A globally intensified hydrologic cycle is an anticipated consequence of global warming. This has well-defined implications, at least qualitatively, for the net water budget of ocean and land areas. The Great Lakes basin, with 32% water by area, represents a hybrid of oceanic and terrestrial surface types. Simplified arguments for enhanced or diminished long-term mean convergence of water vapor over this region will be presented. These will then be used to assess and compare the results of two different methodologies for predicting the net water supply and lake levels of the Great Lakes. The first method is a direct ingestion of general circulation model (GCM) data into a regional hydrologic model, which generally yields drops in net basin supply and lake levels due to global warming. The second method is the application of a regional climate model, with lateral boundary conditions derived from a GCM; this method has shown increased net basin supply and rises in lake levels.