Greenland ice sheet freshwater implementation in NorESM (Deliverable D5.8)

Earth system models are used for a wide variety of experiments aimed at understanding how different components (atmosphere, ocean, land, ice sheets, etc) interact with each other. These interactions can be studied under past, present and future climates. In particular, Earth system models forced by a range of plausible socioeconomic scenarios in the coming century form the basis of future climate projections within the Climate Model Intercomparison Project (CMIP). There are many potentially important feedbacks between model components, depending on time scales and climate state. One such feedback is between ice sheets and climate: the climate influences the magnitude and distribution of freshwater fluxes discharged from ice sheets, while these freshwater fluxes in turn enter the ocean and influence ocean circulation and climate. Past studies have found both positive (self-amplifying) and negative (self-limiting) feedbacks related to ice sheet freshwater fluxes. Although Earth system models could be the ideal tool for quantifying these climate-ice sheet feedbacks, technical difficulties in the computational implementation of interactions (coupling) between an ice sheet and the rest of the Earth system have necessitated some important compromises in how ice sheets are represented. Detailed knowledge of this implementation is thus needed before designing related experiments. This deliverable describes how the Greenland Ice Sheet and its associated freshwater fluxes are represented within two versions of the Norwegian Earth System Model (NorESM2). In one version, both the Greenland and Antarctic ice sheets are represented in the simplest way possible (effectively, as snow-covered mountains). This is the version used for the NorESM2 contribution to the latest CMIP phase (CMIP6). In the second version, the Greenland Ice Sheet is represented by a three-dimensional ice flow model that interacts with other model components, while the Antarctic Ice Sheet remains as a snow-covered mountain. This version is currently under development and is likely to participate in the upcoming CMIP7. We finally provide some examples of potentially important differences between the NorESM2 Greenland Ice Sheet and a real ice sheet.