Evidence is growing that global warming is causing sea ice in the Arctic Ocean to retreat.
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.
Cryosat will fill in the data gaps
But the picture is by no means complete.
Our science reporter Helen Briggs looks at Europe's Cryosat space mission, which aims to build a comprehensive picture of how the Earth's ice surfaces are changing.
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 spacecraft is the first satellite designed specifically to measure the volume of sea ice over the Arctic and to map the ice sheets in unprecedented detail.
The mission has two goals:
How does it work?
- To build a detailed picture of the trends and natural variability in Arctic sea ice, so that scientists can examine the role of the atmosphere, ocean and winds in redistributing ice in the Arctic Ocean from year to year.
- To observe in detail the trend in thinning of the great ice sheets of Antarctica and Greenland.
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:
- Its orbit provides better coverage of the poles.
- It has a double antenna designed to determine the direction of echoes from the margins of ice sheets that have a complex topography and are changing rapidly.
- The resolution of Cryosat's radar altimeter is 10 times better than that on ERS or Envisat: it has a resolution of 1 km rather than 10 km.
Cryosat is the first of Esa's six Earth Explorer missions, a series of fast, relatively low-cost (100m-euro) Earth observing satellites.
KEY CRYOSAT INSTRUMENTS
1 Heat-radiating panel - The path of Cryosat's orbit means it will face extremes of heat and cold. The panel ensures the electronics are kept at an optimum working temperature
2 Star trackers - To get the best out of its radar system, Cryosat must know precisely how it is orientated in the sky and uses three star trackers to get the necessary references
3 Radar antennas - Cryosat's radar altimeter system sends multiple pulses to Earth, and then captures the echoes to determine the heights and angles of ice surfaces
4 "Doris" antenna - The device is a radio receiver that will help work out the precise position in the sky of Cryosat to within a few centimetres. It works off a network of ground beacons
5 Laser retro-reflector - Cryosat's position can also be determined by firing short laser bursts at it and timing the reflections. Both systems are necessary to make sense of the radar data
6 X-band data antenna - This transmits the huge volume of data gathered by the radar system. The data is picked up at the Kiruna receiving station in Sweden
7 S-band telecommand antenna - Engineers also need to speak to Cryosat. Commands can be uploaded. The antenna also transmits status and monitoring information
8 Unlike many spacecraft, which have solar wings, Cryosat has a series of solar panels along its back