Sea-level rise and impacts projections under a future scenario with large greenhouse gas emission reductions

Using projections from two coupled climate models (HadCM3C and HadGEM2-AO), we consider the effect on 21st century sea-level rise (SLR) of mitigation policies relative to a scenario of business-as-usual (BAU). Around a third of the global-mean SLR over the century is avoided by a mitigation scenario under which global-mean near surface air temperature stabilises close to the Copenhagen Accord limit of a 2\textdegreeC increase. Under BAU (a variant of the A1B scenario) the model-averaged projected SLR for 2090\textendash2099 relative to 1980\textendash1999 is 0.29 m\textendash0.51 m (5\%\textendash95\% uncertainties from treatment of land-based ice melt); under mitigation (E1 scenario) it is 0.17 m\textendash0.34 m. This reduction is primarily from reduced thermal expansion. The spatial patterns of regional SLR are fairly dissimilar between the models, but are qualitatively similar across scenarios for a particular model. An impacts model suggests that by the end of the 21st century and without upgrade in defences around 55\% of the 84 million additional people flooded per year globally under BAU (from SLR alone) could be avoided under such mitigation. The above projections of SLR follow the methodology of the IPCC Fourth Assessment. We have, however, also conducted a sensitivity study of SLR and its impacts where the possibility of accelerated ice sheet dynamics is accounted for.