New ideas on high rates of glaciation and deglaciation have suggested changes in currently accepted ideas about the glacial periods and their causes. At the same time, new studies are being done on deep ocean isotope fractionation phenomena. These phenomena are similar to those defining glacial periods, and the new studies have raised questions about paleoclimate analysis for the time span just preceding the glaciers.
The broad variety of explanations for the glacial epochs in the northern hemisphere beginning about 15 million years ago and the lack of sufficient data on the epochs appear to be the result of low precision in correlation between land and ocean methods. Among the many correlations are factors related to the oxygen isotope temperature scale obtained from analysis of marine invertebrate specimens.