Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback

Lake Superior summer ( July - September) surface water temperatures have increased approximately 2.5 degrees C over the interval 1979 - 2006, equivalent to a rate of ( 11 +/- 6) x 10(-2) degrees C 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.