Sea Level Emulator Intercomparison Project (SLEIP) Phase 1: Assessing emulated multi-century global mean sea level projections

Abstract. Simplified sea level modelling approaches calibrated against reference data, also referred to as sea level emulators, can explore future sea level rise and associated uncertainties with low computational cost. Here, we introduce the Sea Level Emulator Intercomparison Project (SLEIP) to assess available sea level emulators and identify future research needs. The SLEIP protocol has a multi-century focus with 2300 as the end year to explore more of the impact-relevant time horizon and also investigate sea level responses under temperature overshoot. SLEIP covers a total of 13 sea level datasets from the participating emulators BRICK, FACTS (7 individual emulator workflows), FRISIA, MAGICC, MP25, ProFSea and SURFER. The sea level components of participating emulators range from statistical models fitted to projections or observational data to physical parametrizations, to coarse spatio-temporal physical models. They also differ in whether and, if yes, how they account for low-confidence ice-sheet processes. To remove the influence of different climate forcings on the sea level responses, we run the participating emulators with both their native climate forcings and a common forcing from the MAGICC climate model. We provide an analysis of the emulator output for the 2300 projection horizon as well as the historical period. SLEIP fills a gap in the IPCC AR6 sea level assessment by providing 2300 projections for three additional policy-relevant pathways. In the MAGICC-forced configuration, the 2300 probability boxes (p-boxes: mean of medians, lowest 17th to highest 83rd percentile) for total global mean sea level rise relative to 1995–2014 are 0.93 m (0.30–3.15 m) under the 1.5 °C-consistent SSP1-1.9, 1.36 m (0.56–10.85 m) under the strong overshoot scenario SSP5-3.4-OS, and 2.08 m (0.97–11.00 m) under SSP2-4.5, which has been used as a proxy for current climate policies. Native-forced projections generally agree closely with the MAGICC-forced results, indicating that structural differences between sea level models dominate over climate forcing, with the Antarctic ice sheet as the largest source of uncertainty. The relative consistency of thermal expansion and glacier contributions largely reflects shared parametrizations and common calibration datasets rather than independent agreement. Under SSP5-3.4-OS, an overshoot sea level rise penalty of roughly 0.1 to 0.3 m persists by 2300 compared to SSP1-2.6, even after global mean surface air temperature has returned to the SSP1-2.6 level by 2150. This first phase of SLEIP provides a lay of the land of current sea level emulation. Future phases will build on this with ScenarioMIP-CMIP7 scenarios, targeted sensitivity experiments, and a potential extension toward regionally resolved projections.