An example of climate-induced geological variability is documented shoreline movement of Virginia's coastal islands over the last 140 years. Records of shoreline position and storm frequency show that changes in the latter have had a large impact on the former and also on local vegetation and human inhabitants [ Hayden et al., 1991]. Measurements of changes in lake level from closed-basin lakes reveal the past balance between precipitation and evaporation. Evidence from geomorphic, sedimentologic, paleoecologic, and archeologic records indicate lake levels, and hence effective precipitation, have changed rather dramatically over the last 12-18,000 years [ COHMAP Members, 1988; Roberts et al., 1993]. Records of glacial advance and retreat are also useful climate proxies that reflect high natural variability [e.g., Oerlemans, 1994].
General circulation models (GCM) have been used to explore the effects of geologic variability over large spatial scales. A GCM experiment by COHMAP Members [1988] showed that changes in solar radiation arising from changes in the orientation of the earth's axis has had a pronounced effect on climate over the last 18,000 years. Likewise, a GCM experiment by Kutzbach et al. [1989] indicated that climate is highly sensitive to orogeny on millennial timescales.