New evidence is helping to solve the mystery surrounding a collapse of the Antarctic ice sheet 23 million years ago.

The surface of the East Antarctic ice sheet is so cold that models can only simulate its collapse by applying a significant climatic warming. Yet numerous lines of evidence suggest that 23 million years ago the Antarctic ice sheet decayed in size as changes in Earth's orbit around the Sun drove more subtle changes in Earth's seasons.

Scientists from Cardiff University's School of Earth and Ocean Sciences analyzed fossil "foraminifera". These microscopic animals live in the ocean - on death their shells collect on the seafloor making a geological record of the past.

The team found that while the ice sheet was expanding, more carbon was buried on the sea floor. Later, while the ice sheet decayed, carbon was introduced back into the oceans. It seems the subtle changes in Earth's orbit around the Sun were amplified by a positive feedback effect, most likely involving the greenhouse gas carbon dioxide.

Dr. Caroline H. Lear  said, "Understanding what caused dramatic collapses of the Antarctic ice sheet in the past is necessary to test the accuracy of current ice sheet models."

Elaine M. Mawbey and Caroline H. Lear (corresponding), School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK. First published on 3 July 2013,; open access online.