DMI Report 21-17 Including a dynamic Greenland Ice Sheet in the EC-Earth global climate model

Recent observations have indicated rapidly increasing mass loss from the Greenland Ice Sheet. To explore the interactions and feedbacks of the ice sheets in the climate system, it is important to develop coupled climate-ice sheet models. The integration of an ice sheet model in a global model is challenging, and, currently, relatively few climate models include a two-way coupling to a dynamical ice sheet model. In this work package, we have continued developing the coupled ice sheet-climate model system comprising the global climate model EC-Earth and the Parallel Ice Sheet Model (PISM) for Greenland. The new model system, EC-Earth3-GrIS, is upgraded to include the recent model versions, EC-Earth3 and PISM version 1.2. In addition, a new module has been developed to handle the exchange of information between the ice sheet model and EC-Earth using the OASIS3- MCT software interface. The new module reads output from the ice sheet model and exchanges the fields with the relevant EC-Earth components. The ice sheet mask and topography are provided to the atmosphere and land surface components. The heat and freshwater fluxes from basal melt and ice discharge are provided to the ocean module via the runoff-mapper that routes surface runoff into the ocean. The new module also prepares the forcing fields for the ice sheet model, i.e., subsurface temperature and surface mass balance. These fields are calculated in EC- Earth3 using a land ice surface parameterization, developed explicitly for the Greenland ice sheet. The parameterization contains a responsive snow and ice albedo scheme and includes land ice characteristics in the calculation of heat and energy transfer at the surface. Experiments with and without the land ice surface parameterization have been carried out for preindustrial and present-day conditions to assess the influence of the surface parameterization on the calculated surface mass balance. The results show that the ice sheet responds stronger and more realistically to forcing changes when the new surface parameterization is used. Besides the model development, the results from experiments with the first model version, EC- Earth-PISM, have been analyzed. These results stress that a decent surface scheme with a responsive snow albedo scheme is necessary for investigating mass balance changes of the Greenland Ice Sheet. Overall, our results indicate that the feedbacks induced by the interactive ice sheet have a significant influence on Arctic climate change under warming conditions. In warm scenarios where the CO2 level is raised to four times the preindustrial level, the coupled model has a colder Arctic surface, a fresher ocean, and more sea-ice in winter

The role of an interactive Greenland ice sheet in the coupled climate-ice sheet model EC-Earth-PISM

AbstractIce sheet processes are often simplified in global climate models as changes in ice sheets have been assumed to occur over long time scales compared to ocean and atmospheric changes. However, numerous observations show an increasing rate of mass loss from the Greenland Ice Sheet and call for comprehensive process-based models to explore its role in climate change. Here, we present a new model system, EC-Earth-PISM, that includes an interactive Greenland Ice Sheet. The model is based on the EC-Earth v2.3 global climate model in which ice sheet surface processes are introduced. This model interacts with the Parallel Ice Sheet Model (PISM) without anomaly or flux corrections. Under pre-industrial climate conditions, the modeled climate and ice sheet are stable while keeping a realistic interannual variability. In model simulations forced into a warmer climate of four times the pre-industrial CO2 concentration, the total surface mass balance decreases and the ice sheet loses mass at a rate of about 500 Gt/year. In the climate warming experiments, the resulting freshwater flux from the Greenland Ice Sheet increases 55% more in the experiments with the interactive ice sheet and the climate response is significantly different: the Arctic near-surface air temperature is lower, substantially more winter sea ice covers the northern hemisphere, and the ocean circulation is weaker. Our results indicate that the melt-albedo feedback plays a key role for the response of the ice sheet and its influence on the changing climate in the Arctic. This emphasizes the importance of including interactive ice sheets in climate change projections.</jats:p

initMIP-Antarctica: an ice sheet model initialization experiment of ISMIP6

Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue

InitMIP-Antarctica:An ice sheet model initialization experiment of ISMIP6

Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue

DMI Report 21-23 A prototype of the coupled EC-Earth-PISM model comprising the Antarctic Ice Sheet

