Greenland ice sheet climate disequilibrium and committed sea-level rise

The Greenland ice sheet's contribution to sea-level rise (SLR) is substantial and continues to grow. Accurate prediction of its future response to climate change is crucial for coastal communities and global climate projections. Current process-based ice sheet models, while sophisticated, face limitations in accurately representing complex interactions between the ice sheet, atmosphere, and ocean. These limitations include uncertainties in atmospheric and oceanic forcing data and the representation of intricate processes like ice flow dynamics, calving, and subglacial drainage. This uncertainty hinders precise predictions of Greenland's future contribution to sea-level rise. To address this, the study proposes a complementary approach that focuses on quantifying the committed SLR contribution arising from the ice sheet's current disequilibrium with the recent climate. This disequilibrium approach leverages readily available observational data to assess the minimum ice loss already locked in due to past climate changes, providing a valuable lower-bound estimate of future sea-level rise that complements the results from process-based ice sheet models.

Existing literature extensively documents the Greenland ice sheet's mass loss and its contribution to sea-level rise. Studies using satellite observations (e.g., GRACE, ICESat) reveal accelerating ice loss over the past decades. Process-based ice sheet models, which incorporate complex physical processes, have been used to simulate the ice sheet's response to various climate scenarios, but they face challenges related to parameterization of sub-grid scale processes and uncertainties in climate forcing data. Previous work has explored the concept of committed sea-level rise, which refers to the inevitable sea-level rise that will occur even if climate change were to cease immediately. However, these studies often lack a precise quantification of committed SLR from the Greenland ice sheet, especially concerning the uncertainties inherent in ice sheet dynamics. The present study bridges this gap by providing a more robust estimate of the committed SLR from Greenland based on observed disequilibrium between the ice sheet geometry and recent climatic conditions.

The study adopts a novel approach that calculates the committed changes in ice sheet area and volume resulting from the current ice sheet's disequilibrium with the climate. Unlike process-based models that solve transient ice flow equations, this approach focuses on determining the ice extent and thickness adjustments needed to achieve equilibrium with the surface mass balance (SMB). Changes in ice flow dynamics are implicitly accounted for through a glaciological power-law scaling function, which relates changes in ice extent to changes in ice volume. To address the complexity of marine-terminating sectors and tidewater outlets where ablation is influenced by iceberg calving, an effective ablation area treatment is introduced. The method utilizes readily available empirical data, including multi-year inventories of tidewater glacier discharge, SMB (snowfall accumulation minus run-off) from observational reanalysis and regional climate data, and the accumulation area ratio (AAR) – obtained from optical satellite imagery. The AAR, the ratio of the glacierized area with net annual mass gain to the total glacierized area, serves as a key indicator of the ice sheet's equilibrium state. The disequilibrium approach exploits the fact that climatically driven SMB perturbations are much faster than the associated dynamic adjustments of the ice mass. The equilibrium line altitude (ELA), which separates accumulation and ablation zones, is used to track changes in the AAR. The ratio between observed AAR and the equilibrium AAR (AAR_eq) quantifies the fractional imbalance (α), which determines the area perturbation needed for the ice mass to reach equilibrium. The adjustment in ice volume (ΔV) and the committed eustatic SLR are then derived using a glaciological scaling theory. While a power-law exponent of 1.24 is observed, a linear exponent of 1 (γ=1) is conservatively used to encompass potential flow interactions between adjacent ice sheet sectors, ensuring a mathematically sound lower bound for committed ice loss. The method is applied to Greenland's entire glacierized area by summing over multiple sectors, providing lower-bound estimates of committed mass loss and resulting SLR. The analysis is performed for three climate scenarios: the average 2000-2019 climate, the high-melt year of 2012, and the low-melt year of 2018. Each scenario is applied in perpetuity to assess the committed ice loss under different climate conditions. Uncertainty analysis is conducted using an ensemble approach that considers variations in SMB from multiple models, albedo, and AAR. The analysis includes detailed regional breakdowns of ice imbalance and committed sea-level rise.

