This TIEE Data Set presents long term data on changes in dates of ice cover in three Wisconsin lakes. Global temperatures have increased by about 1 degree Fahrenheit during the last century, most likely the result of “greenhouse gases” such as carbon dioxide from burning of gasoline, oil, and coal. There are many environmental consequences of warmer temperatures, some unexpected. One challenge to our understanding of environmental effects due to global warming is lack of data collected over long periods of time. In this activity, students plot more than 100 years of data on dates of "ice on" and "ice off" and duration of ice cover and examine patterns of variation at different time scales.