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]
Antarctica Antarctica (UK English /ænˈtɑːktɪkə/ or /ænˈtɑːtɪkə/, US English /æntˈɑːrktɪkə/ ( listen))[note 1] is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14,000,000 square kilometres (5,400,000 square miles), it is the fifth-largest continent. For comparison, Antarctica is nearly twice the size of Australia. About 98% of Antarctica is covered by ice that averages 1.9 km (1.2 mi; 6,200 ft) in thickness,[5] which extends to all but the northernmost reaches of the Antarctic Peninsula.