Airbus Defence and Space

The future of the polar ice caps

CryoSat-2, designed and built by Airbus Defence and Space, is measuring changes in the Earth’s ice masses. The first data were unveiled this June

CryoSat-2: The satellite with an icy mission

Nothing but ice as far as the eye can see. For a trekker in Greenland, looking out over the immeasurably vast white landscape that stretches to the horizon in every direction, the ice seems to extend on and on to infinity. Even downward, the layer of ice is several kilometres thick.

The volume of the Greenland ice sheet has been estimated at an incredible 2.85 million cubic kilometres. After Antarctica, it is the second-largest continental ice mass on Earth. But what cannot be seen from the ground is the fact that it is shrinking. The ice is melting at a rate that can be precisely observed only from space. The Earth observation satellite designed and built by Airbus Defence and Space Friedrichshafen (Germany) for the European Space Agency (ESA) has been orbiting our planet at an altitude of close to 720 kilometres since 8 April, 2010.

CryoSat-2: The satellite with an icy mission

After the initial commissioning phase lasting several months, it entered the operational phase around a year ago. Since then, the satellite’s radar altimeter has been measuring the thickness of the sea ice and collecting elevation data on the continental ice sheets covering the Arctic and Antarctic. It was precisely this gap in our knowledge that the group of 18 scientific investigators led by British climatologist Duncan Wingham (of University College London) had in mind in 1998 when they formulated the mission proposal:

“While it is true that Earth-orbiting satellites such as ERS-2 and Envisat have been mapping the extent of sea-ice cover for some years now, we still know hardly anything about the thickness of the sea ice and hence its total mass. Consequently, we have no reliable information concerning the volume of sea ice that thaws and refreezes each year.”

CryoSat-2: The satellite with an icy mission. © Airbus Defence and Space

There is also an urgent need for precise data on the great continental ice sheets, since the ice masses of Greenland and Antarctica alone contain enough water to cause the sea level to rise by 65 to 70 metres if ever they should melt away completely. Depending on the actual rate of melting, the global sea level could rise by as little as a few dozen centimetres or as much as several metres by the end of this century – with correspondingly dramatic consequences for the many millions of people who live in low-lying coastal regions.

In June 2011, the first provisional ice-thickness map of the northern polar seas generated using CryoSat-2 data was unveiled. “I am very happy with these new results,” said Professor Wingham, ESA’s lead scientific investigator on the CryoSat-2 programme. “They show not only that the hardware is really excellent but that it can deliver the geophysical information we need, too.” From an altitude of just over 700 km and reaching unprecedented latitudes of 88°, CryoSat-2 has spent the last seven months delivering precise measurements to study changes in the thickness of Earth’s ice. According to ESA, the data are exceptionally detailed and considerably better than the mission’s specification.

The Arctic sea ice shown in the ice-thickness map has no direct effect on the sea level because it is already floating on the water. But through the yearly cycle of freezing and thawing it does affect the salinity of the water, which in turn influences the ocean currents. Thanks to the warmth of the Gulf Stream, for example, the average temperature along our European Atlantic coasts is around 4°C higher than at comparable latitudes in the North Pacific. If, however, the freshwater content of the northern oceans keeps increasing due to an accelerated net loss of ice, it is possible that the current that keeps Europe warm could one day change course or even cease to circulate altogether.

CryoSat-2 is capable of detecting changes in this relatively thin layer of ice covering the Arctic Ocean (only a few metres thick) to an accuracy of 1.5 cm per year. In the case of Greenland’s continental ice sheet, the researchers are aiming to achieve an accuracy of 0.7 cm/year. To achieve this kind of precision, the satellite data must be compared with measurements on the ground.

This map showing the sea-ice thickness in the Arctic was generated using CryoSat-2’s exceptionally detailed data. Thanks to CryoSat-2’s particularly high-inclination orbit, ice thickness close to the North Pole can be detailed for the first time. © ESA

This map showing the sea-ice thickness in the Arctic was generated using CryoSat-2’s exceptionally detailed data. Thanks to CryoSat-2’s particularly high-inclination orbit, ice thickness close to the North Pole can be detailed for the first time. © ESA

 “One of the first things we set out to do was to validate the CryoSat-2 data products,” says Dr Veit Helm of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. The institute’s scientists did this by taking a second set of measurements of the same ice observed by the satellite, but this time from an airborne platform. “We investigated Ku-band radar penetration to determine just how accurate CryoSat-2’s measurements were and to identify the dominant scattering surface of the radar echogram: surface roughness or internal interfaces in the topmost metres of ice and snow.”

Dr Helm is very impressed with the results: “The precision of the CryoSat-2 measurements is truly extraordinary.” The Bremerhaven scientists now intend to use the data released for public access by ESA to generate ice-thickness maps that will enable them to show how thicknesses are changing and to improve existing elevation models of Antarctica and Greenland. They also intend to investigate the impact of different properties of snow on the accuracy of the CryoSat-2 data by means of simulation. “The data furnished by CryoSat-2 are helping to considerably widen our knowledge base,” says a delighted Dr Helm.

From an altitude near to 720 km and reaching unprecedented latitudes of 88°, CryoSat-2 has spent the last seven months delivering precise measurements to study changes in the thickness of Earth’s ice.

In two to three years’ time, when CryoSat-2 has observed several complete seasonal cycles and the elevation models are sufficiently exact and reliable, scientists expect to be able to estimate with more confidence the rate at which the ice masses are declining, if that is indeed the case, and whether or not Europe can continue to count on the warming effects of the Gulf Stream.

 

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