Ancient global warming allowed the greening of Antarctica and substantial vegetation sprouted on the frozen continent 15 million years ago, researchers say. Ancient Antarctica was warmer and wetter than previously suspected -- enough to support vegetation, including stunted trees, along its edges -- researchers from the University of Southern California, Louisiana State University and NASA\'s Jet Propulsion Laboratory reported Sunday. Summer temperatures along the Antarctic coast 15 million to 20 million years ago were 20 degrees Fahrenheit warmer than they are today, reaching about 45 degrees Fahrenheit with considerably more precipitation, they said. Scientists began to suspect high-latitude temperatures were warmer than previously believed when LSU researcher Sophie Warny discovered large quantities of pollen and algae in sediment cores taken around Antarctica. \"Deep sea cores are ideal to look for clues of past vegetation as the fossils deposited are protected from ice-sheet advances, but these are technically very difficult to acquire in the Antarctic and require international collaboration,\" Warny said. Fossils of plant life in Antarctica are difficult to come by, scientists say, because the movement of the massive ice sheets covering the landmass grinds and scrapes away the evidence. \"Ice cores can only go back about 1 million years,\" Sarah J. Feakins, USC professor of earth sciences, said. \"Sediment cores allow us to go into \'deep time.\'\" The increased temperatures occurred during a period of global warming in the middle Miocene epoch that coincided with increased levels of atmospheric carbon dioxide, researchers said.