Ice monitoring
Ice extent waxes and wanes to a remarkable extent during Earth's annual journey around the Sun. In the northern hemisphere, Arctic sea ice can stretch across 14 to 16 square kilometres during the months of permanent darkness, drawing back to just half that during each polar summer.
Influenced by the ice
Beyond the area it covers directly, the influence of polar ice extends out much further, as an important factor in oceanic circulation and planetary weather patterns. In particular, warm winter temperatures in Europe result from ocean currents partly driven by meltwater from Arctic ice.
So changes to the state of polar regions could have far wider ramifications, and the latest climate models and evidence on the ground suggests just such a shift is in the offing.
Higher temperatures due to global warming threaten to decrease sea ice and snow cover at high latitudes. Because ice and snow have a high albedo they reflect the majority of solar energy back into space. Sea ice also blocks the transfer of heat energy from the ocean to the atmosphere.
This being so, shrinkage of sea ice will increase temperatures in the region still further. Polar wildlife will lose important habitats and the erosion of coastlines cleared of ice will accelerate markedly.
|  | German ice-breaker RV Polarstern | |
A warmer Arctic
Arctic temperatures are already at their warmest for the last four centuries, and measurements suggest sea ice extent has declined by 10% since the 1960s.
The best means of accurately measuring both ice extent and thickness on an ongoing basis is space-based radar and altimetry instruments of the type flown on ESA's ERS and Envisat spacecraft.
Ice mapping from space
Envisat’s Advanced Synthetic Aperture Radar (ASAR) instrument is able to simultaneously cover an area of the Arctic four times larger than ERS’s SAR. It is also able to distinguish between different types of ice, with a variable angled and polarised radar beam that can show whether ice is solid pack-ice or just thin ‘pancake’ ice.
| | | C-19 iceberg on 18 July 2002 |
The spacecraft's RA-2 Altimeter sends thousands of radar pulses earthward every second to measure sea ice and glacier thickness. It features an innovative ‘four-wheel-drive’ design that enables it to keep better signal contact during previously difficult transitions between ice, land and open sea.
And ESA's forthcoming CryoSat mission will carry an improved twin-antenna radar altimeter design, incorporating synthetic aperture and interferometry techniques for better accuracy. As well as sea ice, it will also measure the thickness of land ice features such as Greenland Ice Shelf.
Of course melting sea ice can do nothing to alter sea levels – as the ice is already floating on the water - but if the ice sheets that rest on land in Greenland and the Antarctic start to melt, that would be a different matter.
Global sea level has risen 18 cm during the past century; it would rise by 70 m if all ice sheets melted together. Climatologists do not think that likely, but Earth Observation will allow them to note the slightest ice sheet retreat or advance across time.
Last update: 19 July 2004 | 
Via broadband from the ArcticAnimated flyover of iceberg B-15AEnvisat monitoring iceberg B-15AIceberg B-15A
