Lake Superior summer (July--September) surface water temperatures have increased approximately 2.50C over the interval 1979--2006, equivalent to a rate of (11 1 6) W 10-20C yr-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.