ESA’s CryoSat satellite — launched in 2010 to measure the thickness of polar sea ice and monitor changes in the Greenland and Antartica ice sheets — is making a difference in efforts to accurately track changes in sea level. Sea level is seen as a very sensitive indicator of climate change, reflecting components of the climate system such as heat, glaciers and the melting of ice-sheets.
Precisely monitoring changes in the average level of oceans is vitally important for understanding not only climate but also the social and economic consequences of any rise in sea level, especially in coastal zones.
Previous radar altimeters have been aimed at measuring oceans and land, but CryoSat’s is the first sensor of its kind designed for ice, and able to map sea levels with unprecedented accuracy. Scientists also discovered that CryoSat — with a footprint 10-30 times smaller than the conventional altimeters on satellites like Envisat and Jason-3 — had the potential to map sea level closer to the coast.
Using satellite altimeters in coastal zones is notoriously difficult. Norway boasts the world’s second longest coastline of some 100,000km, comprising many islands, steep mountains and deep, narrow fjords.
The rugged coastline means that other altimeters produce confused readings close to the coast, showing differences of 10 cm or more. By contrast, CryoSat’s results compare favourably with the Stavanger tide gauge in southwestern Norway, provided by the Norwegian Mapping Authority.
While classical altimetry offers a few tens of observations over a five-year period, more or less near a tide gauge, some 7,000 measurements very close to the gauge are obtainable with CryoSat. The result is a better affinity with the Stavanger data, to within 7 cm for CryoSat, contrasting with the 10–15 cm for classical altimetry. In comparison with the Kabelvåg tide gauge in the Lofoten area in northern Norway, differences as low as 5.4cm were obtained.
ESA now has great expectations from the more recent Sentinel-3 mission, which carries a similar altimeter. Sentinel-3 will measure the temperature, colour and height of the sea surface as well as the thickness of sea ice. Sentinel-3A was launched on 16 February 2016. Sentinel-3B will follow in 2017.