1. NERC Centre for Earth Observation (NCEO), University of Reading, Whiteknights, PO Box 238, Reading, RG6 6AL, UK;
2. International Space Science Institute (ISSI), Hallerstrasse 6, CH-3012 Bern, Switzerland;
3. Leibniz Institute of Marine Sciences, IFM-GEOMAR, D¨usternbrooker Weg 20, D-24105 Kiel, Germany.