when did antarctica's permanent ice cap form
Antarctic ice sheet The icing of Antarctica began in the middle Eocene about 45.5 million years ago[6] and escalated during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm[7] and had been decreasing from earlier levels in the thousands of ppm. Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation.[8] The glaciation was favored by an interval when the Earth's orbit favored cool summers but oxygen isotope ratio cycle marker changes were too large to be explained by Antarctic ice-sheet growth alone indicating an ice age of some size.[9] The opening of the Drake Passage may have played a role as well[10] though models of the changes suggest declining CO2 levels to have been more important.[11]