THE CAUSE OF THE LATE CENOZOIC NORTHERN-HEMISPHERE GLACIATIONS - A CLIMATE CHANGE ENIGMA

The ultimate cause of the onset of glaciations remains elusive, but in the case of northern hemisphere glaciation it is probable that several factors acted in combination. General global cooling resulted from reduction of atmospheric CO2 by weathering of silicate rocks exposed by erosion of late Cenozoic uplifts. Uplifts in south Asia, southwestern North America and Scandinavia occurred at distances appropriate for the generation of quasi-permanent Rossby waves in the atmosphere. The resulting winds, given suitable moisture sources, were favourable for causing large-scale precipitation at mid-latitudes on the northern continents. Moisture sources were provided by the closure of the Central American isthmus. Gulf Stream flow increased, carrying warm subtropical waters to high latitudes. The Denmark Strait deepened permitting greater outflow of deep water from the Norwegian-Greenland Sea. The relative importance of each of these factors should be investigated by additional atmospheric and ocean climate model sensitivity studies.