1. Department of Civil and Environmental Engineering; University of Michigan; Ann Arbor Michigan USA
2. Now at Institute for Mathematics Applied to Geosciences; National Center for Atmospheric Research; Boulder Colorado USA
3. NASA Goddard Space Flight Center; Greenbelt Maryland USA
4. National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences; University of Colorado at Boulder; Boulder Colorado USA
5. Department of Marine Chemistry and Geochemistry; Woods Hole Oceanographic Institution; Woods Hole Massachusetts USA
6. Department of Global Ecology; Carnegie Institution for Science; Stanford California USA