The link between the levels of atmospheric carbon dioxide (CO2) and Antarctic ice sheets has come out in yet another study. The latest study published online in the journal Nature found that the formation of ice sheets was triggered when the atmospheric CO2 levels fell below a threshold level of 750 parts per million by volume (ppmv). This happened some 34 million years ago during the Eocene-Oligocene Transition.

Though geologists have long speculated that the formation of the Antarctic ice-cap was caused by a gradually diminishing of the CO2, the greenhouse gas, concrete proof was not available.

Finding the link between atmospheric CO2 levels and the ice sheet formation became possible as scientists studied geologically well preserved samples of planktonic foraminifera across the transition.

Foraminifera are microscopic organisms that have carbonate shells and live in the surface layer of the sea.

The scientists from Cardiff University, U.K, University of Bristol, U.K and Texas A&M University, U.S., used boron isotope-11 analysis of the carbonate shells. The rationale for using boron isotope-11 to understand the palaeo-surface ocean pH is simple.

The amount of boron isotope-11 present in the shells increases as the pH of the ocean surface water increases. Changes in pH of the ocean surface water in turn are governed by the atmospheric CO2.

pH of the ocean surface water increases (becomes alkaline) when CO2 present in the atmosphere becomes less. Water becomes acidic when the pH is below 7 and alkaline when the pH is more than 7. The more the acidity, the less will be the growth of carbonate shells and vice versa. Elevated levels of CO2 seen today have made the ocean acidic.

“Our data show a significant increase in the boron isotope in the period of global cooling,” notes the paper. “The decline in the atmospheric carbon dioxide in our study may have contributed to global cooling and preconditioned the system for explosive ice sheet growth around 33.5 million years ago.”

According to the paper, the possible role of other greenhouse gases in the formation of the ice caps cannot be ruled out.

Though the atmospheric carbon dioxide subsequently increased to levels seen before the Eocene-Oligocene transition, it did not have great impact on the Antarctic ice sheets.

According to the paper, the ice sheets withstood this increase in atmospheric carbon dioxide as the bright surface of the ice cap reflected the sunlight. “Once an ice cap is formed, melting at its margin is compensated by flow from the cold, high altitude interior,” the paper notes.

This was indeed a slight warming due to increase in carbon dioxide levels after the transition and this resulted in partial melting of the ice cap.