Subtracting the GIA signal from GRACE data would not be a problem if we knew exactly how the ice sheets had changed in the past thousands of years and exactly how the Earth responds to such changes. However, we don’t. The greatest impediment is the uncertainty in reconstructing the post-LGM Antarctic ice sheet. Unlike the formerly-glaciated regions of the Northern Hemisphere, 98% of Antarctic bedrock remains covered by ice and the ice sheet edge is fringed by extensive ice shelves; this hampers the collection of data on ice history and introduces substantial uncertainty in reconstructions. Indeed, previous estimates of the extra volume of Antarctic ice at the LGM, compared to present-day, range from 3 to >30m of equivalent sea level (Bentley, 2010).

These results can be tested directly against global positioning system (GPS) observations of bedrock uplift. These are measurements of how the Antarctic bedrock is moving vertically. The spatial distribution of sites is limited at present – because they need to be placed directly on rocky outcrops – and there are only a few such sites in East Antarctica. In addition, some of the West Antarctic GPS records are campaign-based rather than continuous and hence are less precise than we would like – although this is being tackled through the POLENET programme.