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Role of Low-Level Clouds in the Accelerated Warming of the Great Lakes - A Dual Observational and Regional Modeling Assessment

This is a study of the warming trends in surface temperatures of the Great Lakes since the mid-1980s. Great Lakes surface temperature trends have been upward during this period in all seasons, especially in summer, and the summer surface temperatures in lakes Huron and Michigan have increased by 4 degrees Celsius. This rate of warming, in excess of 1.3C per decade, is greater by a factor of 10 than regional surface air temperature trends over the same period. The warming of the lakes has a number of consequences, including a reduction in lake level due to evaporation, which historically has been a second-order effect compared to rainfall variations but has recently become comparable in magnitude due to the higher temperatures. The prevailing hypothesis for the rapid warming is that reduced winter ice cover leads to earlier springtime stratification and an increase in the number of heating days through the summer, thereby accounting for the large trend in warm season temperature. But the impact reduced ice cover may not be sufficient to account for the observed warming without additional amplifying mechanisms. Research conducted in this project tests the hypothesis that the temperature increase is amplified by feedbacks involving fog and low cloud cover, in which warmer lake temperatures lead to reductions in fog and low cloud, which in turn expose lake surfaces to increased insolation. The impact of changes in fog and low cloud, as well as other warming mechanisms, will be examined using a suite of numerical modeling simulations. Expected outcomes of the research are 1) the identification of various factors that are driving the observed water-surface temperature increases in all of the Great Lakes; 2) estimation of future water-surface temperature increases; 3) assessment of current and future lake-level decreases which will result from increased evaporation; 4) identification of regional climate impacts stemming from global warming induced changes in the Great Lakes' behavior. The work will have broader impacts beyond the scientific questions addressed. Warming lake temperatures have a number of undersirable consequences for the Great Lakes ecosystem and fisheries, including excessive primary production, algal blooms, and consequent hypoxia as excess organic matter decomposes. These effects are associated with "dead zones" and fish kills. The Principal Investigators are active members of the Wisconsin Initiative on Climate Change Impacts (WICCI) and pledge to share their findings with relevant working groups of WICCI including the Coastal Communities, Water Resources, and Coldwater Fish Working Groups. In addition, the project supports and trains two graduate students and a postdoc, thereby providing for the next generation of researchers in this area.

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