Anne Letréguilly

The Greenland ice sheet and greenhouse warming

Increased melting on glaciers and ice sheets and rising sea level are often mentioned as important aspects of the anticipated greenhouse warming of the earths atmosphere. This paper deals with the sensitivity of Greenlands ice mass budget and presents a tentative projection of the Greenland component of future sea level rise for the next few hundred years. To do this, the ‘Villach II temperature scenario’ is prescribed,output from a comprehensive mass balance model is used to drive a high-resolution 3-D thermomechanic model of the ice sheet. The mass balance model consists of two parts: the accumulation part is based on presently observed values and is forced by changes in mean anr tempeerature. The ablation model is based on the degree-day method and accounts for daily and annual temperature cycle, a different degree-day factor for ice and snow melting and superimposed ice formation. Under present-day climatic conditions, the following total mass balance results (in ice equivalent per years): 599.3 × 109m3 of accumulation, 281.7 × 109m3 of runoff assuming a balanced budget, 317.6 × 109m3 of iceberg calving. A 1K uniform warming is then calculated to increase the runoff by 119.5 × 109m3. Since accumulation also increases by 32 × 109m3, this leads to reduction of the total mass balance by 887.5 × 109m3 of ice, corresponding to a sea level rise of 0.22 mm/yr. For temperature increase larger than 2.7 K, runoff, exceeds accumulation, and if ice sheet dynamics were to remain unchanged, this would add an extra amount of 0.8 mmyr to the worls oceans. Imposing the Villach II scenario (warming up to 4.23 K) and accumulating mass balance changes forward in time (static response) would then result in a global sea level rise of 7.1 cm by 2100 AD, but this figure may go up to as much as 40 cm per century in case the warming is doubled. In a subsequent dynamic model involving the ice flow, the ice sheet is found to produce a counteracting effect by dynamically producing steeper slopes at the margin, thereby reducing the area over which runoff can take place. This effect is particularly apparent in the northeastern part of the ice sheet, and is also more pronounced for the smaller temperature perturbations. Nevertheless, all these experiments certainly highlight the vulnerability of the Greenland ice sheet with respect to a climatic warming.

Steady-state characteristics of the Greenland ice sheet under different climates

The Greenland ice sheet is modelled to simulate its extent and volume in warmer climates, and also to find out whether the ice sheet would re-form on theice-free bedrock under present climatic conditions. The ice sheet model is a three-dimensional thermo-mechanical model with a fine resolution grid. Thebedrock surface beneath the ice sheet was mapped using radio-echo-sounding measurements by the Electromagnetic Institute, Copenhagen. The modelexperiments show that increased temperature will result in ice-margin retreat, but the ice sheet is relatively stable; it takes a rather high temperature rise of atleast 6iC for the ice sheet to disappear completely, which indicates that the ice sheet probably survived the last interglacial. Also, it appears that the Greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re-form on the bare bedrock under the present, or evenslightly warmer, climatic conditions.

Space and Time Distribution of Glacier Mass-Balance in the Northern Hemisphere

Long mass-balance series are compared within four areas of the Northern Hemisphere (The Alps, Scandinavia, the Tien Shan in the Soviet Union, and western North America), using Lliboutrys linear model. This model separates the mass-balance into two main additive terms, one of them depending on time only, and the other one being characteristic of the glacier situation. This study shows that for each mountain range the same type of spatio-temporal distribution pattern for the mass balance can be found that classifies the glaciers according to their distance from the main humidity source. Beyond the mountain range scale, this pattern can no longer be recognized. Available reconstructed mass-balance series for the century are then used. They show that secular trends compare well within Europe and the USSR, but not beyond the continent.

