Search

Recent unexpected changes in the Antarctic Ice Sheet, including ice sheet thinning, ice shelf collapse and changes in ice velocities, along with the recent realization that as much as one third of ice shelf mass loss is due to bottom melt, place a new urgency on understanding the processes involved in these changes. Technological advances, including very new or forthcoming satellite-based (e.g. ICESat, CryoSat) remote sensing missions, will improve our ability to make meaningful determinations of changes in Antarctic Ice Sheet mass balance.This paper is the result of a workshop held to develop a strategy for international collaboration aimed at the collection and synthesis of Antarctic Ice Sheet mass balance data, and at understanding the processes involved so that we might predict future change. Nine sets of recommendations are made, concerning the most important and sensitive measurements, temporal ranges and study areas. A final tenth recommendation calls for increased synthesis of ice sheet data and communication between the field measurement, satellite observation and modelling communities. (C) 2004 Published by Elsevier B.V.

A 3-dimensional thermomechanical ice-sheet model is used to simulate the evolution of the geometry of Northern Hemisphere ice sheets through the Last Deglaciation. The ice-sheet model is forced by a time-evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the LMD5.3 atmospheric general circulation model (AGCM) at different periods of the Last Deglaciation (21, 15, 9, 6 and 0 kyr BP). The AGCM is driven by insolation, atmospheric CO2 content, ice-sheet configuration and sea surface temperatures. Although our approach is able to produce the complete continental ice retreat, our simulated deglaciation presents a phase-lag with reconstructions based on observational evidences. This suggests that physical mechanisms related to climate forcing and/or ice-sheet internal dynamics are not properly represented. The influence of millennial-scale forcing, feedback mechanisms between ice-sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments. The rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature-mass balance relationships. Fennoscandia appears to be strongly sensitive to the small-scale ice-sheet instability. Both ice sheets are to some extent sensitive to an increased basal sliding.

Taking into account the low values of average density reported for Comet P/Halley, a model of a porous comet nucleus has been developed. The model includes heat conduction through the solid matrix and heat and mass transfers due to the vapor phase circulating in the pore system. Sublimation and recondensation of volatile species within the nucleus as well as water ice crystallization are represented according to experimental results on the evolution of ice mixtures with time and temperature. The model is based on the resolution of two symmetric diffusion equations through the whole nucleus, one describing the transport of matter and the other the transport of heat. These equations are linked by the source term. The sensible heat term due to gas advection is also integrated into the heat diffusion equation. The calculations are performed for comets on the orbits of P/Halley and P/Churyumov-Gerasimenko. Different nucleus compositions with ice mixtures containing H2O and CO or CO2 are investigated. The consequences of the amorphous to crystalline phase transition of water ice on the thermal profiles and on the evolution of the composition of the nucleus are studied. Results are presented on the evolution of the stratigraphy of the nucleus and of the production rates of CO, CO2, and H2O as a function of the heliocentric distance. Several phenomena are evidenced such as the depletion of volatiles in the subsurface layers and their enrichment in deeper layers. The important role of CO and CO2 in controlling the propagation of the phase transition is emphasized. The relation between the composition of the nucleus and gas production rates is discussed.

The study of ice cores provides the opportunity of access to numerous parameters which are related to climate or environmental changes. Many of these variables are of regional or even global geographical significance. There are now several ice cores which penetrate into ice which has formed at the surface of polar ice sheets during the last ice age and the recently obtained Vostok (Antarctica) record provides detailed information over the last climatic cycle (160 ka). In comparison with current Holocene conditions, the Last Glacial Maximum (20 ka BP) is characterized by much colder conditions (up to 10°C) and reduced precipitation (× 1 2 ). A large increase of continental and marine aerosols is explained by a more vigorous large scale atmospheric circulation associated with changing continental deserts and shelves as well as changes in sea ice extent. The drastic climatic change occurring at the last glacial termination is correlated with a large increase of atmospheric CO 2 (from about 200 to 270 ppmv). The isotopic temperature record from the Vostok ice core depicts two drastic glacial-interglacial terminations (around 15 and 140 ka BP) and a long glacial period (110-15 ka BP) which includes two interstadials. Full glacial stages are characterized by larger aerosol loadings. The CO 2 record of global significance is well correlated with the isotope temperature profile with high (270 ppmv) during interglacials and low (200 ppmv) during full glacial conditions. Spectral analysis of the Yostok temperature record supports the existence of a relationship between the Pleistocene climate and orbital forcing. On the other hand, the existence of a CO 2 -climate correlation suggests that CO 2 changes have had an important climatic role in amplifying the relatively weak orbital forcing.