1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA.
2. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
3. Center for the Remote Sensing of Ice Sheets, Kansas University, Lawrence, KS, USA.
4. Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany.
5. Goddard Earth Science and Technology Center, University of Maryland Baltimore County, MD, and NASA Goddard Space Flight Center, MD, USA.