The future of mountain glaciers, small or major ice caps and their impact on sea level is the focus of much research into the terrestrial cryosphere. The more or less immediate objective of this research is to establish the mass balance of the 200,000 odd glaciers spread over the globe and the two major ice caps (Antarctica and Greenland). To do so, it is necessary to establish the mass balance year after year. Satellite remote sensing instruments play a vital role in these generally inaccessible regions. The data from altimetry missions such as laser ICESat, ESA radar missions and, more recently, AltiKa; from the GRACE gravity measurement mission (since 2002), interferometry missions such as ERS, ENVISAT and ALOS; or photogrammetry missions such as SPOT 5-HRS have helped flesh out our knowledge on these ice mass balances.
A second major objective is to elucidate the processes that explain variations in mass, whether due to the surface component (precipitation, thawing, sublimation or advection of snow by wind) or the dynamic part (calving of icebergs) because this is a prerequisite to numerical modelling and the estimation of future ice mass balances. This requires the use of satellite observations of variables involved in the surface energy budget such as the albedo (MODIS, MERIS and SPOT), surface temperature (MODIS) or surface thaws (AMSR-E and Sentinel-1a). One of the more difficult barriers to overcome is detecting solid precipitations. More generally, a better understanding of snow and weather processes would enable us to improve the characterisation of cryosphere/climate feedback, especially positive feedback linked to the snow’s albedo, mostly responsible for the cryosphere’s increased response to global warming. Satellite observations of the albedo, meteorological variables and atmospheric aerosols would advance knowledge in this domain, which contributes directly to the issues being investigated by the Intergovernmental Panel on Climate Change (IPCC).
Finally, the seasonal snow cover is another key focus of research, whether for hydrological or meteorological applications; to study the subsoil, vegetation, or climate; or to forecast the risk of avalanches, flooding or water resources in mountainous regions. Whatever the regions concerned–from vast boreal expanses to mountain valleys–it is vital to determine the thickness and mass of snow cover. Currently, spaceborne remote sensing instruments shed little light on this subject because limited to mapping snow-covered surfaces using wide-swath optical sensors (MSG-SEVIRI, AVHRR and VGT), high-resolution sensors (SPOT, LANDSAT, ASTER, Pleiades and Sentinel-2A), or passive microwaves on a continental scale (AMSR-E, SSM/I). In addition, ongoing work aims to better model changes in snow cover over time and could be a useful complement for indirectly estimating snow thickness. This work is based on observations of the snow cover’s internal properties (water content, density, grain characteristics, presence of ice etc.) in the field and from space using active radars (Radarsat-2, TerraSAR-X) or passive microwave radiometers.
Beyond the continental cryosphere, studies of sea ice come across the same research issues, such as the snow cover’s role in radiation.
Whatever object in the cryosphere is studied, progress still needs to be made in methodologies. Data assimilation, i.e. the optimal combination of satellite observations, rare but valuable in situ observations and descriptive models of the cryosphere appear to be the ultimate approach, benefitting from the best of each individual approach. In the short term, this involves efforts to develop physical measurement models (such as a radiative transfer model) that link the characteristics of the environment with satellite observations. Such fundamental knowledge of interactions between electromagnetic waves and the environment often also sheds light on the way the environment being studied functions.