Description: 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 models2,3,4,5,6,7,8, 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.

Description: Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (〈500m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of 〈5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.

Description: At the base of a thick ice sheet the temperature locally reaches the pressure melting point and melting generates a thin subglacial water layer. The basal water lubricates the base and thus enhances the sliding of the ice sheet. As a consequence of sliding, the heat source of internal strain heating decreases and the basal ice cools down over time. When frictional heat and heat advection do not counterbalance this, the ice will become frozen to the bedrock again. In addition, strain heating within a temperate ice layer generates a liquid water fraction in the ice, leading to a softer material and enhanced deformation. If the horizontal or vertical advection of cold ice to the base is weak, this positive feedback will lead to a local creep instability. The effect of basal water is thus twofold: it affects the sliding, as well as the rheology and via both ways the ice dynamics. Subglacial water is therefore a crucial component in the dynamic evolution of ice sheets. We present numerical simulations of the present day ice flow using the three-dimensional thermo-coupled full- Stokes model TIM FD 3 on a 2.5 km horizontal grid in the area of the western Dronning Maud Land, Antarc- tica, including the three ice streams Stancomb-Wills, Veststraumen and Plogbreen and the adjacent Brunt and Riiser- Larsen ice shelves. Three different flux routing algorithms for the subglacial meltwater and a modified Weertman-type sliding relation were implemented in the model to account for higher sliding velocities under wet basal conditions. Subsequent to spin-up simulations different sliding simulations considering wet and dry basal conditions were performed. The simulations show a cyclic behaviour on millennial time scale at distinct locations in the model domain. We estimate the distribution of subglacial water based on different flux routing methods and the effect on the ice flow and the basal thermal regime. We further present our analysis of the involved feedback mechanism between ice flow, temperature and rheology, that are related to the simulated cyclic behaviour.

Description: We study the presence and effect of subglacial water on the motion of the inland ice in western Dronning Maud Land. A full-Stokes model including three routing schemes for a thin film of subglacial water and a modification of a Weertman-type sliding relation to account for higher sliding velocities under wet basal conditions were used to perform 200 ka spin-up simulations on a 2.5 km grid. Subsequent 30 ka simulations with wet and dry basal conditions were analysed for the effects of sliding on the thermal regime and velocities. The occurrence of the major ice streams in this area is mainly controlled by the ice and bedrock geometry. Smaller glaciers only appear as pronounced individual glaciers, when subglacial water is taken into account. The thermal regime is affected by creep instabilities produced by an ice rheology including a microscopic water content, leading to a cyclic behaviour on millennial time scales of the ice flow and occurrence of temperate ice at the base.