Constraints on the Dynamic Contribution to 21st-Century Sea Level Rise from Greenland Outlet Glaciers

Abstract

Numerical models currently in use for projections of future ice sheet mass balance lack a mechanistic description of iceberg calving, introducing uncertainty in the future glaciological contribution to global sea level. Constraining dynamic mass loss associated with particular future scenarios can help us parse that uncertainty. We have modified the plastic approximation of Nye (1952) to apply to ocean- terminating glaciers (published derivation: Ultee & Bassis, 2016) and generate physically consistent constraints on dynamic mass loss. Our approach accounts for the interaction of multiple glacier tributary branches (published methods: Ul- tee & Bassis, 2017) and their contribution to sea level. For four large Greenland outlet glacier catchments—Sermeq Kujalleq (Jakobshavn Isbræ), Koge Bugt, Hel- heim, and Kangerlussuaq Glaciers—we find an upper bound of 29 mm on dy- namic contribution to sea level after 100 years of warming. This bound accounts for dynamic loss only and can be summed with surface mass balance projections to bound the total glaciological contribution to sea level from those catchments. The convergence of upper bounds derived from our two strongest forcing scenar- ios agrees with studies that suggest surface mass balance will dominate future mass loss from Greenland. Although our work is motivated by coastal communities’ exposure to rising seas, the constraints we produce here are unlikely to be immediately usable for coastal adaptation. Intermediaries such as extension agents, climate consultants, or re- gional science-policy boundary organizations may be able to tailor our results for use in local adaptation contexts (published commentary: Ultee, Arnott, Bassis, & Lemos, 2018). Understanding the landscape of science intermediation, as well as working directly with stakeholders, can help researchers produce more usable sea level information.