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GLERL 2003 Milestone Reports
GLERL 2003 Milestone Home
GOAL 1: Protect, Restore, and Manage use of Ocean and Coastal Resources through Ecosystem Management Approaches
OBJECTIVE: Increase number and accuracy of forecasts of significant ecological events and trends (e.g., harmful algal blooms, coral bleaching, and population shifts).
Specific Strategy: Assess and predict climate and ecosystems changes over decades by developing models to detect and forecast these changes.
Milestone: Complete analysis of regional climate modeling and Great Lakes basin hydrology for increasing CO2 scenarios.
Scientist: Brent Lofgren
Purpose: The net basin supplies of water for each of the Laurentian Great Lakes (incorporating the inputs of water from runoff and over-lake precipitation and the output to evaporation) are an intermediate result that will yield projected lake levels and outflows into rivers that drain the lakes. The lake levels and outflows of the Great Lakes are important for shippers on the lakes as well as on the St. Lawrence River, along with residents along the lakeshores, recreational boaters, municipal water supplies, and other uses. The efforts leading to this milestone add a new simulation in seeking to quantify the effects that future increases in greenhouse gases may have on average net basin supply and lake levels.
The new model simulations feature coupling between the atmospheric component of the model and the lake (1-dimensional thermodynamic model) and land surfaces. This is in contrast to previous studies for the same purpose, which featured only one-way coupling, with atmospheric models forcing hydrologic models. Two-way coupling accounts not only for the what happens to the surface based on projected changes in the atmosphere, such as changes in air temperature and precipitation, but also accounts for the reverse effects, such as changes in evapotranspiration (input of water vapor to the air) and air temperature moderation by lakes. These latter effects can have a significant influence on the regional climatic system.
Efforts: Model runs using the Coupled Hydrosphere-Atmosphere Research Model (CHARM) to simulate the regional climate of the Great Lakes basin are complete for 10-year time periods centered at 1989, 2030, and 2095. The resultant projected changes to future net basin supplies to the lakes contrast sharply with the results from uncoupled simulations of the Great Lakes hydrology as forced directly by general circulation models (GCMs). Several factors contribute to this difference. One factor accounted for by a coupled model but not an uncoupled one is a trend toward a moister atmosphere overlying warmed lakes that inhibits evaporation from the lakes in the future cases. Another is that evaporation from the lakes can be transferred into precipitation within the drainage basin. A third is an increase in precipitation in the future that is likely to be concentrated in very heavy precipitation events. Because the uncoupled model multiplies all precipitation events in a given month by a fixed ratio, heavy precipitation events are only marginally increased under that method.
Figure 1: Simulated net basin supply for the Lake Superior basin under historical and projected greenhouse gas concentrations.
The figure 1 shows the general increase in net basin supply for the basins of Lakes Superior. For Lake Superior, this increase occurs especially during the winter months (reduced negative net basin supply). This shift may occur at the expense of summertime net basin supply. A shift of runoff toward the winter and early spring is a characteristic that is shared in many simulations of future climate in areas that experience an accumulation of snowpack and peak runoff at the time of snowmelt. The snowpack, as a source for replenishment of soil moisture and groundwater, is spent earlier in the season, which can reduce runoff in the later spring and summer. However, in the cases of Lakes Michigan and Erie, increased precipitation maintains higher net basin supply through most of the summer.
An opposing factor (it would tend to reduce net basin supply in the future) that is currently not well handled by CHARM is the ice cover influence on evaporation. An expectation of reduced ice cover under greenhouse warming conditions would allow greater evaporation from the lakes during late winter. However, because of a wintertime warm bias in the CHARM model, there is little ice formation on any of the lakes even in the 1989 model case, thus negating this effect. Despite this and some other shortcomings of CHARM, it should be regarded as more faithfully depicting the feedbacks that are inherent within the earth-air-water hydrological system than an uncoupled model.
Customers: The results of this study in terms of net basin supply, lake levels, and lake outflows are of interest to all users of the Great Lakes. These include riparians who own shoreline property, shippers, municipalities that draw water from the lakes, recreational boaters, and interests in nearshore wetland and other aquatic habitats. These customers are interfaced to scientific results through a number of institutions, including state, provincial, and local governments, the International Joint Commission, the Great Lakes Commission, Sea Grant, Environment Canada, the Council of Great Lakes Industries, tribal authorities, and non-governmental environmental advocacy groups.
Significance: This study is not the final word on the topic of global climate change effects on Great Lakes levels, but it is significant in re-opening this question based on something beyond using the uncoupled methodology as applied to the newest general circulation model. It is hoped that time will bring improvements to the performance of CHARM and that a new consensus will be reached about the direction and magnitude of change in Great Lakes net basin supplies and water levels due to global warming. As time goes by and computing power increases, longer model runs can be undertaken and questions about changes temporal variability of climate as simulated by models can be addressed.
Success: This study represents a stepping-stone on the way to more reliable regional climate modeling, with more features. Its results contrast to those of previous studies, thus placing the entire question of global warming impacts on Great Lakes water supplies into a state of uncertainty, and opening it up for further significant study
Next steps: Further development of CHARM will help to add to its realism. Planned enhancements include the inclusion of a 3-dimensional dynamical lake model, likely with a flux adjustment scheme to reduce temperature biases in the water. Also, an adjustment to the procedure for off-line simulation of hydrologic effects is being planned, to better take into consideration the anticipated concentration of precipitation into intense events.
Last updated: 2003-11-03 mbl