Human-induced climate change is one of the challenges of our time. The increasing global mean temperature, shifts in precipitation patterns, and the rising sea level threaten ecosystems and natural resources, and pose a great risk on society at large. Policymakers need information about the expected impacts, as accurate as possible, in order to make adequate climate change mitigation and adaptation policies. The Greenland Ice Sheet (GrIS) appears to be sensitive to the changing climate. At present, the GrIS is losing mass at an accelerated pace. This is the focus of this thesis. The key terms of the GrIS mass balance are (1) the Surface Mass Balance (SMB) and (2) the ice discharge at glacier fronts. At the surface, the ice sheet gains mass through precipitation and loses mass through meltwater runoff and through sublimation. Ice discharge is a loss term regulated by ice flow. When the mass balance is negative, the ice sheet loses mass contributing to sea level rise. The GrIS, however, is not an isolated environment. It is an integral part of the Earth system. Interactions and feedback mechanisms between the ice sheet and various parts of the Earth’s system affect the ice sheet’s mass loss. The future behavior of the GrIS is a major source of uncertainty in the projections of 21st century sea level rise. The basis for this lies, among other things, in an incomplete understanding of the interactions between the ice sheets and other components the Earth system.@en

Earth system/ice-sheet coupling is an area of recent, major Earth System Model (ESM) development. This work occurs at the intersection of glaciology and climate science and is motivated by a need for robust projections of sea-level rise. The Community Ice Sheet Model version 2 (CISM2) is the newest component model of the Community Earth System Model version 2 (CESM2). This study describes the coupling and novel capabilities of the model, including: (1) an advanced energy-balance-based surface mass balance calculation in the land component with downscaling via elevation classes; (2) a closed freshwater budget from ice sheet to the ocean from surface runoff, basal melting, and ice discharge; (3) dynamic land surface types; and (4) dynamic atmospheric topography. The Earth system/ice-sheet coupling is demonstrated in a simulation with an evolving Greenland Ice Sheet (GrIS) under an idealized high CO₂ scenario. The model simulates a large expansion of ablation areas (where surface ablation exceeds snow accumulation) and a large increase in surface runoff. This results in an elevated freshwater flux to the ocean, as well as thinning of the ice sheet and area retreat. These GrIS changes result in reduced Greenland surface albedo, changes in the sign and magnitude of sensible and latent heat fluxes, and modified surface roughness and overall ice sheet topography. Representation of these couplings between climate and ice sheets is key for the simulation of ice and climate interactions.

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A new set of stratospheric aerosol geoengineering (SAG) model experiments has been performed with Community Earth System Model version 2 (CESM2) with the Whole Atmosphere Community Climate Model (WACCM6) that are based on the Coupled Model Intercomparison Project phase 6 (CMIP6) overshoot scenario (SSP5-34-OS) as a baseline scenario to limit global warming to 1.5 or 2.0&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C above 1850-1900 conditions. The overshoot scenario allows us to applying a peak-shaving scenario that reduces the needed duration and amount of SAG application compared to a high forcing scenario. In addition, a feedback algorithm identifies the needed amount of sulfur dioxide injections in the stratosphere at four pre-defined latitudes, 30<span classCombining double low line"inline-formula">ĝ</span>&thinsp;N, 15<span classCombining double low line"inline-formula">ĝ</span>&thinsp;N, 15<span classCombining double low line"inline-formula">ĝ</span>&thinsp;S, and 30<span classCombining double low line"inline-formula">ĝ</span>&thinsp;S, to reach three surface temperature targets: global mean temperature, and interhemispheric and pole-To-Equator temperature gradients. These targets further help to reduce side effects, including overcooling in the tropics, warming of high latitudes, and large shifts in precipitation patterns. These experiments are therefore relevant for investigating the impacts on society and ecosystems. Comparisons to SAG simulations based on a high emission pathway baseline scenario (SSP5-85) are also performed to investigate the dependency of impacts using different injection amounts to offset surface warming by SAG. We find that changes from present-day conditions around 2020 in some variables depend strongly on the defined temperature target (1.5&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C vs. 2.0&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C). These include surface air temperature and related impacts, the Atlantic Meridional Overturning Circulation, which impacts ocean net primary productivity, and changes in ice sheet surface mass balance, which impacts sea level rise. Others, including global precipitation changes and the recovery of the Antarctic ozone hole, depend strongly on the amount of SAG application. Furthermore, land net primary productivity as well as ocean acidification depend mostly on the global atmospheric <span classCombining double low line"inline-formula">CO2</span> concentration and therefore the baseline scenario. Multi-model comparisons of experiments that include strong mitigation and carbon dioxide removal with some SAG application are proposed to assess the robustness of impacts on societies and ecosystems.

