Using data from ice cores, Antarctic scientists shared "In the last 2,000 years there was another period around about the 1500s where we also saw a temperature warming similar to the rate of today, but the temperature cooled again. And then at about 400 A. D., we also saw not quite as great a warming, but then it cooled again." At a news conference the scientist stated that the sharp warming trend we're seeing today is quite unusual but not entirely unique. What evidence did the scientist use to support this report?