Reconciling global mean and regional sea level change in projections and observations

Abstract:

The ability of climate models to simulate 20th century global mean sea level (GMSL) and regional sea-level change has been demonstrated. However, evaluation of sea-level projections with observations has not been rigorously tested, as the overlapping period (2007-2018) is short. Here, we evaluate the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) and the latest Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) sea-level projections with satellite and tide gauge observations, in terms of trend and acceleration. After considering the impacts of natural variability and local vertical land motion (VLM), the observed trends from both GMSL and regional weighted mean at 177 tide-gauge stations distributed globally confirm the projections under three Representative Concentration Pathway (RCP) scenarios within 90% confidence level during the overlapping period (2007-2018). Because of the complication of natural variability, we extend the data period for both observations (1970-2018) and sea-level projections (2007-2032) so that they have sufficiently long data length for a robust derivation of the acceleration. The central values of the observed GMSL and regional weighted mean over 1970-2018 imply larger accelerations than that from the RCP2.6 and lie between (or even above) the accelerations for RCP4.5 and RCP8.5 over 2007-2032, but are not yet statistically different from any scenario. The confirmation of the projections with observations gives us confidence on our current understanding of near future sea-level change but leave open questions concerning future non-linear accelerations of ice sheet contributions.