These animations of Antarctica (above) and Greenland, derived from the radar altimeter instrument on ESA’s Envisat satellite, illustrate the variations in the surface height on each ice sheet from 2003 to 2010.

Each map in the animations represents a 35-day integration of Envisat radar altimeter (RA2) measurements; the date on each map indicates the middle of the 35-day period.

The colour-coded values indicate the surface height difference between that particular time and the average over the Envisat observation period. Many glaciers in West Antarctica and Greenland show red values at the beginning and deep blue at the end of the time series, indicating that they are thinning dramatically.

The polar ice caps have a very significant role in the study of climate, since they are both polar records and climatic indicators. Both Antarctic and Greenland ice sheets have an uneven bedrock, almost entirely covered by ice. With a surface of 12 million sq km and an average ice thickness of 2200 m, the Antarctic represents 90% of terrestrial ice. If it were to melt it would lead to an equivalent sea level rise of up to 70 m. The Greenland ice sheet is smaller, representing 9% of the terrestrial ice.

Envisat monitoring Greenland ice sheet

"These maps are possible to realise every 35 day thanks to the regular and well repeated Envisat RA2 measurements," said Benoit Legresy, a researcher with the LEGOS laboratory for geophysical studies in Toulouse, France. "In addition to fast changing glaciers, a large inter-annual signal is detected with large areas going up and down at inter-annual scales with variations in precipitation. These events are not at all found to be usual."

"A comparison of these data with German/American GRACE satellite observed gravity field variations shows coherent patterns between the two measurements and allows the qualification of the volume/mass variations of the ice sheets in terms of ice dynamics and surface mass balance on a larger scale," he added.

Noticeably, in 2005 a large accumulation event is seen to temporarily compensate (for that year only) the dynamic mass over-loss by the glaciers in the Amundsen Sea - a region changing faster than anywhere else on the West Antarctic Ice Sheet.

Legresy said Envisat allows for the relatively high resolution (here about 50km for global scale maps) mapping of these fluctuations. Legresy, Frédérique Remy and Fabien Blarel presented these findings at ESA’s Living Planet Symposium today.