Antarctic ice loss adjusted downwards, but melting continues to increase

After 20 years of occasionally contradictory results, an international team of 47 researchers, which included scientists from TU Delft and Utrecht University, has reached agreement on the question of how quickly the polar icecaps in Greenland and Antarctica are shrinking. This now appears to be occurring three times quicker than in the 1990s. The melting has caused sea levels to rise by 11.1 millimetres in the previous 20 years. This amounts to a fifth of the total sea-level rise in this period. The researchers will publish their results in Science on 30 November.

The amount of ice loss for Greenland since 1990 appears to match earlier reports. For Antarctica, the researchers have adjusted the loss of ice mass downwards. This means that the loss of ice mass in Greenland is now twice as great as in Antarctica between 1992 and 2011.

However, the increase in the speed of icecap melting is unequivocal: from 0.27 mm per year in 1992 to 0.95 mm per year at present. 'This means that the icecaps of Antarctica and Greenland contribute significantly to the present rise of sea levels and that this is almost certain to continue in coming decades,' according to the head of the research project, professor Michiel van den Broeke of Utrecht University.

11 satellite missions

Since the beginning of the 1990s, various satellites have been measuring changes in the thickness and flow rate of the ice in Greenland and Antarctica. Since 2002, changes in the mass have also been measured. TU Delft also contributed to the research by interpreting the gravity field measurements of the so-called GRACE satellites and translating this data into changes in the ice mass in Greenland and Antarctica. Lecturer and researcher Ernst Schrama of TU Delft: 'We are now able to see a level of melting that is half of what a few years ago was considered realistic for Antarctica. The new balances of mass for Greenland are practically the same as what we found before.'

The research combined ice measurements from 11 different satellite missions. This meant that an immense amount of measurement data had to be "translated". 'One technology, for example, measured more accurately but for shorter and another technology less accurately but for longer,' explains Van den Broeke. Now that the combination of the 11 satellite missions has succeeded, the researchers have a complete picture for the first time of the development of the icecaps for the whole area of both regions and over the whole of the research period. Van den Broeke: 'The fact that this project made it possible to combine all of the available data has been enormously valuable. I hope that this methodology will become the standard in future. The space agencies carrying out these sorts of missions will at least be using the experience gained in this research in future satellite missions.' 

Urgency

Urgent attention is also needed for new satellite missions. Two of the satellites used for measuring ice levels, the American ICESat and the European Envisat, recently ceased taking measurements. A third mission, GRACE, which is carrying out gravity field measurements, will be completed shortly and will probably only be followed up in 2017. A new European satellite for measuring ice thickness, named CryoSat-2, was launched in 2010, but the data from this still needs to be analysed better. 'It's important to continue as many measurements as possible, otherwise gaps will occur in the data. And this will make it even more difficult to predict future developments,' according to Van den Broeke.

Provided by Delft University of Technology