An ancient crater lake in Central Otago has given a Kiwi scientist a unique insight into how quickly climate change could cause the Antarctic ice sheet to melt.

Fossilised leaves found at Foulden Maar near Middlemarch have been found to contain evidence of a sharp increase in atmospheric CO2 levels associated with a major collapse of the ice sheet 23 million years ago.

University of Waikato paleoclimatologist Dr Beth Fox and colleagues from Columbia University in New York calculated the CO2 levels by studying stomatal cells and carbon isotope ratios in the fossils.

They found CO2 levels rose from about 500 parts per million (ppm) to up to 1550ppm over a geologically short span of less than 10,000 years.

"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," Fox said.

"Once the process of destabilisation of the ice-sheet was kick-started, it could keep going by itself."

Fox said they were surprised at how such a large CO2 fluctuation happened over such a relatively short time.

The research could have important implications for scientists studying current CO2 concentrations and the melting ice in Antarctica.

"But we need to build on this new information by doing more analysis and modelling," Fox said.

"We don't yet know at which point between 500 and 1550 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."

Fossilised leaves found at Foulden Maar near Middlemarch. Photo / Supplied
Fossilised leaves found at Foulden Maar near Middlemarch. Photo / Supplied

Dr Tammo Reichgelt of Columbia University said some models showed the Antarctic ice sheet might reach a critical tipping point and start destabilising very quickly. Now, they have evidence that's happened before.

Fox first had the idea to reconstruct historical CO2 levels from stomata in leaf fossils while studying for her PhD at Otago University, but she didn't follow it up until she completed her doctorate.

She said the study was made possible by the fantastic level of fossil preservation in Foulden Maar, where soft tissues were preserved right down to the cellular level, along with the fact that the sediment contains annual layers, allowing for much more accurate dating than is normal for such an ancient deposit.

"It's an amazing site, with many more stories to tell us about how climate has changed in the past."