Topic: Can we (yet) predict how fast Greenland is going to melt?
Abstract: The Greenland ice sheet has been losing mass in response to the rapid warming of the Arctic, and is contributing to sea level rise at an increasing rate. Fluctuations in ocean and atmosphere circulations are not only affecting the amount of melting and runoff at the ice sheet surface, they also produce acceleration, thinning and retreat of multiple outlet glaciers around Greenland. Numerical models are the best tools to assess the vulnerability of the Greenland ice sheet to climate warming and to make projections of the future of the ice sheet under different scenarios of CO2 emissions. Yet, predicting how fast the ice sheet will be melting has proven to be challenging, primarily because of (1) the poor knowledge of the bed topography and bathymetry in the vicinity of the ice sheet margin and (2) our limited understanding of calving dynamics and (3) knowledge of ocean conditions in the fjords is very limited. Bed topography is indeed a fundamental control on ice dynamics and ocean circulation along Greenland’s periphery, while calving dynamics and ocean-induced melting control the rate of retreat of marine terminating glaciers.
I will first present recent advances made in mapping the bed topography and ocean bathymetry around the periphery of the ice sheet. By assimilating seafloor bathymetry and ice thickness data through a mass conservation approach, we produced a new 150-m resolution bed topography/bathymetric map of Greenland with seamless transitions at the ice/ocean interface: BedMachine v3. I will then show how this new dataset helps in turn improve our ability to model ice front dynamics. I will focus on modeling the response of Northwest Greenland (from 72.5ºN to 76•N) to ocean forcing. Warm and salty Atlantic water, which is typically found at a depth below 200-300 m, has the potential to trigger ice-front retreats of marine-terminating glaciers, and the corresponding loss in resistive stress leads to glacier acceleration and thinning. It remains unclear, however, which glaciers are currently stable but may retreat in the future, and how far inland and how fast they will retreat. We rely on the ice melt parameterization from Rignot et al. 2016, and use ocean temperature and salinity from high-resolution ECCO2 simulations on the continental shelf to constrain the thermal forcing. I then investigate the sensitivity of Northwest Greenland to enhanced ocean thermal forcing and subglacial discharge. The model confirms that ice-ocean interactions are the triggering mechanism of glacier retreat, but the bed controls its magnitude.
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