A new framework to explore high-end sea-level rise for the UK: updating H++

The UK high-plus-plus (H++) scenario for high-end sea-level rise is used in sensitivity testing for significant infrastructure (e.g. nuclear facilities) and forms part of the Environment Agency planning guidance in England. However, the existing H++ scenario, developed as part of the UK Climate Projections in 2009 (UKCP09), does not reflect the latest science knowledge on ice sheet instability processes and has limitations, as revealed in consultations with users of this information. We outline a new H++ framework to inform coastal planning and decision-making. The first step involves users screening decisions using an updated H++ scenario that reflects major scientific advances since UKCP09. For decisions found to be sensitive to high-end sea-level rise in the screening step, the second step involves users evaluating adaptation options and damage costs against a wider library of alternative, plausible storylines. Our H++ screening scenario is based on the Intergovernmental Panel on Climate Change Sixth Assessment Report low-likelihood high-impact sea-level rise assessment. In response to stakeholder needs, all storylines within the H++ framework provide time-continuous, geographically-specific sea-level rise projections to 2300 and information on rates of sea-level rise.