Mountain uplift and the glaciation of North America – a sensitivity study

Abstract. The Miocene (~24 to ~5 million years ago) was a period of relative global warmth compared to the Quaternary (~2 million years ago to present; e.g. Zachos et al., 2001) and was characterised by the intermittent glaciation of Antarctica only. Paradoxically, the majority of available proxy data suggest that during the Miocene, pCO₂ was similar, or even lower, than the pre-industrial levels (280 ppmv; Pagani et al., 1999; Pearson and Palmer, 2000; Kürschner et al., 1996, 2008) and at times probably crossed the modelled threshold value required for sustained glaciation in the Northern Hemisphere (DeConto et al., 2008). Records of ice rafted debris and the oxygen isotope composition of benthic foraminifera suggest that at several times over the last 25 million years substantial amounts of continental ice did build up in the Northern Hemisphere but none of these led to prolonged glaciation. In this contribution, we review evidence that suggests that in the Miocene the North American Cordillera was, at least in parts, considerably lower than today. We present new GCM simulations that imply that small amounts of uplift of the North American Cordillera result in significant cooling of the northern North American Continent. Offline ice sheet modelling, driven by these GCM outputs, suggests that with a reduced topography, inception of the Cordilleran ice sheet is prohibited. This suggests that uplift of the North American Cordillera in the Late Miocene may have played an important role in priming the climate for the intensification of Northern Hemisphere glaciation in the Late Pliocene.