Even as the sea ice on the Arctic Ocean looks to be headed for a possible record meltback this year, scientistsreported Wednesday that the land-based ice sitting atop Greenland has already melted more than any time in the past 30 years — and that’s with another four weeks left in the melting season. The new study, based on satellite observations, expands on a report in July that showed a record 97 percent of Greenland’s icy surface had undergone at least some melting during the summer season.