VIENNA— Scientists have drilled a record-setting ice core stretching back 1.2 million years. The ancient air it contains reveals sharp swings in carbon dioxide that could help explain a mysterious shift in the rhythm of Earth’s ice ages. The core, described this week at the general assembly of the European Geosciences Union here, is the culmination of 10 years of work and 2.8 kilometers of drilling in Antarctica by the European project Beyond EPICA. It provides the first direct, detailed look at how greenhouse gases varied during a critic al climatic window between 800,000 and 1.25 million years ago, when Earth ’ s ice ages shifted from 40,000-year-long cycles to longer, more intense sequences of 100,000 years. “ It ’ s really important,” says paleoclimatologist Edward Brook of Oregon State University.…