An exceptional melting of the Greenland Ice Sheet in summer 2012 was the result of unusual changes in atmospheric jet stream circulation, scientists say. An international team led by researchers at Britain\'s University of Sheffield, using computer modeling and satellite data, said they determined more than 90 percent of the ice-sheet surface melted by July 11, 2011, a record melt for the last 50 years. \"The GrIS [Greenland Ice Sheet] is a highly sensitive indicator of regional and global climate change, and has been undergoing rapid warming and mass loss during the last 5-20 years,\" Sheffield geology Professor Edward Hanna said. \"Our research found that a \'heat dome\' of warm southerly winds over the ice sheet led to widespread surface melting,\" he said in a Sheffield release Monday. In comparison, ocean temperatures and Arctic sea-ice cover were relatively unimportant factors in causing the extra Greenland melt, the researchers said. The unusually high melt in 2012 may have been an anomaly, Hanna said. \"According to our current understanding, the unusual atmospheric circulation and consequent warm conditions of summer 2012 do not appear to be climatically representative of future \'average\' summers predicted later this century.\"