Changes in the extent and duration of winter ice cover may influence lake levels via water loss through evaporation. Loss of lice cover earlier in the spring can lead to higher water temperatures by affecting the onset of summer warming. Overall, the spatial extent of Great Lakes ice cover has decreased by 71% in the past 40 years.1
- Where detailed records are available, the duration of winter ice cover has decreased (e.g., Grand Traverse Bay, MI; Bayfield, WI).
- The annual maximum percentage of lake surface covered by ice is highest in Lake Erie.
- Lake Superior has the second-highest maximum percentage of ice cover, due to colder air temperatures.
- Evidence to date and future projections suggest climate change will bring reductions in the extent and duration of winter ice cover, and Lakes Erie and Superior may show the greatest changes.
Mapping reduced winter ice cover as a Great Lakes stressor
Ice cover data were obtained from NOAA's Great Lakes Ice Atlas (mostly the National Ice Cover (NIC) dataset, but Canadian Ice Service data were used for dates when NIC data were not available).
- These data consisted of total expanse (fraction of surface area covered) of ice across the Great Lakes basin, observed or interpolated daily in 2.5-by-2.5 km pixels throughout the winter (Dec. 1 - May 31).2 We considered a pixel on a given day to be ice-covered if 50% or more of its area was occupied by ice.
- For each winter from 1973 to 2002, we calculated the cumulative duration of ice cover (number of days having ice cover ≥ 50%).
- Since the data in many pixels were not normally distributed, to determine whether there was a significant decrease in ice cover in a given pixel, Kendall's tau and p-values were calculated using the nonparametric (rank-based) Mann-Kendall test.
- For pixels that exhibited a significant decrease in ice cover (defined as Mann-Kendall p < 0.10 and trend direction negative), we expressed the rate of change in duration (i.e. days of ice duration lost per year) using the slope from simple linear regression or duration versus calendar year since 1973.
Spatial distribution of reduced ice cover as a stressor in the Laurentian Great Lakes. (Inset: Western Lake Superior).