A dynamical systems perspective on the Mid-Pleistocene Transition.

Long-term cooling of Earth's climate led to repeated glaciations that from the Pliocene (5.3-2.6 Ma) into the Pleistocene (2.6 Ma-11.7 ka) became increasingly cold and asymmetric. Whether this progressive change in global climate was forced by external drivers, or arose entirely from internal feedbacks, is debated. Much of this debate has centred on the so-called 'Mid-Pleistocene Transition' (MPT, 1.25 Ma to 0.75 Ma), when these changes become most apparent in palaeo-environmental proxies. Here we first establish a context for this period of change by summarising the key characteristics of Plio-Pleistocene climate cycles. We then use simple conceptual dynamical systems models to (1) simulate the asymmetric pattern of Late Pleistocene temperature changes and (2) investigate the mechanism driving increasing asymmetry through the MPT. Our models reveal that climate cycle asymmetry could have arisen from steadily increasing coupling between components of the global climate system. We speculate that through the MPT, increasing synchronisation between Earth system elements, entrained at 100 kyr frequency, allowed internal feedbacks to dominate over shorter-period orbitally-regulated insolation variability. These feedbacks pushed global temperatures towards glacial maximum states far from equilibrium that were terminated by environmental cascades of collapsing earth system connectivity.