The Antarctic ice sheet retreated inland millions of years ago, when atmospheric carbon dioxide levels were not that much higher than they are now.

Credit: Jonathan & Angela Scott/Getty

A team led by Richard Levy of GNS Science in Lower Hutt, New Zealand, analysed a drill core of sediment from McMurdo Sound, Antarctica, to reveal climate history between 21 million and 13 million years ago. The greatest ice-sheet shrinkage was seen when CO2 levels were 500 parts per million or more: today's level is about 400 p.p.m., and rising. The researchers conclude that Antarctica (pictured) may respond more quickly to changing CO2 levels than once thought.

A related study by some of the same authors modelled how the Antarctic ice sheet responded to shifts in climate and found similar changes.

Proc. Natl Acad. Sci. USA http://doi.org/bcv4 (2016); http://doi.org/bcv5 (2016)