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The most baddest case at Atrantic! Obrigado! Ms Kika Da Silva & Mr. Bill Smith! Badman Nishioka/rainforest action group/HUTAN Group/　 Recently, Deconto and Pollard (2016) included the potential for break-up of Antarctic ice shelves in a dynamical ice model showing that Antarctica could contribute to global mean sea level by up to 114 cm with a 1σ of 36 cm in 2100 relative to 2000 under the Representative Concentration Pathway RCP8.5 scenario.

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LETTER • THE FOLLOWING ARTICLE IS OPEN ACCESS • THE FOLLOWING ARTICLE IS FEATURED ARTICLE/
A high-end sea level rise probabilistic projection including rapid Antarctic ice sheet mass loss
Dewi Le Bars1,4, Sybren Drijfhout1,2,3 and Hylke de Vries1
Published 3 April 2017 • © 2017 IOP Publishing Ltd
Environmental Research Letters, Volume 12, Number 4　/Article PDF

Abstract
The potential for break-up of Antarctic ice shelves by hydrofracturing and following ice cliff instability might be important for future ice dynamics. One recent study suggests that the Antarctic ice sheet could lose a lot more mass during the 21st century than previously thought. This increased mass-loss is found to strongly depend on the emission scenario and thereby on global temperature change. We investigate the impact of this new information on high-end global sea level rise projections by developing a probabilistic process-based method. It is shown that uncertainties in the projections increase when including the temperature dependence of Antarctic mass loss and the uncertainty in the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble. Including these new uncertainties we provide probability density functions for the high-end distribution of total global mean sea level in 2100 conditional on emission scenario. These projections provide a probabilistic context to previous extreme sea level scenarios developed for adaptation purposes.

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1. Introduction
Sea level rise is one of the main consequences of global warming (Arnell et al 2016). Knowing how fast it can develop given a scenario of future greenhouse gas emissions is crucial for both mitigation and adaptation choices (Wong et al 2014). Unfortunately there remains considerable uncertainty. The Antarctic ice sheet is potentially the largest contributor to future sea level rise but also the most uncertain (Meehl et al 2007, Church et al 2013, Levermann et al 2014, Ritz et al 2015, Deconto and Pollard 2016). In 2013 the fifth assessment report (AR5) from the Intergovernmental Panel on Climate Change (IPCC) assessed the likelihood for an extensive grounding line retreat of the Antarctic ice sheet, that would contribute significantly to sea level rise, to less than 34% (Church et al 2013). In such a case, there was a medium confidence that the magnitude would be 'several tenths of a meter'. However, for long term projects that have a high risk aversion, low probability events also need to be taken into account (Veerman 2008, Ranger et al 2013, Hinkel et al 2015). This can be done in different ways: convening an expert committee to develop extreme scenarios (Katsman et al 2011, Ranger et al 2013), conducting a large expert assessment survey (Horton et al 2014) or combining expert assessment of ice sheet contribution (Bamber and Aspinall 2013, de Vries and van de Wal 2015) with climate models projections (Jevrejeva et al 2014, Grinsted et al 2015). It is difficult with these approaches to capture the correlation between ice sheet mass loss and all the other processes. Also important subjective choices are involved in each of these methods (de Vries and van de Wal 2015). Until recently such choices were unavoidable as climate projections with an ice sheet model were either not available at all, or carried out by models that did not include processes that become important when identifying the high end of the distribution. However, as ice sheet models continue to improve and include new processes it has now become timely to carry out a probabilistic assessment of the high end of the distribution. Therefore we propose here an alternative method where extreme mass loss from numerical ice sheet simulations is used in a 'process-based' method (Church et al 2013).

Recently, Deconto and Pollard (2016) included the potential for break-up of Antarctic ice shelves in a dynamical ice model showing that Antarctica could contribute to global mean sea level by up to 114 cm with a 1σ of 36 cm in 2100 relative to 2000 under the Representative Concentration Pathway RCP8.5 scenario. This estimate is much higher than the IPCC AR5 upper bound of the likely range, which was set at 14 cm. Another important conclusion from Deconto and Pollard (2016) is that future mass losses strongly depend on the emission pathway, with an order of magnitude less melt occurring in the RCP2.6 scenario. In particular, Pollard et al (2015) and Deconto and Pollard (2016) argued that ice fracturing of the ice shelves by surface melt (hydrofracturing) and ice cliff failure when cliffs become too high could increase Antarctic mass loss dramatically after they included these effects in their dynamical ice sheet model. These findings need to be taken with caution because they are not confirmed by other studies (Clark et al 2015). Nevertheless the Deconto and Pollard (2016) projections haven't been demonstrated to be unphysical. Therefore it is timely to explore the impact of rapid mass loss from the Antarctic ice sheet on global mean sea level rise and to update the techniques to project future total sea level rise in a high-end scenario.