The evolution of UK sea-level projections

Abstract

The methods used to generate process-based global and local mean sea-level projections have evolved substantially over the last fifteen years, including improved process understanding, advances in ice-sheet modelling, the use of emulators and further development of high-end scenarios. During this time, two sets of UK national sea-level projections have been generated as part of the UK Climate Projections in 2009 (UKCP09; Lowe et al 2009) and in 2018 (UKCP18; Palmer et al 2018b). UKCP18 presented local mean sea-level projections for the UK coastline for the 21st century rooted in Coupled Model Intercomparison Project Phase 5 (CMIP5) models and in methods used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5), with an emulator-based methodology to provide extended projections to 2300 (Palmer et al 2018a; 2020). We compare UKCP18 global and local mean sea-level projections with those presented in the IPCC Sixth Assessment Report (AR6, IPCC 2021a). We find the likely range projections (characterising the central two-thirds of the distribution) are broadly similar at 2150 to within 0.1 m, except at Edinburgh, where the maximum difference is 0.22 m under medium emissions. Differences arise due to higher contributions from sterodynamic processes and the Antarctic ice sheet, and higher or lower vertical land movement, in AR6 compared to UKCP18. We also compare high-end sea-level rise estimates, presented in AR6 and UKCP09, finding reasonable global and UK local agreement over the 21st century. We explore future paths for UK sea-level science considering both user needs for information and developments in modelling capability. Future UK sea-level projections would benefit from updated high-end sea-level rise scenarios which extend beyond 2100 and continued efforts to build understanding of observed sea-level change drivers. Alongside close collaboration with user groups this would enhance the utility of local sea-level projections by UK coastal practitioners and decision-makers.