The role of orbital forcing in the Early Middle Pleistocene Transition

The Early Middle Pleistocene Transition (EMPT) is the term used to describe the prolongation and intensification of glacial–interglacial climate cycles that initiated after 900,000 years ago. During the transition glacial–interglacial cycles shift from lasting 41,000 years to an average of 100,000 years. The structure of these glacial–interglacial cycles shifts from smooth to more abrupt ‘saw-toothed’ like transitions. Despite eccentricity having by far the weakest influence on insolation received at the Earth's surface of any of the orbital parameters; it is often assumed to be the primary driver of the post-EMPT 100,000 years climate cycles because of the similarity in duration. The traditional solution to this is to call for a highly nonlinear response by the global climate system to eccentricity. This ‘eccentricity myth’ is due to an artefact of spectral analysis which means that the last 8 glacial–interglacial average out at about 100,000 years in length despite ranging from 80,000 to 120,000 years. With the realisation that eccentricity is not the major driving force a debate has emerged as to whether precession or obliquity controlled the timing of the most recent glacial–interglacial cycles. Some argue that post-EMPT deglaciations occurred every four or five precessional cycle while others argue it is every second or third obliquity cycle. We review these current theories and suggest that though phase-locking between orbital forcing and global ice volume may occur the chaotic nature of the climate system response means the relationship is not consistent through the last 900,000 years.