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Spinning up a highly complex, coupled Earth system model (ESM) is a time consuming and computationally demanding exercise. For models with interactive ice sheet components, this becomes a major challenge, as ice sheets are sensitive to bidirectional feedback processes and equilibrate over glacial timescales of up to many millennia. This work describes and demonstrates a computationally tractable, iterative procedure for spinning up a contemporary, highly complex ESM that includes an interactive ice sheet component. The procedure alternates between a computationally expensive coupled configuration and a computationally cheaper configuration where the atmospheric component is replaced by a data model. By periodically regenerating atmospheric forcing consistent with the coupled system, the data atmosphere remains adequately constrained to ensure that the broader model state evolves realistically. The applicability of the method is demonstrated by spinning up the preindustrial climate in the Community Earth System Model Version 2 (CESM2), coupled to the Community Ice Sheet Model Version 2 (CISM2) over Greenland. The equilibrium climate state is similar to the control climate from a coupled simulation with a prescribed Greenland ice sheet, indicating that the iterative procedure is consistent with a traditional spin-up approach without interactive ice sheets. These results suggest that the iterative method presented here provides a faster and computationally cheaper method for spinning up a highly complex ESM, with or without interactive ice sheet components. The method described here has been used to develop the climate/ice sheet initial conditions for transient, ice sheet-enabled simulations with CESM2-CISM2 in the Coupled Model Intercomparison Project Phase 6 (CMIP6).

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The Greenland Ice Sheet (GrIS) mass balance is examined with an Earth system/ice sheet model that interactively couples the GrIS to the broader Earth system. The simulation runs from 1850 to 2100, with historical and SSP5-8.5 forcing. By the mid-21st century, the cumulative GrIS contribution to global mean sea level rise (SLR) is 23 mm. During the second half of the 21st century, the surface mass balance becomes negative in all drainage basins, with an additional SLR contribution of 86 mm. The annual mean GrIS mass loss in the last two decades is 2.7-mm sea level equivalent (SLE) year−1. The increased SLR contribution from the surface mass balance (3.1 mm SLE year−1) is partly offset by reduced ice discharge from thinning and retreat of outlet glaciers. The southern GrIS drainage basins contribute 73% of the mass loss in mid-century but 55% by 2100, as surface runoff increases in the northern basins.@en

The Greenland ice sheet (GrIS) is now losing mass at a rate of 0.7 mm of sea level rise (SLR) per year. Here we explore future GrIS evolution and interactions with global and regional climate under high greenhouse gas forcing with the Community Earth System Model version 2.1 (CESM2.1), which includes an interactive ice sheet component (the Community Ice Sheet Model v2.1 [CISM2.1]) and an advanced energy balance-based calculation of surface melt. We run an idealized 350-year scenario in which atmospheric CO₂ concentration increases by 1% annually until reaching four times pre-industrial values at year 140, after which it is held fixed. The global mean temperature increases by 5.2 and 8.5 K by years 131–150 and 331–350, respectively. The projected GrIS contribution to global mean SLR is 107 mm by year 150 and 1,140 mm by year 350. The rate of SLR increases from 2 mm yr⁻¹ at year 150 to almost 7 mm yr⁻¹ by year 350. The accelerated mass loss is caused by rapidly increasing surface melt as the ablation area expands, with associated albedo feedback and increased sensible and latent heat fluxes. This acceleration occurs for a global warming of approximately 4.2 K with respect to pre-industrial and is in part explained by the quasi-parabolic shape of the ice sheet, which favors rapid expansion of the ablation area as it approaches the interior “plateau.”.

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Ice sheets are a major component of the Earth System, however they are not yet interactively coupled to most global climate models. Here we present past achievements in this front with the CESM1.0 version as well as first results and challenges with the upcoming CESM2.0, where the Community Ice Sheet Model 2.1 is bi-directionally coupled to the atmospheric and ocean components, as opposed to only one-way coupling in CESM1.0. In both CESM versions, the surface mass balance (SMB) is calculated in the land component (CLM) with explicit albedo and refreezing calculations, and downscaled to the ice sheet model resolution via elevation classes and bi-linear (horizontal) and linear (vertical) interpolations. A major highlight of CESM is that the most important coupling process between ice sheet and atmosphere, the albedo feedback, is explicitly modeled, as opposed to state-of-the-art parameterizations of albedo and/or surface melt. Regarding future Greenland ice sheet projections with CESM1, we summarize our results on: 1) doubled end-of-the-century melt rates under RCP8.5 from increased incoming thermal radiation and turbulent fluxes despite decreased incoming solar radiation over Greenland from more clouds, 2) increase in SMB variability due to reduced accumulation to ablation area ration, 3) bimodal emergence of an anthropogenic signal on the SMB due to both increasing melt and snow accumulation, 4) reduction in ice discharge from marginal thinning. We also outline work in progress in preparation for our contribution to the Ice Sheet Model Intercomparison Project 6 (ISMIP6) and paleo-research on the last deglaciation, e.g., on model initialization, improved SMB calculation, evaluation of CESM2.0 climate over Greenland, and parameter optimization of the higher-order CISM2.1. @en