Future Changes in the Annual Sea‐Level Cycle

Abstract Recent projections indicate that the range of the annual sea‐level cycle (ASLC) may increase, affecting flood risk, groundwater, and ecosystems. However, existing projections have several limitations, such as their exclusion of the inverse‐barometer effect, their uncertainty due to internal variability, and/or their regional focus. Furthermore, the historical ASLC in the climate models used for these projections was not extensively evaluated. Here, we address these limitations using a large ensemble of simulations from the Coupled Model Intercomparison Project 6. Compared to observations, we find that four climate models perform particularly poorly. Excluding these, our multi‐model median projections for 2071–2100 indicate an average increase in the ASLC range at tide‐gauge locations of 7.9% under a high emissions scenario. Changes under lower scenarios are similar spatially but have a smaller magnitude. Regions with above‐average changes include Northwestern Europe, the Mediterranean, Asia, and the North Pacific. In several regions, the inverse‐barometer effect substantially contributes to the total changes. The median projected shifts in the peak month of the ASLC are mostly small, but like for the projected range changes, larger changes cannot be excluded given the substantial uncertainties. Finally, we find that the ASLC changes translate to differences between seasonal and annual mean sea‐level changes of up to 5.4 cm under the highest emissions scenario. Consequently, sea‐level projections that exclude seasonal changes under‐ or overestimate total sea‐level change in specific seasons, sometimes by more than 8%. This will influence projections of hazards and impacts tied to specific seasons.