The evolution of UK sea-level projections

The methods used to generate global to local sea-level projections have evolved significantly since the publication of the first set of UK national sea-level projections in 2009 (UKCP09; Lowe et al, 2009), including improved process understanding, ice-sheet modelling advances, the use of emulators and development of high-end scenarios. The UK Climate Projections in 2018 (UKCP18) presented local mean sea-level projections for the UK coastline for the 21st century based on CMIP5 models, with an emulator-based methodology to provide traceable extended projections to 2300 (Palmer et al, 2018). First, we present the evolution of UK sea-level projections since UKCP09 and discuss how these projections have been used by UK stakeholders. Second, we compare UKCP18 global and local sea-level projections with those recently presented in the IPCC Sixth Assessment Report (AR6). We find that although the likely range projections (i.e the characterisation of the central two-thirds of the distribution) are broadly similar, larger AR6 contributions from oceanographic processes and the Antarctic ice sheet give rise to discrepancies at selected tide gauge locations of up to 30%. In AR6, high-end scenarios for sea-level rise were presented as low-likelihood high-impact storylines. These offer some comparison with the high-end H++ range presented at 2095 for UKCP09, showing reasonable agreement for London. Future UK sea-level projections would benefit from updated scenarios for high-end sea-level change which extend beyond 2100 as well as an improved understanding of observed sea-level change drivers. This will enhance the usability of these local sea-level projections by UK stakeholder groups and coastal decision-makers.