Melting Greenland ice reveals ancient warm period and threats of future sea level rise

Long-lost ice core shows large parts of Greenland were ice-free about 400,000 years ago, with climate similar to today.
Previous assumptions of Greenland being frozen for millions of years have been overturned by the new ice core evidence.
Past melting during moderate warming suggests Greenland ice sheet is highly sensitive to human-caused climate change.
Complete melting of Greenland ice sheet could raise global sea levels by around 7 meters (23 feet), with severe impacts.
The newly analyzed ice core provides evidence of Greenland's warm, ice-free past and highlights concerns about stability of current ice sheet.