Over several years, the US Environmental Protection Agency performed a series of studies, directed by
Congress, examining the potential impacts of climate change on the health and environment of the
United States. The Great Lakes region was one of the four regions on which EPA focused. In the
studies, government and academic scientists examined issues that may be affected by global
warming, including lake levels, ice cover, shorelines, shipping, and lake thermal structure.
The studies utilized three general circulation models (GCMs) for the regional climate data. Though the GCMs do not accurately predict regional climate change, they do provide internally consistent and plausible scenarios of climate change. Thus, the data from EPA studies is dependent to a certain extent upon accuracy of the GCMs, but are also accurate to the extent that the GCMs are internally consistent. The GCMs that were used were the Goddard Institute for Space Studies (GISS), Geophysical Fluid Dynamics Laboratory (GFDL), and Oregon State University (OSU). The models were set up to provide data for carbon dioxide levels doubled over preindustrial levels.
The data provided by the models was not all in agreement. Each model predicted an increase in average temperatures, though they disagreed on the magnitude. They also agreed on increased annual precipitation, though the direction of seasonal changes were not consistent. However, the range of possible changes provided by the three models provides a range of data to examine for predicting regional changes in the Great Lakes area.
Average lake levels were estimated to fall in all of the Great Lakes based on each of the three models. Decline in lake levels would be caused by two main contributors. The first is reduced snowpack, which is virtually certain to occur with higher temperatures. The second is increased evaporation due to increased air temperatures. Increased evaporation is less certain, however, because evaporation is also affected by wind speed and humidity. Changes in these factors might be enough to offset increased air temperatures such that lake levels would even increase. A third, indirect factor affecting lake levels is the prediction that demand for withdrawals of water from the Great Lakes for irrigation and power plant cooling would increase with higher air temperatures.
Ice cover on Lakes Erie and Superior was also significantly affected by hypothetical increased air temperatures. A study by Assel (1990) found that ice cover on Lake Erie could be slightly reduced until 2009, and significantly reduced in later years. Ice cover on Lake Superior could be reduced from about four months to between one and two and a half months a year. The changes in lake levels and ice cover could have large effects on shipping over Erie and Superior as well. Without dredging, ships would be forced to reduce cargoes to sail through channels shallower due to lower lake levels. Decreased ice coverage would lengthen shipping seasons, however. These factors offset each other to a certain extent, but the three models differ on whether the net change in shipping would be positive or negative for ports in Lake Erie.
Changes in air temperature would also affect lake water temperature and water quality. Normally, the Great Lakes stratify to a warmer top layer and a cold lower layer during the summer. Circulation between these layers is limited, and thus nutrient levels in the upper layer are essentially fixed until cooler weather arrives, the layers break up and the lakes become well- mixed. Increased air temperatures could greatly lengthen the stratification times for the various lakes. This would affect circulation patterns in the lakes, and might have major negative ecological impacts.
The results of this EPA study shows how important it is to continue applying research and expanding knowledge to the area of global climate change. Though the effects on the Great Lakes region are mostly indeterminate from the data obtained in this study, they demonstrate that many of the effects are indeed negative. As GCMs are improved and their accuracy on regional scales increases, and as our knowledge of the natural world increases, we need to continue exploring the possible effects of global warming on our environments, and determine whether the predicted changes can wait for us to reply, or whether they are serious enough that they require an immediate response.
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