. This temperature change is representative of the ablation region over the ice margin where there is a sharp contrast between temperature changes over land and the much larger changes over the interior of the ice sheet. Ice sheet extent is also found to be dependent on the initial bed topography, but is found to be weakly dependent on changes to the sea level and the base accumulation distribution outside the ice sheet 'elevation-desert' effect. The existence and extent of a large ice shelf in the Arctic Ocean however, is found to be strongly dependent on the net accumulation rate. An estimate of eustatic sea level change through the last glacial cycle based on the modelling of the northern hemisphere ice sheets also agrees reasonably well with other sea level change reconstructions. The timing and extent of the 'fresh water pulse' from the maximum rate of deglaciation exhibits some correspondence with the initiation of the Younger Dryas cooling episode. The Antarctic is shown to be sensitive to changes in sea level and accumulation rate, and it is found that these influences have largely opposing effects. While the sea level change strongly influences ice thickness in regions of significant basal sliding, variation in the accumulation rate offsets this effect and there is little modelled grounding line movement during the glacial cycle. This leads to only a small net contribution to post glacial maximum sea level change from the Antarctic, in the order of 1m. The West Antarctic region is very sensitive to the sliding parameterisation and grounding line dynamics and consequently the results are not as robust as those for East Antarctica. The derived present state of the ice sheet shows that in central East Antarctica, there is ~20% positive mass balance still remaining, whereas nearer the coast the ice sheet is closer to balance. It is concluded that prior to the onset of recent anthropogenic influences, the net Antarctic mass balance was near to zero with only a very small positive net balance contributing slightly towards sea level lowering.