The leaf fossils found at Foulden Maar, in the Otago region of South Island, held evidence of a sharp increase in atmospheric carbon dioxide (CO2) levels associated with a major collapse of the ice-sheet, Waikato University paleoclimatologist Beth Fox said, Xinhua news agency reported.

Her study with US scientists found that changes in the stomatal cells and carbon isotope ratios in the leaves indicated a major increase in the levels of CO2, rising from about 500 parts per million (ppm) to between 750 and 1,550 ppm over a span of less than 10,000 years.

"What surprised us was how such large CO2 fluctuations happened over geologically, relatively short time scales," Fox said.

"We found that atmospheric CO2 levels began to rapidly increase around the same time as the ice-sheet began to decline, and more importantly, even when the CO2 levels dropped back to previous levels, the ice kept on melting. Once the process of destabilisation of the ice-sheet was kick-started, it could keep going by itself."

Some models had shown that at the current rate, the Antarctic ice-sheet might reach a critical tipping point and start destabilising very quickly which had happened before.

"We don't yet know at which point between 500 and 1,550 ppm that destabilisation of the ice took place and we'd also like to look at different plant species to confirm what we've found so far," said Fox.