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Anthropogenic greenhouse gas emissions are causing unprecedented changes to the climate. In 2015, at the United Nations (UN) Conference of the Parties in Paris, France, countries agreed to limit the global mean temperature (GMT) increase to 2°C above preindustrial levels, and to pursue efforts to limit warming to 1.5°C. Due to the long‐term irreversibility of sea level rise (SLR), risks to island and coastal populations are not well encapsulated by the goal of limiting GMT warming by 2100. This review article investigates the climate justice implications of temperature targets in light of our increasing understanding of the spatially variable impact and long temporal commitment to rising seas. In particular we highlight the impact that SLR will have on island states and the role of the Alliance of Small Island States (AOSIS) in UN climate negotiations. As a case study we review dual impacts from the Antarctic Ice Sheet under a changing climate: (a) recent climate and ice sheet modeling shows that Antarctic melt has the potential to cause rapid SLR with a distinct spatial pattern leading to AOSIS nations experiencing SLR at least 11% higher than the global average and up to 33% higher; and (b) future ice sheet melt will result in a negative feedback on GMT, thus delaying temperature rise. When considering these impacts in conjunction, justice concerns associated with the Paris Agreement are exacerbated.

 

Much of the knowledge of Antarctic Ice Sheet variations since its inception ~34 Ma derives from marine sediments on the continental shelf, deposited in glacimarine or sub-ice environments by advancing and retreating grounded ice, and observed today by seismic profiling and coring. Here we apply a 3-D coupled ice sheet and sediment model from 40 Ma to the present, with the goal of directly linking ice-sheet variations with the sediment record. The ice-sheet model uses vertically averaged ice dynamics and parameterized grounding-line flux. The sediment model includes quarrying of bedrock, sub-ice transport, and marine deposition. Atmospheric and oceanic forcing are determined by uniform shifts to modern climatology in proportion to records of atmospheric CO2, deep-sea-core δ18O, and orbital insolation variations. The model is run continuously over the last 40 Myr at coarse resolution (80 or 160 km), modeling post-Eocene ice, landscape evolution and off-shore sediment packages in a single self-consistent simulation. Strata and unconformities are tracked by recording times of deposition within the model sediment stacks, which can be compared directly with observed seismic profiles. The initial bedrock topography is initialized to 34 Ma geologic reconstructions, or an iterative procedure is used that yields independent estimates of paleo bedrock topography. Preliminary results are compared with recognized Cenozoic ice-sheet variations, modern sediment distributions and seismic profiles, and modern and paleo bedrock topographies. 

The use of a boundary-layer parameterization of buttressing and ice flux across grounding lines in a two dimensional ice-sheet model is improved by allowing general orientations of the grounding line. This and another modification to the model’s grounding-line parameterization are assessed in three settings: rectangular fjord-like domains – the third Marine Ice Sheet Model Intercomparison Project (MISMIPC) and Marine Ice Sheet Model Intercomparison Project for plan view models (MISMIP3d) – and future simulations of West Antarctic ice retreat under Representative Concentration Pathway (RCP) 8.5-based climates. The new modifications are found to have significant effects on the fjord-like results, which are now within the envelopes of other models in the MISMIP+ and MISMIP3d intercomparisons. In contrast, the modifications have little effect on West Antarctic retreat, presumably because dynamics in the wider major Antarctic basins are adequately represented by the model’s previous simpler one-dimensional formulation. As future grounding lines retreat across very deep bedrock topography in the West Antarctic simulations, buttressing is weak and deviatoric stress measures exceed the ice yield stress, implying that structural failure at these grounding lines would occur. We suggest that these grounding-line quantities should be examined in similar projections by other ice models to better assess the potential for future structural failure. 

The Antarctic Ice Sheet contains enough ice above floatation to contribute ∼57 m of sea-level rise. About a third of that ice rests on bedrock far below sea level, in deep subglacial basins. In places where the bedrock slopes downward, away from the coast (retrograde bed) or where groundling lines terminate in deep water, the ice-sheet margin is susceptible to dynamic instabilities (related to both viscous and brittle processes) that can cause rapid retreat. Here, we use a numerical ice sheet-shelf model that captures these processes to demonstrate that the current configuration of the Antarctic Ice Sheet, with marine-terminating grounding lines poised on the edges of deep subglacial basins, combined with current, early 21st century atmospheric and oceanic tempera- tures, maximizes the danger of rapid retreat and sea-level rise. The potential retreat rates from the modern state greatly exceed those starting from other past configurations such as glacial maxima and smaller terrestrial ice cover. We also explore the importance of various negative feedbacks on the pace of retreat, including the effects of mélange, meltwater-climate feedback, and bedrock rebound. We find that today’s Antarctic Ice Sheet is capable of contributing faster sea-level rise than at any other time in the ice sheet’s recent (three million-year) geologic history, and possibly its entire 34 million-year history. This potential for unprecedented sea-level rise from Antarctica, coinciding with the recent expansion of human population centers and infrastructure along low-lying coastlines maximizes the risk of a future sea-level catastrophe. 

