We produce projections of global mean sea-level rise to 2500 for low and medium emissions scenarios (Shared Socioeconomic Pathways SSP1-2.6 and SSP2-4.5), based on extending and combining model ensemble data from current literature. We find that emissions have a large effect on sea-level rise on these long timescales, with a difference of 0.95 m at 2300 and 1.40 m at 2500 between the medians for the two scenarios. The largest and most uncertain component is the Antarctic ice sheet, projected to contribute median and 5-95% intervals of 0.86 [0.40, 1.57] m by 2500 under SSP1-2.6 and 1.44 [0.68, 2.71] m under SSP2-4.5. We discuss how the simple statistical extensions used here could be replaced with more physically-based methods for more robust predictions. We show that, despite their uncertainties, current multi-centennial projections combined into multi-study projections as presented here can be used to avoid future ‘lock-ins’ in terms of risk and adaptation needs to sea-level rise.

Ensemble-based simulations of future shoreline evolution to 2100, including sea-level rise driven erosion, are performed and analysed  Future shoreline projections uncertainties are initially controlled by modelling assumptions and after 2060 by sea-level rise uncertainties  The choice of wave-driven equilibrium modelling approach and incident wave chronology are critical to future shoreline projections 1 Abstract Most sandy coasts worldwide are under chronic erosion, which increasingly put at risk coastal communities. Sandy shorelines are highly dynamic and respond to a myriad of processes interacting at different spatial and temporal scales, making shoreline predictions challenging, especially on long time scales (i.e. decades and centuries). Shoreline modelling inherits uncertainties from the primary driver boundary conditions (e.g. sea-level rise and wave forcing) as well as uncertainties related to model assumptions and/or misspecifications of the physics. This study presents an analysis of the uncertainties associated with future shoreline evolution at the high-energy, cross-shore transport dominated, sandy beach of Truc Vert (France) over the 21 st century. We explicitly resolve wave-driven shoreline change using two different equilibrium modelling approaches to provide new insight into the contributions of sea-level rise, and free model parameters uncertainties on future shoreline change in the frame of climate change. Based on a Global Sensitivity Analysis, shoreline response during the first half of the century is found to be mainly sensitive to the equilibrium model parameters, with the influence of sea-level rise emerging in the second half of the century (~2050 or later), in both Representative Concentration Pathways 4.5 and 8.5 scenarios. The results reveal that the seasonal and interannual variability of the predicted shoreline position is sensitive to the choice of the wave-driven equilibrium based model. Finally, we discuss the importance of the chronology of wave events in future shoreline change, calling for more continuous wave projection time series to further address uncertainties in future wave conditions.

Sea-level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientist and practitioners, builds on a framework of discussing physical evidence to quantify high-end global SLR for practice. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2 ˚C in 2100 (SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for +5 ˚C (SSP5-8.5) we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2 ˚C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while we emphasize the timing of ice-shelf collapse around Antarctica, which is highly uncertain due to low understanding of the driving processes.