Asymmetric development of the polar ice sheets changed ice age cycles

The MPT took place around one million years ago over a period of several hundred thousand years and still has an effect on the strength and frequency of ice ages and warm periods that our ancestors were exposed to during the development towards modern humans. During that time, the rhythm of cold and warm periods changed and a new glacial dynamic emerged, characterized by a relatively slow build-up of ice ages and their comparatively abrupt end. While a complete cycle of warm and ice ages lasted around 41,000 years before the MPT, this rhythm slowed down to around 100,000 years after the MPT. “There are various hypotheses as to what could have triggered the MPT,” says Dr Christian Stepanek, co-author of the study and researcher at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “One of these considers the size of the ice sheets in the Northern Hemisphere, which became larger around the time of the MPT. These could then have become more vulnerable to lower frequent changes in solar radiation due to ongoing natural variations in the Earth's axis of rotation and orbit around the sun. Together with downstream processes in the Earth and climate system, it is such changes in isolation that explain the origin and end of ice ages.”

The study, which was led by An Zhisheng from the Chinese Academy of Sciences (CAS), describes a possible cause for the changes in the ice sheets in the Northern Hemisphere during the Pleistocene: according to the study, a slowly intensifying glaciation of the Antarctic could have brought about conditions under which the Arctic was able to freeze over increasingly. “We not only found that the Antarctic ice sheets grew prior to their counterparts in the Northern Hemisphere during the Pleistocene. Using numerical climate modelling, we also show that the increased ice volume in the Southern Hemisphere could have shaped the environmental conditions for the later growth of the ice sheets in the Northern Hemisphere,” says Christian Stepanek. The expansion of the ice sheets and sea ice in the Southern Hemisphere had influence on the dynamics of the oceans and the atmosphere. In particular, they brought about a cooling of northern polar regions and an increased transport of moisture to the north. Together with carbon dioxide-induced feedbacks, this could have led to the growth of ice sheets in the Northern Hemisphere and thus favored the formation of the MPT. The latter overlaps with the expansion of the ice masses in the northern hemisphere around 1.25 million years ago, while the ice sheets in the Southern Hemisphere expanded much earlier according to this study.