The contribution of the Greenland ice sheet to global sea-level rise has increased rapidly during the last two decades and is currently ∼ 0.8 mm/year. As observations show a clear, accelerating increasing trend in both global temperature and ice mass loss from the Greenland ice sheet, how much mass the Greenland ice sheet is going to lose over the next century and beyond is one of the most urgent questions in understanding the implication of climate change. Estimating future ice sheet contributions to sea-level rise is currently an active area of research and numerical ice sheet modeling is our best tool to address this question.

This thesis provides an estimate of sea-level contribution from Greenland with a new generation ice sheet model that fully accounts for changes of 200+ Greenland glaciers. First, we introduce modeling of calving dynamics which is one of the most important processes contributing to mass loss from outlet glaciers around the coast of Greenland. We test and compare calving laws in an ice sheet model and assess which calving law has better predictive abilities for each glacier. We then apply the best calving law to Nioghalvfjerdsfjorden and Zachariae Isstrøm glaciers in northeast Greenland to investigate the response of these fast-changing glaciers to future climate forcing.

We extend our model to the entire Greenland ice sheet to estimate the future sea-level contribution from Greenland. Compared to previous studies, we calibrate our model at the individual glacier scale with a moving boundary capability to better constrain the retreat of marine-terminating glaciers. We find that the Greenland ice sheet will contribute 79.2 to 167 mm to sea-level between 2007 and 2100 under the most extreme warming scenarios. Our simulations show that discharge from ice dynamics will contribute to the total mass loss from Greenland more than previously estimated, implying that future scientific focus should remain on not only atmospheric processes but also the ice front of marine-terminating glaciers.

Totten Glacier, the primary ice discharger of the East Antarctic Ice Sheet (EAIS), contains 3.85 m sea level rise equivalent ice mass (SLRe) and has displayed dynamic change driven by interaction of its ice shelf with the Southern Ocean. To project Totten Glacier's evolution, it is critical that sub-shelf ocean processes are properly resolved in dynamic ice sheet models. First, we combine an ocean box model with a buoyant plume parameterization to create PICOP, a novel melt parameterization that resolves sub-shelf vertical overturning and produces melt rates that are in excellent agreement with observations. We then use this parameterization to make century-scale mass balance projections of the EAIS, forced by surface mass balance and ocean thermal anomalies from ten global climate models. Although increased snowfall offsets ice discharge in high emission scenarios and results in ~10 mm SLRe gain by 2100, significant grounded ice thinning (1.15 m/yr) and mass loss (~6 mm SLRe) from Totten Glacier is projected. To investigate whether PICOP misses important processes, such as the advection of warm water into the ice shelf cavity, we develop a fully coupled ice-ocean model and find that warm water is able to access Totten Glacier's sub-shelf cavity through topographic depressions along the central and eastern calving front. By mid-century in high emission scenarios, warm water intrusions become strong enough to overcome topographic barriers and dislodge Totten Glacier's southern grounding line, triggering abrupt acceleration in ice discharge (+185%). Overall, the timing and extent of Totten Glacier's retreat is predominately controlled by the sub-shelf ocean circulation, highlighting the importance of studying dynamic glaciers in fully coupled ice-ocean models.

Petermann Glacier is a major outlet glacier of northern Greenland that drains a marine-based basin vulnerable to destabilization from enhanced oceanic and atmospheric forcings. Satellite observations show significant grounding line retreat of ∼7 km in a central region of the glacier, with at least 1 km of retreat elsewhere along the grounding line. This representsa significant shift from the glacier’s previously stable grounding line position mapped in the 1990s. Satellite observations also show a seasonal ice acceleration for Petermann of 15% in the summer, from 1,250 to 1,500 m/yr measured close to the grounding line. We use a subglacial hydrology model (GlaDS) and an ice sheet model (ISSM) with asynchronous coupling to evaluate the role of subglacial hydrology as a physical mechanism explaining the seasonal speedup of ice velocity. Results show an excellent agreement between the observed and modeled velocity in terms of timing and magnitude when an applied lower limit on effective pressure of 6% of ice overburden pressure is imposed in the ice flow model. We conclude that seasonal changes in subglacial hydrology are sufficient to explain the observed seasonal speed up of Petermann Glacier. Current projections of glacier dynamics under 21st century climate forcings do not include seasonality or subglacial hydrology, so it is unknown if either will play any important role in evolving glacier dynamics under different climate change scenarios. We use climate forcings through 2100 to investigate how the subglacial hydrologic system may evolve in a warmer climate, and to test if including hydrology changes the stability of Petermann under future climate scenarios using ISSM and the GlaDS model in both an asynchronous and synchronous coupled configuration. Results show that including subglacial hydrology in projections of Petermann’s evolution yield larger predictions of future sea level rise by the end of the century. However, modeled results of both present day and future ice dynamics with and without subglacial hydrology included do not reproduce the observed grounding line retreat. To better understand grounding line migration of Petermann, we apply a newly published theory of seawater intrusion below grounded ice. By incorporating ocean driven basal melting in the grounding zone, we achieve a significantly improved match to the observed grounding line behavior that previous model setups failed to reproduce. This underscores the importance of considering ocean-driven melting to accurately capture grounding line behavior. These studies contribute to a deeper understanding of the observed behavior of Petermann Glacier, particularly its seasonal acceleration and grounding line migration. Subglacial hydrology and seawater intrusion both emerge as influential short time scale processes on ice dynamics, with potential long term implications on glacier stability and sea level rise.