The study's key findings reveal a significant commitment to sea-level rise from Greenland's ice sheet, even under current climate conditions. Using the average 2000-2019 climate as a baseline, the ice sheet is found to be significantly out of equilibrium, committing at least 274 ± 68 mm of global eustatic SLR. This commitment stems from a 3.3 ± 0.8% loss of ice sheet area and volume. This imbalance is primarily driven by increased surface ablation through meltwater run-off, which is found to have a stronger impact than tidewater ice flow discharge on both the trend and interannual variability of the Greenland ice sheet mass budget. The high-melt year of 2012, used as a proxy for a sustained warmer future climate, projects a dramatically larger commitment of 782 ± 135 mm SLR. Conversely, the low-melt year of 2018 shows the ice sheet to be in near mass budget equilibrium, highlighting the sensitivity to climate extremes. Regional analysis reveals significant variations in ice imbalance across Greenland, with western sectors showing much higher disequilibrium than eastern sectors. This regional contrast is attributed to differences in snowfall accumulation, topographic relief, and proximity to North Atlantic storms. The southwestern sector is identified as particularly sensitive to climate warming, showing a substantial commitment to SLR (186 ± 30 mm) under the 2012 high-melt scenario. The study further demonstrates that even small areas (less than 30 km²) contribute to overall sea-level rise, leading to further refinement of the overall commitment estimate. The analysis consistently points to surface ablation as the dominant driver of current ice loss, particularly in land-terminating sectors, which disproportionately contribute to the overall imbalance. The findings highlight the crucial role of considering both the average climate and extreme climate events in projections of sea-level rise from Greenland's ice sheet.

The study's findings emphasize the substantial and unavoidable contribution of Greenland's ice sheet to future sea-level rise. The committed SLR of at least 274 ± 68 mm, based on the recent (2000–2019) climate, highlights the significant impact of past climate changes, independent of future climate trajectories. The finding aligns with expert judgment that places the committed SLR contribution from Greenland in the range of 330–490 mm by 2100. The projection of 782 ± 135 mm SLR based on a perpetual 2012 high-melt year demonstrates the potential for even greater sea-level rise under a sustained warming scenario, corroborating expert assessments predicting meter-scale SLR under unabated warming. This study underscores the high sensitivity of the Greenland ice sheet to both average and extreme climate conditions. The observed large interannual variability in Greenland's climate, as exemplified by the 2012 and 2018 extremes, is often overlooked in aggregated mass balance analyses. The research implies that modeling efforts should explicitly account for time-varying extreme events to accurately capture the full range of potential future ice sheet responses. The study offers a lower-bound estimate of committed SLR, not explicitly addressing future changes in ice discharge resulting from processes beyond geometric adjustments to climate. Future research should focus on integrating these additional dynamic processes and feedback mechanisms into the methodology to refine the estimations of committed SLR and to assess the associated timescales. Furthermore, improved integration of the presented findings with process-based models may greatly enhance predictive capabilities in assessing Greenland's contribution to future sea-level rise.

This study provides a novel approach for quantifying the committed sea-level rise contribution from the Greenland ice sheet based on observed ice sheet disequilibrium with recent climate. The findings reveal a substantial committed SLR of at least 274 ± 68 mm, even without considering future climate change, emphasizing the significance of past climate impacts. The high-melt year scenario dramatically increases this commitment, highlighting the potential for even greater sea-level rise under continued warming. Future research should focus on refining the methodology to incorporate additional dynamic processes and feedback mechanisms, improving the integration with process-based models, and exploring the associated timescales of ice sheet response. Understanding this commitment is critical for developing effective strategies to mitigate and adapt to the impacts of future sea-level rise.

The study provides a lower-bound estimate of committed SLR, neglecting potential future changes in ice discharge that are not directly related to geometric adjustments to climate. This approach implicitly assumes that the relationship between area and volume changes remains constant and that the power law exponent used remains valid. Furthermore, the study does not explicitly address the timescale over which the committed ice loss will occur, which may range from centuries to millennia. The uncertainties associated with the observational data used in the analysis also contribute to the limitations of the findings. Finally, the study focuses primarily on the effects of surface mass balance and does not explicitly consider dynamic changes such as accelerated ice flow, which may further exacerbate ice loss and sea-level rise.