The role of hydraulic conductivity in the Pine Island Glacier's subglacial water distribution.

Global climate warming leads to ever-increasing glacier mass loss. Pine Island Glacier in Antarctica is one of the largest contributors to global sea level rise (SLR). One of the biggest uncertainties in the assessment of glacier contribution to SLR at present are subglacial hydrology processes which are less well known than other ice dynamical processes. We use the Glacier Drainage System (GlaDS) model which couples both distributed and channelized components to simulate the basal hydrology of Pine Island Glacier with basal sliding and meltwater production taken from a full-Stokes Elmer/Ice model fitting observed surface velocities. We find ≈100 km long Rothlisberger channels up to 26 m in diameter extending up glacier from the grounding line along the main trunk of Pine Island Glacier delivering 51 m3 s-1 of fresh water to the grounding line. Channelization occurs at high water pressure because of high basal melt rates (maximum of 1 m a-1) caused by high rates of shear heating in regions with fast ice flow (>1000 m a-1). We simulate a shallow "swamp" of 0.8 m water depth where flow transitions from a distributed system into the channels. We performed a set of 38 sensitivity experiments varying sheet and channel conductivity over 4 orders of magnitude. We find a threshold behavior in distributed sheet conductivity above which basal water pressures are unaffected by changing channel conductivities. Our findings suggest a strong need to better understand controls on basal water conductivity through the distributed system. This issue is critical to improve model-based predictive capability for the Pine Island Glacier and, more generally, the Antarctic Ice Sheet.

Projected land ice contributions to twenty-first-century sea level rise.

The land ice contribution to global mean sea level rise has not yet been predicted1 using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models2-8, but primarily used previous-generation scenarios9 and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12 using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

Ocean-driven thinning enhances iceberg calving and retreat of Antarctic ice shelves.

Iceberg calving from all Antarctic ice shelves has never been directly measured, despite playing a crucial role in ice sheet mass balance. Rapid changes to iceberg calving naturally arise from the sporadic detachment of large tabular bergs but can also be triggered by climate forcing. Here we provide a direct empirical estimate of mass loss due to iceberg calving and melting from Antarctic ice shelves. We find that between 2005 and 2011, the total mass loss due to iceberg calving of 755 ± 24 gigatonnes per year (Gt/y) is only half the total loss due to basal melt of 1516 ± 106 Gt/y. However, we observe widespread retreat of ice shelves that are currently thinning. Net mass loss due to iceberg calving for these ice shelves (302 ± 27 Gt/y) is comparable in magnitude to net mass loss due to basal melt (312 ± 14 Gt/y). Moreover, we find that iceberg calving from these decaying ice shelves is dominated by frequent calving events, which are distinct from the less frequent detachment of isolated tabular icebergs associated with ice shelves in neutral or positive mass balance regimes. Our results suggest that thinning associated with ocean-driven increased basal melt can trigger increased iceberg calving, implying that iceberg calving may play an overlooked role in the demise of shrinking ice shelves, and is more sensitive to ocean forcing than expected from steady state calving estimates.