Earth’s changing ice
One of the most dramatic signs of climate change is in the extent of Arctic sea ice. Since 2000, the area covered by sea ice in the summer has reduced drastically. The absolute minimum recorded occurred in September 2007, and 2008 would have been a new record if it had not been for 2007, and 2009 was similar.
These great reductions in the area of sea ice in the middle of the summer are much worse than expected if we simply extrapolated from the previous trends. There are several factors involved: the prevailing winds have a great influence (floating ice can be simply blown out of the Arctic Ocean, passing Greenland and Iceland and disappearing into the Atlantic).
Thermodynamics is also playing a role: as the amount of ice reduces, more heat is absorbed by the ocean in summer, and consequently less ice formed by freezing in winter, accelerating the trend in reducing ice cover.
While these reductions in the area of sea ice are readily observable using a variety of satellite remote-sensing techniques, there is only one practical way of converting this knowledge of sea-ice area into the amount of sea ice. We need information about the thickness of the ice, and the only way to measure that on a large scale is by satellite. This is where CryoSat comes in.
Apart from floating sea ice, the other characteristic manifestations of ice in polar regions are the ice caps: thick domes of ice resting on land, from relatively small islands up to the complete continent of Antarctica. The two largest, Antarctica itself and Greenland, are several kilometres thick and, at the summits, very cold.
Thus they may seem immune to the influence of a few degrees of global temperature rise. Indeed, prior to 2000 the indications were that these major ice caps were largely stable, at least in their interiors. The principal means of determining this was satellite altimetry. However, the capabilities of such instruments to measure change at the ice cap margins, where most change is expected, is limited by their design.
But, already by 2006, skilled analysis of the existing altimeter data was teasing out details which were beginning to cast doubt on this picture of stability. A very large glacial basin at the coastal boundary of West Antarctica, the Pine Island Glacier, was sufficiently large that it could be resolved in the conventional altimeter data. And the ice was thinning.
In the years since 2006, this thinning has been characterised and linked to Synthetic Aperture Radar interferometry measurements of ice flow, which show an increased flow rate into the sea. The rate of thinning is stunning, at about 16 m per year.
While this has been reported by several groups, it has been put into perspective by the late-2009 report by the Scientific Committee on Antarctic Research Antarctic Climate Change and the Environment, which projects a sea level rise of about 1.4 m by 2100, significantly higher than the well-known 28–43 cm projections of the Intergovernmental Panel on Climate Change. The difference is largely attributable to melting of the ice caps at their base by warming oceans.
Change is not limited to Antarctica. Greenland, being smaller and at lower latitudes, may be even more vulnerable. Gravity measurements from the NASA/German Aerospace Centre (DLR) GRACE mission have revealed large-scale mass changes and even sensitive GPS receivers placed around the coast show signs of uplift as the burden of ice is reduced.
So indeed, in the decade since the CryoSat mission was proposed, the signs of change in the polar regions have become unambiguous, and this trend is also clear in the four years since CryoSat-2 was approved.