1. Department of Earth Sciences, University College LondonGower Street, London WC1E 6BT, UK
2. Environment Institute, University College LondonGower Street, London WC1E 6BT, UK
3. National Centre for Atmospheric ResearchPO Box 3000, Boulder, CO 80307-3000, USA
4. Potsdam Institute fur Klimatology14473 Potsdam, Germany