Cryosat’s mission is dedicated to monitoring very precise changes in the elevation and thickness of polar ice sheets and floating sea ice over a three-year period. The observations made by Cryosat will determine whether or not our ice masses are thinning due to global warming.
The question of whether global climate change is causing the polar ice caps to shrink is one of the most hotly-debated environmental issues we currently face. By monitoring precise changes in the thickness of the polar ice sheets and floating sea ice, the Cryosat mission aims to answer this question.
Almost 80% of the Earth’s fresh water is locked up in the cryosphere, i.e. in snow, ice and permafrost. The cryosphere plays an important role in moderating the global climate, and as such, the consequences of receding ice cover due to global warming are far-reaching and complex. Due to their high albedo, ice masses directly affect the global energy budget by reflecting about 80% of incident sunlight back out to Space. Thus, once formed, ice tends to be maintained. However, if ice cover were to decrease, less solar radiation would be reflected away from the surface of the Earth – causing the ice to absorb more heat and consequently melt faster still. Around the North Pole, an area of sea ice the size of Europe melts away every summer and then freezes again over the winter. The thickness of Arctic sea ice plays a central role in the polar climate as it moderates heat exchange by insulating the ocean from the cold polar atmosphere.
The cryosphere has a central role in the Earth’s radiation budget. Loss of sea ice is predicted to cause greater greenhouse-gas warming in the Arctic than the rest of the Earth. Ice sheets and glaciers are a control on sea level. They are the largest uncertainty factor in determining the source of the present rise in sea level. The central questions concerned with the cryosphere are: How is deepwater formation and polar and sub-polar ocean exchange affected by sea ice? What are the imbalances in the Antarctic and Greenland ice sheets? Ocean-atmosphere-ice models and ocean-ice-solid Earth models demand spatially- and temporally-continuous estimates of ice mass fluxes on regional and global scales. Current observations are deficient and only satellites can eliminate this deficiency. The Cryosat mission and various international programmes will be undertaking a decade of focused study on the roles of the cryosphere.
To meet the challenges of measuring ice, Cryosat will reach latitudes of 88° and will carry a sophisticated radar altimeter called SIRAL (Synthetic Aperture Radar Interferometric Radar Altimeter). It is based on the heritage of existing instruments, but incorporates several major enhancements designed to overcome the difficulties intrinsic to the precise measurement of ice surfaces.
Fundamentally, there are two types of polar ice: the ice that floats in the oceans and the ice that lies on land. Not only do these two forms of ice have different consequences for our planet and its climate, they also pose different challenges when trying to measure them from Space.
Floating sea ice
Sea ice is relatively thin – up to a few metres thick, but it influences regional temperature and the circulation of ocean currents, and consequently the Earth’s climate. Cryosat will acquire precise measurements of the thickness of floating sea ice so that annual variations can be detected.
Cryosat will determine the thickness of floating sea ice by measuring the freeboard of ice floes; that is the height by which the ice extends above the water’s surface. This technique was demonstrated with the ERS-1 radar altimeter, but that instrument, as with all conventional radar altimeters, was hampered by its relatively low spatial resolution of about 5 km. Cryosat will achieve improved spatial resolution of 250 m in the along-track direction using the Synthetic Aperture technique.
Ice-shelf break up
In contrast, the ice sheets that blanket Antarctica and Greenland are several kilometres thick. The growth and shrinkage of these ice masses have a direct influence on sea level. The approach used for measuring these vast thicknesses is to determine the height of the surface accurately enough to detect small changes.
The improved resolution of Cryosat’s radar compared with that of its pulse-limited predecessors, coupled with its interferometric capability, will allow for the first time spatially- and temporally-continuous measurements of the ice-sheet margins and smaller ice masses.