John J. Magnuson,^1* Dale M. Robertson,² Barbara J. Benson,¹ Randolph H. Wynne,³ David M. Livingstone,⁴ Tadashi Arai,⁵ Raymond A. Assel,⁶ Roger G. Barry,⁷ Virginia Card,⁸ Esko Kuusisto,⁹ Nick G. Granin,¹⁰ Terry D. Prowse,¹¹ Kenton M. Stewart,¹² Valery S. Vuglinski¹³

Freeze and breakup dates of ice on lakes and rivers provide consistent evidence of later freezing and earlier breakup around the Northern Hemisphere from 1846 to 1995. Over these 150 years, changes in freeze dates averaged 5.8 days per 100 years later, and changes in breakup dates averaged 6.5 days per 100 years earlier; these translate to increasing air temperatures of about 1.2°C per 100 years. Interannual variability in both freeze and breakup dates has increased since 1950. A few longer time series reveal reduced ice cover (a warming trend) beginning as early as the 16th century, with increasing rates of change after about 1850.

¹ Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706, USA.
² U.S. Geological Survey, Water Resources Division, 8505 Research Way, Middleton, WI 53562, USA.
³ Department of Forestry, Virginia Polytechnic Institute and State University, 319 Cheatham Hall, Blacksburg, VA 24061, USA.
⁴ Department of Environmental Physics, Swiss Federal Institute of Environmental Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland.
⁵ Department of Geography, Rissho University 4-2-16 Osaki, Shinagawa-Ku, Tokyo 141, Japan.
⁶ Great Lakes Ecosystem Research Laboratory, National Oceanic and Atmospheric Agency, 2205 Commonwealth Boulevard, Ann Arbor, MI 48105-1593, USA.
⁷ World Data Center for Glaciology, University of Colorado at Boulder, Boulder, CO 80309-0449, USA.
⁸ College of Arts and Sciences, Metropolitan State University, 700 East 7 Street, St. Paul, MN 55106, USA.
⁹ Finnish Environment Institute, Post Office Box 140, FIN-00251 Helsinki, Finland.
¹⁰ Limnological Institute, Post Office Box 4199, Irkutsk 664033, Russia.
¹¹ National Water Research Institute, Environment Canada, 11 Innovation Boulevard, Saskatoon, SK S7N 3H5, Canada.
¹² Department of Biological Science, State University of New York at Buffalo, Buffalo, NY 14260, USA.
¹³ State Hydrological Institute, 23 Second Line, St. Petersburg 199053, Russia.
^*   To whom correspondence should be addressed. E-mail: jmagnuson{at}mhub.limnology.wisc.edu