A prototype of a coupled climate-ice sheet model has been developed by the work package 1.1.3 "IskappeANT." The coupled system comprises the climate model EC-Earth and the Parallel Ice Sheet Model (PISM), representing Antarctica. Since the direct implementation of the involved processes, such as the implementation of ice shelf geometries, the ocean-ice shelf interaction, or the computation of the surface mass balance, would exceed the funding period of one year, we exploit state-of-the-art parameterizations. However, the robust system is open for enhancements in consecutive steps afterward and allows exploring scientific frontiers. The coupled system is one of the first state-of-the-art global climate models where the climate system interacts with the Antarctic ice sheet and its fringing ice shelves. This ambitious package includes these tasks: infrastructure to run the Parallel Ice Sheet Model (PISM) version 1.1.4 and version 1.2, setup and configuration of PISM to simulate Antarctica as a standalone model, coupling infrastructure, and first coupled simulations. This document describes the design decisions of the coupling. It presents the analysis of the preindustrial climate state in the Southern Ocean and across Antarctica. These states are subject to sufficiently large biases suggesting anomaly coupling between the climate model and the ice sheet model as an adequate coupling strategy

DMI Report 21-33 Strengthening DMI's contribution to CMIP6 and climate change assessment

Assessment of the projected future climate change relies on experiments using complex climate models (i.e., Earth System Models) that can realistically represent the Earth climate system. In this work package we made great efforts to enhance our capability in climate modelling using the family of the EC-Earth3 Earth System Model, and to increase our contribution to the international Coupled Model Intercomparison Project phase 6 (CMIP6). The contributions include performing another set of climate change simulations for the historical period (https://www.dmi.dk/fileadmin/Rapporter/2021/DMI_Report_21-33.pdf1850-2014) followed by four future scenarios (i.e., SSP5-8.5, SSP3-7.0, SSP2-4.5, SSp1-2.6, respectively) using the Earth System Model EC- Earth3-Veg; Extending the climate change simulation under the high-end emission scenario SSP5- 8.5 to year 2300; and performing the new CMIP6-endorsed CovidMIP aiming at assessing the climate impact of the emission reduction due to COVID19 pandemic. Archiving the CMIP6 experiment data following the CMIP6 standards and compliance on the Earth System Federation Grid (ESGF) data nodes is important to ensure the contribution to CMIP6 is available for scientists worldwide to access the data for a variety of applications from assessment and understanding of climate change, to studies of climate impacts and mitigation. We have worked a great deal to prepare the above mentioned and other CMIP6 simulation data for publishing on the DMI's ESGF data node. The processes involve in reformatting the simulation data into the CMIP standard and quality control the reformatted data to ensure their correctness. With our efforts and contributions to CMIP6, we have joined several multi-model multi-member ensemble analyses using the CMIP6 experiment ensembles. We have quantified the future climate changes under variety of future scenarios, and analyzed climate response to the COVID19 related emission reduction. We have also participated in the documentation of the EC-Earth3 model development and performance. These studies have led to a number of scientific papers. We foresee our contributed simulation ensembles, as subsets in the CMIP6 experiment ensembles, have contributed and will continue to contribute to many climate change assessments and studies for many years ahead. www.dmi.d

Uptake of natural and anthropogenic carbon by the Labrador Sea

We apply to Classical Labrador Sea Water (CLSW) the transit-time distribution (TTD) method to estimate the inventory and uptake of anthropogenic carbon dioxide (Cant). A model of TTDs representing bulk-advection and diffusive mixing is constrained with CFC11 data. The constrained TTDs are used to propagate Cant into CLSW, allowing the air-sea disequilibrium to evolve consistently. Cant in the Labrador Sea (LS) surface waters cannot keep pace with increasing atmospheric CO2 and is highly undersaturated. Our best estimate for 2001 is an anthropogenic inventory of 1.0 Gt C and an uptake of 0.02 Gt C/year. By additionally using the constraint of present-day CO2 measurements, we estimate that the preindustrial LS was neutral or a weak source of CO2 to the atmosphere. Our estimates are subject to possible error due to the assumption of steady-state transport and carbon biochemistry. Copyright 2007 by the American Geophysical Union