Global warming is expected to drive increasing extreme sea levels (ESLs) and flood risk along the world’s coastlines. In this work we present probabilistic projections of ESLs for the present century taking into consideration changes in mean sea level, tides, wind-waves, and storm surges. Between the year 2000 and 2100 we project a very likely increase of the global average 100-year ESL of 34–76 cm under a moderate-emission-mitigation-policy scenario and of 58–172 cm under a business as usual scenario. Rising ESLs are mostly driven by thermal expansion, followed by contributions from ice mass-loss from glaciers, and ice-sheets in Greenland and Antarctica. Under these scenarios ESL rise would render a large part of the tropics exposed annually to the present-day 100-year event from 2050. By the end of this century this applies to most coastlines around the world, implying unprecedented flood risk levels unless timely adaptation measures are taken.

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Two degrees of global warming above the preindustrial level is widely suggested as an appropriate threshold beyond which climate change risks become unacceptably high. This "2°C" threshold is likely to be reached between 2040 and 2050 for both Representative Concentration Pathway (RCP) 8.5 and 4.5. Resulting sea level rises will not be globally uniform, due to ocean dynamical processes and changes in gravity associated with water mass redistribution. Here we provide probabilistic sea level rise projections for the global coastline with warming above the 2°C goal. By 2040, with a 2°C warming under the RCP8.5 scenario, more than 90% of coastal areas will experience sea level rise exceeding the global estimate of 0.2 m, with up to 0.4 m expected along the Atlantic coast of North America and Norway. With a 5°C rise by 2100, sea level will rise rapidly, reaching 0.9 m (median), and 80% of the coastline will exceed the global sea level rise at the 95th percentile upper limit of 1.8 m. Under RCP8.5, by 2100, New York may expect rises of 1.09 m, Guangzhou may expect rises of 0.91 m, and Lagos may expect rises of 0.90 m, with the 95th percentile upper limit of 2.24 m, 1.93 m, and 1.92 m, respectively. The coastal communities of rapidly expanding cities in the developing world, and vulnerable tropical coastal ecosystems, will have a very limited time after midcentury to adapt to sea level rises unprecedented since the dawn of the Bronze Age.2°warming | coastal sea level rise | probabilistic sea level projections | regional sea level rise T he threshold for dangerous climate change is widely reported to be about 2°C above preindustrial temperature; therefore, international efforts have been generally aimed at keeping average global temperatures below this (1, 2). Fragile coastal ecosystems (3) and increasing concentrations of population and economic activity in maritime cities (4) are reasons why future sea level rise is one of the most damaging aspects of the warming climate (5). Furthermore, sea level is set to continue to rise for centuries after greenhouse gas emissions concentrations are stabilized due to system inertia and feedback time scales (5-7). Impact, risk, adaptation policies, and long-term decision-making in coastal areas depend on regional and local sea level rise projections, and local projections can differ substantially from the global one (6,(8)(9)(10)(11). Coastal sea level projections should also take into account the local vertical land movement that is caused both by glacial isostatic adjustments (GIAs) due to redistribution of masses subsequent to the end of the last ice age (12-14), and subsidence due to groundwater extraction, urbanization, tectonics, and river delta sedimentation rates (15).Global sea level is an integrated climate system response to changes in radiative forcing that alters the dynamics and thermodynamics of the atmosphere, ocean, and cryosphere. In a warming climate, global sea level will rise due to melting of land-based glaciers and ice sheets and from the thermal expansion o...