Ice freeze and breakup quantifies long-term global responses of lakes and rivers to climate change and variability. Each year lakes and rivers at northern latitudes freeze over in Autumn and breakup in Spring. Taken as single points in space and time, these events lack context and reveal little to the observer. Yet generations of observers have recorded such events at sufficient regularity since the middle of the last century that long-term global changes can be analyzed and significant inferences drawn of change around the Northern Hemisphere. What do such long-term records tell us (Magnuson et al. in press)? First they tell us that at the North Temperate lakes LTER site, the ice duration on Lake Mendota in the winter of 1997-98 was the shortest over the period of record beginning in the 1850's. Also, the average duration of ice cover has declined from about 4 to 3 months or by 25%. Step changes occurred that correspond to the end of the "little ice age" and to interdecadal changes in the strength of the Aleutian low. Springs with the earliest breakups occurred in the year following the onset of an El Niño. The long-term trend corresponds to a warming of about 1°C in 100 years. These dynamics in the timing of ice breakup and freezing are driven by climate drivers, originating far distant from Wisconsin in the Southern and Northern Pacific, and in the case of the long-term warming trend from the globally dispersed drivers behind that warming.
The Lake Mendota patterns above are observable around the Northern Hemisphere with some variation in pattern (Magnuson et al. 2000). The ability to infer long-term and regional pattern from these events, puts the observations at one site in one year in a context more useful and meaningful to us as we attempt to deal with global change. The particular events have been moved from behind the mask of the "invisible present" (Magnuson 1990) and the "invisible place" and as a result can be used to understand and respond more appropriately to the changes in the world around us.
Magnuson, John J., R. H. Wynne, Barbara J. Benson, and D. M. Robertson. 2000. Lake and river ice as a powerful indicator of past and present climates. Verh. Internat. Verein. Limnol. 27:2749-56
Magnuson, J. J., Robertson, D. M., Benson, B. J., Wynne, R. H., Livingstone, D. M., Arai, T., Assel, R. A., Barry, R. G., Card, V., Kuusisto, E., Granin, N. G., Prowse, T. D., Stewart, & K. M., Vuglinski, V. S. 2000. Historical Trends in lake and river ice cover in the Northern Hemisphere. Science 289: 1743- 1746.
Magnuson, J.J. 1990. Long-term ecological research and the invisible present. BioScience 40(7):495-501