1. Potsdam Institute for Climate Impact Research, Post Office Box 601203, D–14412 Potsdam, Germany.
2. Max Planck Institute for Biogeochemistry, Post Office Box 100164, D–07745 Jena, Germany.
3. Department of Geography, Boston University, Boston, MA 02215, USA.
4. Laboratoire des Sciences du Climat et de'l Environment, F–91198 Gif-sur-Yvette Cedex, France.
5. Department of Physical Geography and Ecosystems Analysis, Lund University, S-22362 Lund, Sweden.