1. Department of Physical Geography and Ecosystem Analysis, Lund University, 22362 Lund, Sweden.
2. Department of Physics, University of Helsinki, 00014 Helsinki, Finland.
3. NASA Goddard Institute for Space Studies, Center for Climate Systems Research, Columbia University, New York, NY 10025, USA.
4. Max Planck Institute for Chemistry, Biogeochemistry Department, 55020 Mainz, Germany.