Varying amounts of glacial debris in a core of ancient sediment show the ice cover grew and shrank until airborne carbon dioxide levels fell below 600 parts per million, spurring steady growth.

The Antarctic ice cap can withstand only so much carbon dioxide in the atmosphere, and scientists might have found this limit. In a new paper published today in Science, researchers present the first physical evidence that conditions about 33 million years ago may have led the vast southern ice sheet to become a massive, stable block.

“What we see now is that the strengthening of the ice sheet works only with CO2 levels below 600 ppm.” A million or so years before then, the Earth was much hotter than it is today, and atmospheric carbon dioxide (CO 2 ) concentrations exceeded 750 parts per million (ppm)—much higher than they are today. But when CO 2 concentrations declined to 600 ppm, the southern ice cap became much more long-lived.

“What we see now is that the strengthening of the ice sheet works only with CO 2 levels below 600 ppm,” said Simone Galeotti, a paleoclimatologist at the University of Urbino, Italy, and lead author on the paper. He added that if we reach those levels again by continuing to pump greenhouse gases into the atmosphere, humans could sentence the ice sheet to an inevitable collapse.

Icehouse Planet

When the Antarctic ice cap started forming about 35 million years ago, it was a tumultuous time in Earth’s past during which a lot of carbon, mainly in the form of carbon dioxide, quickly left the atmosphere. Among several factors contributing to this carbon shift, the Antarctic continent peeled away from South America and Australia, forming the Southern Ocean. Now unencumbered by land, strong westerly winds were free to blow around Antarctica, pushing water northward and pulling water from great depths.

Upwelling of this deep, nutrient-rich water led to a flurry of biological activity in the Southern Ocean for the first time. Blooms of phytoplankton pulled CO 2 from the atmosphere as organisms made their shells. As the organisms died, this carbon became trapped in the ocean.

This sinking of carbon into the ocean helped to plunge the Earth into an “icehouse” phase—as opposed to its previous “greenhouse” phase—and glaciers on Antarctica bloomed.

Clues in Ocean Sediment

To investigate the behavior of Antarctica’s ancestral ice sheet, the researchers turned to a sediment core bored into the bottom of the Ross Sea 16 years ago. The core contains many layers, and the isotopic composition of these layers reflects Earth’s natural heating and cooling phases. Known as Milankovitch cycles, those phases result from changes in Earth’s orbit and the tilt of its rotational axis.

The core also contains evidence of ice advance and retreat, both in the types of sediment present and in patterns within the layered material indicating what sort of erosion took place at the time. In the layers corresponding to ice advance, the researchers found larger pieces of sediment, or clasts, that the growing ice transported. In the layers corresponding to ice retreat, the researchers found more marine sediment and other types of features typical of a continental shelf lacking ice above.

The researchers compared the layers in the sediment core with an already existing data set of CO 2 concentrations taken from ancient plankton shells in marine sediment so they could pinpoint the state of the ice sheet as CO 2 levels changed. When CO 2 concentrations were above 750 ppm, around 35 million years ago, some glaciers likely existed at high elevations but they didn’t extend to the ocean. But when CO 2 concentrations fell between 750 and 600 ppm, the fledgling ice sheet was unstable and highly responsive to the Milankovitch cycles, extending and retreating over 20,000- to 40,000-year cycles, the team reported.

When the CO 2 concentration reached 600 ppm 32.8 million years ago, the number of cycles of advance and retreat dropped, implying the existence of a widespread, stable ice sheet that was less responsive to Milankovitch cycles and that extended and retreated on 100,000- to 400,000-year timescales.

“When CO 2 dropped below 600 ppm is when we see first evidence of the ice sheet expanding and growing into the ocean and onto the continental shelf,” said coauthor Timothy Naish, a glaciologist and director of the Antarctic Research Centre at the Victoria University of Wellington in New Zealand.

“This paper strengthens our understanding that CO 2 affects climate including ice and sea level, that smaller ice masses respond more rapidly, and that large ice-sheet changes affect other aspects of the climate and in turn influence themselves,” said Richard Alley, a glaciologist at the Pennsylvania State University in University Park, who wasn’t involved in the new research.

Antarctica’s Future

Today’s research reveals that the entire ice sheet could revert back to its unstable roots if the 600-ppm threshold is crossed once again. The current concentration of atmospheric CO 2 sits well below this threshold; it recently surpassed 400 ppm. Although recent research has found signs that the West Antarctic ice sheet is already in irreversible decline, today’s research reveals that the entire ice sheet could revert back to its unstable roots if the 600-ppm threshold is crossed once again—like replaying the ice sheet’s evolution backward through time.

Models run by the Intergovernmental Panel on Climate Change show that this threshold could be reached by the end of the century. However, Naish emphasized that even if we were to reach this crucial threshold, it would be thousands of years before the ice sheet melted completely. Although many other greenhouse gases contribute to warming, he added, they would contribute little to this potential deterioration of the Antarctic ice cap because they remain so briefly in the atmosphere compared with CO 2 , which can last for centuries.

At the 600-ppm threshold, Naish continued, we start to commit to continent-wide ice loss that “we can’t stop.”

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2016), Scientists find the point of no return for Antarctic ice cap, Eos, 97, doi:10.1029/2016EO047929. Published on 10 March 2016.