Lake Superior summer (July -- September) surface water temperatures have increased approximately 2.5 over the interval 1979 -- 2006, equivalent to a rate of (11 $\pm$ 6) 10 2 1, significantly in excess of regional atmospheric warming. This discrepancy is caused by declining winter ice cover, which is causing the onset of the positively stratified season to occur earlier at a rate of roughly a half day per year. An earlier start of the stratified season significantly increases the period over which the lake warms during the summer months, leading to a stronger trend in mean summer temperatures than would be expected from changes in summer air temperature alone.
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