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For the second time in a month, leading scientists have closely tied the ancient history of the vast Antarctic ice sheet to a key planetary parameter that humans are now controlling - the amount of carbon dioxide in the atmosphere.

Last month, new research showed that during the Miocene era, some 14 to 23 million years ago, Antarctica gave up huge volumes of ice, equivalent to tens of meters of sea level rise, when levels of carbon dioxide in the atmosphere are thought to have been around 500 parts per million. We're at a little over 400 parts per million now.