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Almost 80% of the Earth's fresh water is locked up in the cryosphere, i.e. 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.

Concept Science
Concept Science

Due to its 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 during the winter. The thickness of Arctic sea ice plays a central role in polar climate as it moderates heat exchange by insulating the ocean from the cold polar atmosphere.

A decrease in sea ice could disturb ocean circulation in patterns in the North Atlantic As sea ice forms, the salinity and therefore the density of the upper ocean increase. The density increase causes the surface waters to sink – in essence acting as a pump, driving cold, deep ocean currents from the polar regions towards the Equator. A reduction in Arctic sea ice could significantly disrupt the Gulf Stream which transports warm surface waters northwards from the Gulf of Mexico to the sub-polar waters east of Greenland. It is thanks to the Gulf Stream that north-west Europe currently enjoys annual temperatures of about 9° C higher than average for the latitude.

As well as influencing how much sunlight is reflected back to space, continental ice has an impact on sea level. The large ice sheets covering Antarctica and Greenland amount to about 28 million km3, which means that sea level is about 65 m lower than it would be if these ice sheets didn't exist. Whilst evidence suggests that these ice sheets are relatively stable, there are indications that rapid changes are occurring around their margins.

CryoSat-2's icy mission is dedicated to monitoring very precise changes in the elevation and thickness of polar ice sheets and floating sea ice over a 3-year period. The observations that CryoSat-2 makes will determine whether or not our ice masses are thinning due to global warming.

Sea ice

Sea ice typically covers up to 15 million square kilometres of the Arctic Ocean, and up to 19 million square kilometres of the Southern Ocean around Antarctica, during their respective winter seasons. This seasonal cycle of sea ice is one of the most dynamic components of the Earth's climate system.

Floating sea ice
Floating sea ice

Unlike icebergs, which are chunks of ice that have broken off the edges of ice-shelves or fronts of glaciers where they reach the sea, sea ice is seawater that has frozen. It contains little salt as most of it is rejected as it forms. Sea ice covers the Arctic Ocean more or less permanently above the latitude of about 75°N. This permanent ice cap is composed of pack ice, which is kept in continuous motion by the wind, tides and ocean currents.

Regional sea-ice models have been successfully developed over the last decades. However, given the impact that sea ice has on the climate, it is essential to acquire more comprehensive data on sea ice thickness to improve sea ice models for their implementation in general climate studies.

To further our understanding of the impact that sea ice has on climate and to ascertain whether there is currently a trend towards reduced sea ice cover, CryoSat-2 will provide new and authoritative data on fluctuations in Arctic and Antarctic sea ice.

Ice sheets

Sea levels rose by 18 cm during the last century alone, and there are major concerns about what a warmer climate means for sea-level rise for this century.

An obvious source for this extra water is from the melting of ice sheets and glaciers overlying land. The ice sheets that blanket Antarctica and Greenland are up to around four kilometres thick, and it is the melting of these large ice masses that have the potential to cause a significant rise in global sea level.

Glacier calving
Glacier calving

However, observations from ERS indicate that the great central plateaux of the Antarctic and Greenland ice sheets are relatively stable (Wingham et al., 1998). Nevertheless, there are indications that changes are occurring at the margins of the ice sheets and it is these apparent changes that need to be quantified. Also, dotted around the globe, smaller glaciers play a significant role in local hydrology, sea level rise and regional climate. However, evidence from a variety of sources suggests that the majority of the world's glaciers are currently retreating.

The improvement in resolution of the CryoSat-2 radar over that of its pulse-limited predecessors, coupled with its interferometric capability, will make spatially and temporally continuous measurements of the ice-sheet margins and smaller ice masses possible for the first time.

Polar climate

Sea-ice thickness plays a central role in climate processes. Ice extent and thickness have important stabilising effects on world climate, insulating large areas of the oceans from solar radiation in the summer and preventing heat loss to the atmosphere in the winter.

Disappearing Arctic sea ice predicted under CO2 doubling The effects of a warming climate are expected to be felt more strongly at the poles, and particularly in the Arctic. However, current global ocean-ice-atmosphere models are not yet able to accurately reproduce observed sea-ice coverage.
Disappearing Arctic sea ice predicted under CO2 doubling

Arbitrary flux corrections in these models, which essentially hold the sea surface temperature at freezing in regions where there is sea ice, make the calculation of the effect of 'perturbations' such as CO2 –induced warming questionable (Gates et al, 1996).

The physics of sea ice in global climate models is, at the moment, oversimplified. With the recognition that variations in thermohaline circulation may have important consequences for poleward-bound heat transport – the next years will see more complete sea-ice physics within global ocean-atmosphere models. These developments demand more information on sea-ice extent and volume changes – which will be provided by the CryoSat-2 mission.