The intensifying global warming is increasingly affecting our planet. The Arctic and Antarctic are showing serious changes with far-reaching consequences for the global climate system. While the Greenland ice sheet and the sea ice in the Arctic are melting comparatively quickly today, the ice in the Antarctic is reacting more slowly and with a delay to climate change. Asymmetrical developments of the ice sheets have also occurred in the Earth's past. An international research team has now shown for the first time how such an asymmetric development of the northern and southern polar ice masses could have triggered one of the most important changes in recent Earth history in the Pleistocene, around 2 million years before today: the so-called mid-Pleistocene Transition (MPT). The Alfred Wegener Institute was involved in the study, which was published in the journal Science.
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.
“What's new about our approach is that we show the development of ice volume in the hemispheres separately during the Pleistocene,” adds Gerrit Lohmann, co-author of the study. By linking geological records with numerical climate simulations, in which the AWI researchers played a key role, the study illustrates how processes and developments in the Southern Hemisphere could have controlled conditions in the Northern Hemisphere that were favorable for the build-up of ice sheets and for the formation of the MPT. “For me personally, it was impressive how well the ice sheet volumes reconstructed in our study for the two hemispheres matched the total global ice sheet volume previously derived using other methods,” says Christian Stepanek.
The results clearly show the profound effects of the asymmetric development of the polar ice sheets on the global climate, especially on the climate of the Northern Hemisphere. They illustrate that changes in the ice masses in the north and in the Antarctic, which are spatially widely separated, can nevertheless influence each other and lead to deviations in the global climate. “This conclusion could be important for today's climate change, in which the Northern Hemisphere has so far been ahead in its response to global warming, but the Southern Hemisphere now seems to be catching up in terms of the decline in the cryosphere," says Gerrit Lohmann.
Original publication:
An Zhisheng, Weijian Zhou, Zeke Zhang, Xu Zhang, Zhonghui Liu, Youbin Sun, Steven C. Clemens, Lixin Wu, Jiaju Zhao, Zhengguo Shi, Xiaolin Ma, Hong Yan, Gaojun Li, Yanjun Cai, Jimin Yu, Yuchen Sun, Siqi Lis, Yu'ao Zhang, Christian Stepanek, Gerrit Lohmann, Guocheng Dong, Hai Cheng, Yu Liu, Zhangdong Jin, Tao Li, Yifei Hao, Jing Lei, and Wenju Cai: Mid-Pleistocene climate transition triggered by Antarctic ice sheet growth, Science, RAabn4861/AA/ATMOS, in press. DOI: 10.1126/science.abn486.