PALEOCLIMATIC EVIDENCE THAT FUTURE SEA LEVEL RISE COULD BE LARGER AND FASTER THAN COMMONLY BELIEVED

Current estimates suggest that the recently accelerating rates of sea level rise could yield about 0.5 to 1m of sea level rise within the next one hundred years, with most of this rise resulting from thermal expansion of the ocean rather than ice sheet melting. Geological constraints suggest a more troubling future. First, the evidence (e.g., well-dated corals from tectonically stable locations) is strong that global sea level was 4 to 6m higher than today during the last interglacial period (“LIG” or Eemian or MIS 5e). Second, we have combined a review of the geological evidence with new climate simulations for the LIG. Our simulations, made with a state-the-art NCAR coupled atmosphere-ocean GCM indicate that the Arctic was as warm 130,000 to 128,000 years ago (start of peak LIG sea level high-stand) as the same region will be in about 100 years in the absence of greenhouse gas emission reductions; in contrast, Antarctic summers were not warmer than present at ca. 130,000 years ago. This suggests that Greenland was the source of the LIG sea level rise above present. However, geological (ice core) evidence and ice sheet modeling indicates that a reduced Greenland Ice Sheet can only explain part of the LIG sea level anomaly. We conclude that a large portion of the West Antarctic Ice Sheet also melted at the beginning of the LIG, and that this melting was driven by Northern Hemisphere ice sheet melting coupled with regional ocean warming. This conclusion is supported by a range of theoretical and empirical evidence, and also suggests that future sea level rise could take place at a rate in excess of 1m per 100 years. For several reasons, the LIG sea level rise of 4 to 6m above present should be considered a low-end bound for the magnitude of future sea level rise unless greenhouse gas emissions are much reduced.