Spatial and temporal variability in twenty-first century sea-level changes

Mass loss from polar ice sheets is becoming the dominant contributor to current sea-level changes, as well as one of the largest sources of uncertainty in sea-level projections. The spatial pattern of sea-level change is sensitive to the geometry of ice sheet mass changes, and can deviate from the global mean sea level change due to gravitational, Earth rotational, and deformational (GRD) effects. The pattern of GRD sea-level change associated with the melting of an ice sheet is often considered to remain relatively constant in time outside the vicinity of the ice sheet. However, ice sheet simulations predict that the geometry of ice mass changes across a given ice sheet and the relative mass loss from each ice sheet will vary during the 21st century, producing sea-level changes that are spatiotemporally variable.

To explore this effect, we adopt a sea-level model that includes shoreline migration and GRD effects to calculate time-varying sea-level patterns associated with projections of the Greenland and Antarctic Ice Sheets during the coming century (Golledge et al., 2019; Seroussi et al., 2020; DeConto et al., 2021). We find that in some cases, sea-level changes can be substantially amplified above the global mean early in the century, with this amplification diminishing by 2100. We explain these differences by simulating the contributions from Earth rotation as well as gravitational and deformational effects separately, finding, for example, that ice gain on the Antarctic Peninsula can cause an amplification of up to 2.9 times the global mean sea level equivalent along South American coastlines due to positive interference of GRD effects. To explore the uncertainty introduced by differences in predicted ice mass geometry, we predict the sea level changes following end-member mass loss scenarios for various regions of the Antarctic ice sheet from the ISMIP6 model ensemble (Seroussi et al., 2020), and find that sea level amplification above the global mean between different ice mass projections differ by up to 1.9. This work suggests that assessments of future sea level hazard should consider not only the integrated mass changes of ice sheet, but also the geometry of the ice mass changes across the ice sheets.