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

We examine the response of the Community Earth System Model Versions 1 and 2 (CESM1 and CESM2) to abrupt quadrupling of atmospheric CO 2 concentrations (4xCO2) and to 1% annually increasing CO 2 concentrations (1%CO2). Different estimates of equilibrium climate sensitivity (ECS) for CESM1 and CESM2 are presented. All estimates show that the sensitivity of CESM2 has increased by 1.5 K or more over that of CESM1. At the same time the transient climate response (TCR) of CESM1 and CESM2 derived from 1%CO2 experiments has not changed significantly-2.1 K in CESM1 and 2.0 K in CESM2. Increased initial forcing as well as stronger shortwave radiation feedbacks are responsible for the increase in ECS seen in CESM2. A decomposition of regional radiation feedbacks and their contribution to global feedbacks shows that the Southern Ocean plays a key role in the overall behavior of 4xCO2 experiments, accounting for about 50% of the total shortwave feedback in both CESM1 and CESM2. The Southern Ocean is also responsible for around half of the increase in shortwave feedback between CESM1 and CESM2, with a comparable contribution arising over tropical ocean. Experiments using a thermodynamic slab-ocean model (SOM) yield estimates of ECS that are in remarkable agreement with those from fully coupled Earth system model (ESM) experiments for the same level of CO 2 increase. Finally, we show that the similarity of TCR in CESM1 and CESM2 masks significant regional differences in warming that occur in the 1%CO2 experiments for each model. Plain Language Summary Computer models of the Earth's climate system are complex. Our best guess scenarios for how the climate system will change due to human activity over the next century are also complex. They include estimates of changing greenhouse gas (e.g., CO 2) levels in the atmosphere, aerosol (e.g., smog and haze) emissions, and land use changes (e.g., deforestation and urbanization). To help understand this complex system, the climate modeling community has designed two simplified experiments: abrupt CO 2 quadrupling (4xCO2) and 1% annual CO 2 increase (1%CO2). In these experiments all human-induced factors in the climate system are held constant (at preindustrial levels) except for CO 2 in the atmosphere. Results of these experiments from different climate models can be compared to gain insight into the climate system. We look at two versions of the Community Earth System Model (CESM1 and CESM2). The warming simulated in the 4xCO2 experiment (climate sensitivity) has increased substantially in CESM2. This is related to changes in clouds over the Southern Ocean and tropics. At the same time warming in in the 1%CO2 experiment has not increased. This is related to differences in how CESM1 and CESM2 simulate northern oceans (Arctic, North Atlantic, and North Pacific).

Section: Comparison With 1%co2 Experiments and Relation To Tcrmentioning

confidence: 99%

CO₂ Increase Experiments Using the CESM: Relationship to Climate Sensitivity and Comparison of CESM1 to CESM2

Bacmeister

Hannay

Medeiros

et al. 2020

“…In the first 98 years, the ablation area expands by around 0.1% yr −1 . The anthropogenic‐forced signal emerges from background variability in year 46, when the global mean temperature anomaly is 1.1 K. In a CESM2.1‐only simulation (without an interactive ice sheet) under the same scenario forcing (Sellevold & Vizcaino, 2020), this ablation area signal emerges before the SMB signal due to lower variability. During years 99–192, the rate of expansion triples to 0.3% yr −1 ; by years 131–150, the ablation region makes up 24.2% (4.8 × 10 5 km 2 ) of the total ice sheet area.…”

Section: Resultsmentioning

confidence: 95%

“…The slightly reduced snowfall together with increased rain impacts refreezing, as more rain can be refrozen while the refreezing capacity remains nearly constant. Sellevold and Vizcaino (2020) give a more detailed analysis with spatial maps for a 150‐year CESM2.1‐only simulation under the same scenario forcing but with prescribed GrIS topography.…”

Section: Resultsmentioning

confidence: 99%

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

Muntjewerf

Sellevold

Vizcaíno

et al. 2020

Self Cite

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.

“…CESM2-CISM2 is one of a small number of coupled ice sheet-climate models participating in the CMIP6-endorsed ISMIP6 experiments (Nowicki et al, 2016), with studies of 1,850-2,100 historical and SSP5-8.5 simulations , and of preindustrial and transient 1% CO 2 simulations (Muntjewerf, Sellevold, et al, 2020). These simulations are an important contribution to the international effort to include ice sheets as interactive components of Earth System Models.…”

Section: Discussionmentioning

confidence: 99%

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

Muntjewerf

Sacks

Löfverström

et al. 2021

Self Cite

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.