Anderson, Leif SÌý1Ìý;ÌýRoe, Gerard HÌý2Ìý;ÌýAnderson, Robert SÌý3
1ÌýUniversity of Colorado INSTAAR and Department of Geological Sciences
2ÌýUniversity of Washington Department of Earth and Space Sciences
3ÌýUniversity of Colorado INSTAAR and Department of Geological Sciences
Glacial moraines are commonly used to infer climate at the time of moraine formation. But because glaciers respond to year-to-year (interannual) changes in near-surface temperature and snow accumulation, former glacier lengths recorded by moraines can represent ice advances solely due to interannual variability (neoglacial advances?) or with interannual variability superimposed on changes in the mean climate (glacial age advances). We use a linearized glacier model to examine the effect of inherent climate variability on last glacial maximum (LGM), ~21ka, glacier lengths in the Colorado Front Range, USA. Modern climate data from Colorado and global compilations of relevant atmospheric variables are used as model inputs. Not surprisingly, the models confirm that the maximum LGM glacier extents cannot be explained solely by interannual variability around the modern climate, and require a true climate change relative to the present. The models demonstrate that mean glacier lengths in the LGM were likely 15% upvalley from the LGM terminal moraines. This offset between maximum glacier length and mean glacier length implies that using the moraines to infer paleoclimate results in estimates that are ~1°C too cold or ~25% too high in snow accumulation. We also find that small LGM glaciers less than 4 km2Ìýin area were transient features, flicking in and out of their valleys due to interannual climate variability. Finally, we find that the occurrence of terminal moraines, typically described as recessional moraines, ~15% upvalley from maximum LGM moraines can be explained by LGM climate variability and do not require climate changes for emplacement.
Roe, G.H., and O’Neal, M.A., 2009, The response of glaciers to intrinsic climate variability: observations and models of late-Holocene variations in the Pacific Northwest: Journal of Glaciology, v. 55, no. 193, p. 839–854, doi: 10.3189/002214309790152438.
Reichert, B.K., Bengtsson, L., and Oerlemans, J., 2002, Recent glacier retreat exceeds internal variability: Journal of Climate, v. 15, no. 21, p. 3069–3081, doi: 10.1175/1520-0442(2002)015<3069:RGREIV>2.0.CO;2.