Note: the overview is written for faculty but can be used as the basis for an introduction to the data set for students.
Ice Ridge over Lake Mendota during the winter of 1987-1988
This activity uses ice cover records from three lakes in Madison, Wisconsin. Students work in groups to make predictions based on prior knowledge and assumptions, manipulate and summarize data, interpret the data by suggesting trends, and construct arguments from the data related to evidence of global change. In the main exercise, students work in small groups and attempt to see patterns in 20 years of data; all groups then combine data and only then can the long-term trend be seen.
Climatic observations have been made regularly around the world. Many of the records of these observations date back to several centuries. Recent publications have used data from lakes and rivers in the northern hemisphere dating back to the mid 1800's (“Shorter lake and river ice seasons confirm global warming.” September 7, 2000. CNN.com http://www.cnn.com/2000/NATURE/09/07/global.warming/index.html). The data used in this exercise are the dates of fall "ice-on" (the initial formation of ice cover), spring "ice-off" (the break-up of winter ice cover), and duration of ice cover on Wisconsin's lakes Mendota, Monona, and Wingra, which are part of the North Temperate Lakes Long-Term Ecological Research site (Assel and Robertson 1995).
Magnuson et al. (2000) looked at river ice cover data from 39 locations in the northern hemisphere including sites in Russia, Finland, Japan, and the U.S. The authors conclude that over the 150-year period from 1846-1995, average rate of change in freeze dates was 5.8 days per 100 days later and that change in breakup averaged 6.5 days per 100 years earlier. These changes translate into increasing air temperature of about 1.2 degrees Celsius per 100 years.
The longest time series is from Lake Constance in central Europe, which has data from the 9th through the 20th centuries (Magnuson et al. 2000). "Total ice cover" is the date on which a Madonna statue could be carried between a church in Germany to another in Switzerland on opposite sides of the lake. These data indicate a warming trend which began during the early 19th century with an increase in rate of change after about 1850.
In the 1970’s, ecologists realized that many ecological questions could not be answered by individual or small groups of scientists conducting short-term research. Therefore, in 1980 the National Science Foundation established the Long-Term Ecological Research (LTER) network to support research on long-term ecological phenomena in the U.S. As of 2004, the LTER Network includes 26 sites representing different biomes across the U.S. and Antarctica (http://www.lternet.edu/).
The vision of the North Temperate Lakes LTER is “...to gain a predictive understanding of the ecology of lakes at longer and broader scales than has been traditional in limnology. Thus, we analyze and interpret data we collect over long periods on suites of lakes” (http://lter.limnology.wisc.edu).
For background on limnology and lake ecology see:
Global Warming Science
Global Warming: Physical Limnological Effects
Other Climate Change Data
Lake Mendota, Monona, and Wingra