Risk estimation of collapse of the West Antarctic Ice Sheet

Complete collapse of the West Antarctic Ice Sheet (WAIS) would raise global sea level by around 5 m, but whether collapse is likely, or even possible, has been `glaciology's grand unsolved problem' for more than two decades. Collapse of WAIS may result from readjustments continuing since the last gl acial maximum, or more recent climate change, but it is also possible that collapse will result from internal flow instabilities, or not occur at all in the present inter-glacial. Such complexity led the Intergovernmental Pan el on Climate Change to conclude in its Second Assessment Report that `esti mating the likelihood of a collapse during the next century is not yet poss ible'. However, a refusal by scientists to estimate the risk leaves policy- makers with no sound scientific basis on which to respond to legitimate pub lic concerns. Here we present a discussion of the likelihood of WAIS-collap se, drawing input from an interdisciplinary panel of experts. The results h elp to summarise the state of scientific knowledge and uncertainty. While t he overall opinion of the panel was that WAIS most likely will not collapse in the next few centuries, their uncertainty retains a 5% probability of W AIS causing sea level rise at least 10 mm/year within 200 years. Since this uncertainty reflects both the unpredictability of the physical system and the scientific uncertainty, it will undoubtedly change as a better understa nding is established.