Ice Progression (IMAGE) Penn State Caption Modeled Antarctic ice sheet at particular times through the warm Marine Isotope Stage 31 event, around 1.07 million years ago. Ice-sheet elevations and floating ice-shelf thicknesses shown by two different color scales, "yr BP" is "years before present." Credit David Pollard, Penn State Usage Restrictions Non-advertising use only License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.