25 years (1981-2005) of equilibrium-line altitude and mass-balance reconstruction on Glacier Blanc, French Alps, using remote-sensing methods and meteorological data

Annual equilibrium-line altitude (ELA) and surface mass balance of Glacier Blanc, Ecrins region, French Alps, were reconstructed from a 25 year time series of satellite images (1981–2005). The remote-sensing method used was based on identification of the snowline, which is easy to discern on optical satellite images taken at the end of the ablation season. In addition, surface mass balances at the ELA were reconstructed for the same period using meteorological data from three nearby weather stations. A comparison of the two types of series reveals a correlation of r > 0.67 at the 0.01 level of significance. Furthermore, the surface mass balances obtained from remote-sensing data are consistent with those obtained from field measurements on five other French glaciers (r ¼ ¼ 0.76, p < 0.01). Also consistent for Glacier Blanc is the total mass loss (10.8mw.e.) over the studied period. However, the surface mass balances obtained with the remote-sensing method show lower interannual variability. Given that the remote-sensing method is based on changes in the ELA, this difference probably results from the lower sensitivity of the surface mass balance to climate parameters at the ELA.

A new, detailed ice-age oxygen-18 record from the ice-sheet margin in central West Greenland

Abstract A new detailed oxygen-18 record measured on surface-ice samples from a West Greenland ice-margin location reveals the hitherto longest climatic record from the Greenland ice sheet, spanning the last c. 150,000 years. The new record implies that the Greenland deep ice-core records from Dye3 and Camp Century need to be re-interpreted. A comparison with the deuterium record from the Vostok deep ice core, Antarctica indicates that climate behaved differently in the northern and southern hemispheres during the last glacial/interglacial cycle, with major differences occurring in Emiliani isotopic stage 5.

Bilans de masse des glaciers alpins et scandinaves, leurs relations avec l'oscillation du climat de l'Atlantique nord

This paper considers the relations between the North Atlantic Oscillation, reflecting the climate variation patterns on the North Atlantic region, and mass balance variations of Alpine and Scandinavian glaciers. This study first enhances the mass balance representativeness of climate variations on the massif scale. However, dealing with glaciers in the North and the South of Europe, the mass balance variations are inversely correlated. Analyses comparing the mass balance variations and the NAO index show that annual trends between these two components are really poor, but at the decadal scale, the two signals are well correlated.

A century reconstruction of the mass balance of Glacier de Sarennes, French Alps

The 50 year time series of mass balance on Glacier de Sarennes is one of the longest in the French Alps, and so is often used as a reference for glacier variations in the French Alps. Meteorological data can be used to extend the series backwards in time. Martin (1978) proposed such a reconstruction for the 1882-1977 period. With 50 years of observations, we show that the classical method used by Martin is too dependent on the calibration period. We therefore try to improve the accuracy of this reconstruction using the Vincent and Vallon (1997) method which takes into account the albedo change of the surface during the ablation period (this is called the daily method). This new method appears to be stable in time. Once calibrated, the daily method is applied to reconstruct the 1881-1949 period. The new reconstruction is compared to a volumetric balance between two maps from 1906 and 1981. It appears that both reconstructions (classical and daily) fail to render the trend correctly over a long period of time. The cumulative centred mass balance correlates well (r 2 = 0.62) with the hydrological mass-balance series of Aletschgletscher, Switzerland.

Greenland ice-sheet mass-balance distribution: a variance analysis of existing field data

A large number of mass-balance measurements have been carried out on the Greenland ice sheet, but few of the series obtained are well suited for statistical studies. Rather than looking at the global mass-balance value, this paper deals with the spatial and temporal mass-balance variability on the ice sheet. Two sorts of analysis are possible: direct comparisons of the series measured at a given site, or a broader approach involving comparisons between different sites using variance analysis. For glaciers in the southwest ablation area, a significant interannual variability (around 1.0 m w.e.) is found. This variability is spatially consistent. In the accumulation area, the results are more complex. For example, there is consistent evidence of year-to-year variations on the west-east Expeditions Glaciologiques Internationales au Groenland (EGIG) profile, but other closer sites are weakly correlated. These results emphasize the need for a better coverage of measurements over the entire ice sheet, as well as longer and more continuous measurement series.