Accelerated Greenland Ice Sheet Mass Loss under High Greenhouse Gas Forcing as Simulated by the Coupled CESM2.1-CISM2.1

Abstract

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 CO 2 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."Plain Language Summary Observations show that the Greenland ice sheet (GrIS) has been losing mass at an accelerating rate over the last few decades and is currently one of the main contributors to global sea level rise. To understand the causes of GrIS mass loss, we must consider the Earth system as a whole. This study uses an Earth system model with an interactive GrIS model to explore (1) the extent to which the GrIS responds to warming and (2) the main processes that govern this response. The model is forced with an idealized greenhouse gas scenario in which the atmospheric CO 2 concentration increases by 1% per year until reaching four times the pre-industrial level; the CO 2 concentration is then kept constant for another two centuries. The GrIS responds nonlinearly to climate warming. The GrIS contributes about 100 mm of sea level rise by year 150, when CO 2 is stabilized and the global mean temperature has increased by 5.2 K. By year 350, when global warming has increased to 8.5 K, the GrIS contributes more than 1 m of additional sea level rise. The accelerated mass loss is mostly driven by summertime warming and increased melting of a darkening ice surface.

Global Warming Threshold and Mechanisms for Accelerated Greenland Ice Sheet Surface Mass Loss

“…The SMB of the GrIS decreases with 994 Gt yr −1 in our simulation. In a similar CESM2.1 study, though with a dynamically evolving ice sheet, the SMB decreases with 952 Gt yr −1 by the same time (Muntjewerf et al., 2020). This SMB decrease represents a 2.8 mm yr −1 contribution to global mean sea‐level rise, assuming that the pre‐industrial ice sheet SMB would give no change in sea‐level rise.…”

“…A longer simulation with the same greenhouse gas forcing and a dynamical GrIS is analyzed in Muntjewerf et al. (2020). In the current study, the main focus is on the coupling between the atmosphere and the GrIS SMB, with detailed analysis of SMB and SEB components.…”

Description and Demonstration of the Coupled Community Earth System Model v2 – Community Ice Sheet Model v2 (CESM2‐CISM2)

“…The substantial mass loss between the PI and hCO 2 simulations (−13 Gt yr −1 compared to −1,506 Gt yr −1 ; Table 1 ) reflects rapid GrIS deglaciation in the hCO 2 case. Conversely, ice discharge decreases in the hCO 2 case, as the margin thins and migrates inland from the coast (Muntjewerf, Sellevold, et al., 2020 ).…”

“…In the hCO 2 run, the integrated SMB decreases to −1,293 Gt yr −1 (Table 1 ). The ablation areas expand, and the average ELA rises above 2,000 m. The SMB slightly increases in the high interior regions because of enhanced snowfall (Muntjewerf, Sellevold, et al., 2020 ; Sellevold & Vizcaino, 2020 ).…”

“…Within the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) (Eyring et al., 2016 ; Nowicki et al., 2016 ), this study and a companion paper on model spin‐up (Lofverstrom et al., 2020 ) serve as the primary background documentation for coupled “AOGCM‐ISM” (atmosphere‐ocean general circulation model—ice sheet model) ISMIP6 experiments using CESM2 (Muntjewerf, Petrini, et al., 2020 ; Muntjewerf, Sellevold, et al., 2020 ). Beyond ISMIP6, CESM2‐CISM2 is intended to simulate past, present, and future climates, in which ice sheet dynamics and ice‐sheet/Earth system interactions are considered important aspects of the overall system.…”