A two-mile long ice core drilled out of an Antarctic ice sheet shows that levels of the greenhouse gases carbon dioxide and methane are higher now than at any time in the past 420,000 years. The ice-core record is the longest obtained to date. The findings — gathered by a U.S.-Russian-French team at Russia’s Vostok research station, the coldest spot on Earth — were reported in last Thursday’s issue of the journal Nature. “This study is probably the most convincing evidence to date that humans are making some really large changes to the Earth’s climate system,” said Jonathan Overpeck, head of the paleoclimatology program at the National Geophysical Data Center in Boulder, Colo.