Climate predictions suggest the thin crust of sea ice around the North Pole could disappear entirely by the end of the century, at least in summer.
But the picture is by no means complete.
Our science reporter Helen Briggs looks at Europe's Cryosat-2 space mission, which aims to build a comprehensive picture of how the Earth's ice surfaces are changing.
Click here to see a graphic of the satellite
What will the mission do?
Although satellites have provided dramatic images from space, large areas remain uncharted, particularly at the centre of the poles and at the edge of the great ice sheets.
Europe's Cryosat-1 spacecraft was the first satellite designed specifically to measure the volume of sea ice over the Arctic and to map the ice sheets in unprecedented detail.
Cryosat-2 has the same mission goals:
Cryosat uses an instrument known as a radar altimeter. In simple terms, it bounces a radar pulse off the ground, and studies the echoes.
The device will measure the difference in height between the floes of sea ice and the surrounding ocean, and, on land, the elevation of ice sheets. The thickness and mass of the ice can then be calculated from the known densities of ice and seawater.
What data will it provide?
Cryosat will provide data on the ice that covers the Arctic Ocean and the ice that covers the landmasses of Greenland and Antarctica. It will provide a 3D map of the world's ice caps and details of the thickness of the ice.
Studying how the ice changes over the course of the three-year mission should give a picture of how the ice ebbs and flows from season to season and from year to year.
Why is the data needed?
Our understanding of the planet's ice fields comes from four main sources: military submarines, field studies, satellite data and climate models.
Military submarines patrolling under the ice during the Cold War provide historical records of ice thickness. But the declassified information available covers only a small fraction of the Arctic Ocean.
Hundreds of scientists visit the Arctic Ocean and the Antarctic and Greenland ice sheets each year to carry out field studies. But the sheer size of these regions, and their extreme weather and geology, makes it difficult to study them in detail.
Data from the US space agency (Nasa) and European Space Agency (Esa) satellites (Envisat, ERS) gives the most accurate record of changes in ice cover.
But the orbits of these satellites, which are designed to study the whole of the planet, leave vast areas uncharted, in particular the nine degrees of latitude nearest the poles. Historical records suggest that this sector may be subject to the greatest thinning.
Climate models predict what may happen to ice as the planet warms. These rely on a number of assumptions - if ice melts, the ocean will become warmer, as there will be less ice to reflect radiation back into space. Scientists need hard data to feed into the models to firm up their predictions.
Cryosat has been developed to fill in some of the gaps in the data. It will do this in a number of ways:
Cryosat-1 was the first of Esa's six Earth Explorer missions, a series of fast, relatively low-cost (100m-euro) Earth observing satellites.
The second Cryosat mission is expected to be launched in March 2009.
KEY CRYOSAT INSTRUMENTS
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Cryosat (Esa)
ICESat
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Centre for Polar Observation and Modelling
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