UKCP18 Marine Report

Changes in global and regional sea level arise from a wide variety of geophysical processes that operate on different time and space scales (the sea level "jigsaw puzzle"). Global mean sea level (GMSL) rise occurs from thermal expansion of seawater and the addition of water to the ocean from the loss of land-based ice and water. Changes in land-based ice and water storage result in spatial patterns of regional sea level change through the associated impact on Earth?s gravity field and other effects. Local changes in seawater density and ocean circulation also give rise to a spatial pattern of change, which varies markedly among climate models, and is therefore highly uncertain. In addition, the ongoing response of the Earth system to the last deglaciation brings about a spatial pattern of regional sea level change across the UK that is dominated by the effect of vertical land motion. At local scales, the impacts of coastal sea level change typically arise primarily from extreme water level events. These deviations from the regional mean water level are often associated with storm surges and extreme wave conditions combined with the local tide. The UKCP18 sea level work focuses on 21st century projections of: (i) regional time-mean sea level; (ii) changes in surge extremes; (iii) potential changes in tide and surge characteristics; and (iv) changes in local wave climate. In addition, we present exploratory projections of regional time-mean sea level change out to 2300. All projections are rooted in, or traceable to, CMIP5 climate model simulations under the RCP climate change scenarios.