In two new studies, scientists say that the vast ice continent of Antarctica seems to have given up tremendous volumes of ice — even sprouting considerable plant life — during an era over 10 million years ago when concentrations of atmospheric carbon dioxide don’t seem to have been all that much higher than they are now.
That period was known as the Miocene. And during its early and middle phases, between 23 and 14 million years ago, carbon dioxide concentrations are believed to have sometimes reached around 500 parts per million or somewhat higher — not so very far from the 400 parts per million (and rising) where we stand today.
During this same era, finds the research, the continent is believed to have lost volumes of ice equivalent to tens of meters of sea level rise around the globe. Overall, Antarctica currently contains enough ice to raise seas by some 60 meters, or 200 feet, were it to melt entirely.
“It’s doing it during this time interval when CO2 levels are not all that high,” says David Harwood, one of the study’s authors and a geologist at the University of Nebraska, Lincoln. “It’s showing that dynamic behavior of advance and retreat under pretty modest CO2 concentrations.”
The paper therefore infers that there the ice sheet may be quite sensitive to atmospheric carbon dioxide at levels not far from where we are now. Or as the scientists put it in their final sentence: “Given current atmospheric Co2 levels have risen above 400 ppm and are projected to go higher, paleoclimate reconstructions such as this one for the early to mid-Miocene imply an element of inevitability to future polar warming, Antarctic ice sheet retreat, and sea level rise.”