#Coupling of Southern Ocean and Antarctica during a past greenhouse

A new study published in Nature Geoscience shows that temperature in the Southern Ocean was more tightly linked to the extent of Antarctic glaciation during past greenhouse climates than previously thought. This affects how we see the complex mechanisms driving climate change around Antarctica, a region that is considered especially vulnerable to future changes.

Around 15 million years ago, in the Miocene, the Earth experienced high global temperatures and a greenhouse climate similar to that expected for the future. The warm period was followed by an abrupt transition towards cooler conditions and an expansion of the Antarctic ice sheet.

Although these changes went along with a drop in atmospheric CO₂ concentrations, it was previously thought that the main reason for the ice sheet growth were changes in the Southern Ocean surrounding Antarctica. This is because previous data suggested a pronounced cooling in that ocean prior to the ice expansion, implying only an indirect role of CO₂ for the ice sheet behavior.
“However, estimating ocean temperatures from the Miocene epoch, millions of years ago, is a major challenge,” says Thomas Leutert, lead author of the new study.
Together, researchers with the Bjerknes Center of Climate Research and the University of Bergen and colleagues from the Max Planck Institute for Chemistry in Mainz, Germany, have applied not just one, but two independent methods for reconstructing temperatures in the upper waters of the Southern Ocean.
Two independent methods—The new results show that ocean temperature in the Southern Ocean cooled in lock-step with the expansion of the Antarctic ice sheet, challenging the previous notion that Southern Ocean surface waters cooled first and thereby triggered ice sheet growth on Antarctica, Thomas Leutert says.
The study is part of his doctoral thesis at the University of Bergen and the Bjerknes Center for Climate Research. Together with his supervisor Nele Meckler, Thomas Leutert studied the composition of tiny shells of microorganisms called foraminifera, found in the sediment cores collected from the Southern Ocean sea floor.