The Mid-Pleistocene Transition (MPT) is the interval of change from the 41 ka obliquity-paced climate cycles to the 100 ka cycles of the late Pleistocene. Marine Isotope Stage [MIS] 31, centered at 1.07 Ma, is an unusual interglacial that lies at the cusp of the MPT. It is still within the 41 ka world, but geologic records have demonstrated that this interglacial was unusually long and unusually warm, especially in Antarctic records, and corresponds with some of the highest high latitude insolation of the last several million years.
The fortuitous coincidence of peak Northern Hemisphere (NH) insolation and peak warmth in paleoclimate records with a geomagnetic reversal (the base of the Jaramillo subchron) allows confident correlation and comparison of high resolution paleoclimate records and calculated insolation at all latitudes around the globe.
Antarctic continental shelf and Southern Ocean records (Scherer et al., 2008) show that MIS-31 was, indeed, remarkably warm. Sea surface temperatures were ∼3-5°C warmer than now and were accompanied by little inshore sea ice and the likely absence of the WAIS. Furthermore, evidence of warmth and ice sheet retreat at ∼1.08 Ma in the Antarctic preceded warmth and ice sheet retreat in the NH by about 10 ka; both peaks directly in phase with local insolation maxima. A coupled GCM-ice shelf-ice sheet model of the MIS-31 orbital configuration (DeConto et al., 2007; Pollard and DeConto, 2008) reproduces the changes in the Antarctic inferred from geologic data.
Kawamura et al. (2007) showed that "orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation." However, our results from early Pleistocene MIS-31 indicate significant warming in the Antarctic and Southern Ocean directly in phase with Southern Hemisphere (SH) high latitude insolation. Thus, in this particular case warming in the SH is shown to lead the NH. So did the MPT mark the suppressed significance of precession paced insolation in the Antarctic, or is MIS-31 simply an anomaly - a paleoclimatic red herring in the debate?
References: DeConto et al., Eos, 352 Abst. PP41F-07 (2007). Kawamura et al., Nature, 448, 912-916 (2007). Pollard and DeConto, Geophys. Res. Abstr. 10, EGU2008-A-04400 (2008). Scherer et al., GRL, L03505 (2008).