Science Express: Plio-Pleistocene Ice Volume, Antarctic Climate, and the Global δ18 O Record

Although glacial-interglacial cycles of the past 3 million years represent some of the largest and most studied climate variations of the past, the physical mechanisms driving these cycles in ice volume are not well understood. For the past thirty years, the prevalent theory has been that fluctuations in the large land based glacial ice sheets were caused by variations in the amount of solar radiation received at high northern latitudes during the summer melt season. However, a long standing problem is that high latitude summer radiation is controlled by the 23 thousand year period of the precession of the equinoxes, whereas in the interval from ~3 to 1 million years ago the ice volume records are dominated by a period of 41 thousand years, the primary period of variations in the tilt of the Earth’s axis.

In this research article Nisancioglu and his colleagues use a imple model of ice volume change to explore the origins of the mysterious 41 thousand year glacial cycles. The crucial difference between their study and previous attempts is that the model allows for a dynamic Antarctic ice sheet, as supported by sea level data from the time period in question. The researchers propose that from ~3 to 1 million years ago ice volume changes occurred in both the northern and southern hemispheres, each controlled by local summer solar radiation. In this new model, the individual ice sheets are dominated by both obliquity (41 thousand year cycles) and precession (23 thousand year cycles) partly due to the dependence of ice melt on summer temperatures. However, because the response of orbital precession is exactly out of phase between hemispheres, the 23 thousand year cycles in ice volume in each hemisphere cancel in globally integrated proxies for ice volume, leaving the in-phase obliquity (41 thousand year) component of solar radiation to dominate the record.

In the context of global climate change this new theory, which suggests a critical role for the Antarctic ice sheet in past glacial-interglacial cycles, poses the question of the vulnerability of Antarctica in a future warm climate. As argued in the article, Antarctica could have contributed as much as ~10-20 meters of sea level rise during past warm interglacials, as its margins melted back onto the continent.

Reference:
Raymo, Maureen E., Lisiecki, L. E., Nisancioglu K.H. (2006): “Plio-Pleistocene Ice Volume, Antarctic Climate, and the Global δ 18 O Record”, in Science June 22, 2006, DOI: 10.1126