September 2012 (Great Lakes Environmental Research Laboratory) – Ice cover on the Great Lakes has been declining since 1973. This figure shows the linear trends for the six lakes. The linear trend was estimated using the least squares regression (LSR). The linear equation is in the form: x=a+bt, where x is the ice cover (in %), t is the year starting in 1973, a is the x-intercept constant (the value of x for t=0 (i.e., 1973), and b is the slope of the line (namely, the rate of change in x with a time increment of t).
Winter mean ice cover in all lakes shows a significant negative trend, indicating that the ice extent in the Great Lakes has been decreasing since the 1970s. The negative trends vary from lake to lake (-0.56%/yr to -1.96 %/yr, Table 4). Lake Superior has the largest negative trend (-1.96 %/yr), Lakes Ontario and Michigan place second (~-1.6%/yr), and Lakes Erie and St. Clair have the smallest negative trend: -0.74%/yr and -0.56%/yr, respectively. This translates to the total loss of winter lake ice coverage over the entire 39-year record (from 1973-2011) relative to 1973 in Table 4 (last row), which varies from 22% in Lake St Clair to 76% in Lake Superior.
The total loss for overall Great Lakes ice coverage is 63% relative to the 1973 value. Another metric of loss of seasonal average ice cover over the 1973-2011 winters is the difference (1973 minus 2011) in the regression values for 1973 and 2011. The loss in seasonal average ice cover from 1973 to 2011 using this metric is 24.4% for Lake Superior, 12.9% for Lake Michigan, 15.5% for Lake Huron, 15.2% for Lake St. Clair, 11.1% for Lake Erie, 6.5% for Lake Ontario, and 14.9% for the entire Great Lakes basin.
Note that the trends calculated within a specific period of time such as 1973-2011 can only be applicable to the same period, and cannot be extrapolated to the future and back to the past. It should not be interpolated to a period shorter than the time series of the data from which the trends are derived, since there are decadal and multi-decadal changes in lake ice cover.
To search factors responsible for the lake ice trend, the winter surface air temperature (SAT) trend over the Northern Hemisphere was calculated for the period 1973-2010 (Figure 6). The SAT trend over the Great Lakes ranges from ~0.4 °C per decade over the lower lakes to ~0.6 °C per decade over the upper lakes, with Lake Superior being the highest (0.6 °C per decade). This is consistent with the upward trend of Lake Superior water temperature (Austin and Colman 2007). They found that summer (July–September) surface water temperatures have increased approximately 2.5°C over the period 1979– 2006, significantly higher than regional atmospheric warming. This excessive warming of lake water temperature relative to the local surface air temperature has caused a positive ice/water albedo feedback (Wang, et al. 2005) due to the declining ice cover (Austin and Colman 2007).