The loss or thinning of buttressing ice shelves and accompanying changes in grounding-zone stress balance are commonly implicated as the primary trigger for grounding-line retreat, such as that observed in Amundsen Sea outlet glaciers today. Ice-shelf thinning is mostly attributed to the presence of warm ocean waters beneath the shelves. However, climate model projections show that summer air temperatures could soon exceed the threshold for widespread meltwater production on ice-shelf surfaces. This has serious implications for their future stability, because they are vulnerable to water-induced flexural stresses and water-aided crevasse penetration, termed ‘hydrofracturing’. Once initiated, the rate of shelf loss through hydrofracturing can far exceed that caused by sub-surface melting, and could result in the complete loss of some buttressing ice shelves, with marine grounding lines suddenly becoming calving ice fronts. In places where those exposed ice fronts are thick (>900m) and crevassed, deviatoric stresses can exceed the strength of the ice and the cliff face will fail mechanically, leading to rapid calving like that seen in analogous settings such as Jakobshavn on Greenland. Here we explore the implications of hydrofacturing and subsequent ice-cliff collapse in a warming climate, by parameterizing these processes in a hybrid ice sheet-shelf model. Model sensitivities to meltwater production and to ice-cliff calving rate (a function of cliff height above the stress balance threshold triggering brittle failure) are calibrated to match modern observations of calving and thinning. We find the potential for major ice-sheet retreat if global mean temperature rises more than ~2ºC above preindustrial. In the model, Antarctic calving rates at thick ice fronts are not allowed to exceed those observed in Greenland today. This may be a conservative assumption, considering the very different spatial scales of Antarctic outlets, such as Thwaites. Nonetheless, simulations following a ‘worst case’ RCP8.5 scenario produce rates of sea-level rise measured in cm per year by the end of this century. Clearly, the potential for brittle processes to deliver ice to the ocean, in addition to viscous and basal processes, needs to be better constrained through more complete, physically based representations of calving. 

Theory, modeling and observations point to the prospect of runaway grounding-line retreat and marine ice loss from West Antarctica and major East Antarctic basins, in response to climate warming. These rapid retreats are associated with geologic evidence of past high sea-level stands, and pose a threat of drastic sea-level rise in the future. Rapid calving of ice from deep grounding lines generates substantial downstream melange (floating ice debris). It is unknown whether this melange has a significant effect on ice dynamics during major Antarctic retreats, through clogging of seaways and back pressure at the grounding line. Observations in Greenland fjords suggest that melange can have a significant buttressing effect, but the lateral scales of Antarctic basins are an order of magnitude larger (100's km compared to 10's km), with presumably much less influence of confining margins. Here we attempt to include melange as a prognostic variable in a 3-D Antarctic ice sheet-shelf model. Continuum mechanics is used as a heuristic representation of discrete particle physics. Melange is created by ice calving and cliff failure. Its dynamics are treated similarly to ice flow, but with little or no resistance to divergence. Melange provides back pressure where adjacent to grounded tidewater ice faces or ice-shelf edges. We examine the influence of the new melange component during rapid Antarctic retreat in warm-Pliocene and future warming scenarios. 

Seismic tomography models indicate highly variable Earth structure beneath Antarctica with anomalously low shallow mantle viscosities below West Antarctica. An improved projection of the contribution of the Antarctic Ice Sheet to sea‐level change requires consideration of this complexity to precisely account for water expelled into the ocean from uplifting marine sectors. Here we build a high‐resolution 3‐D viscoelastic structure model based on recent inferences of seismic velocity heterogeneity below the continent. The model serves as input to a global‐scale sea‐level model that we use to investigate the influence of solid Earth deformation in Antarctica on future global mean sea‐level (GMSL) rise. Our calculations are based on a suite of ice mass projections generated with a range of climate forcings and suggest that water expulsion from the rebounding marine basins contributes 4%–16% and 7%–14% to the projected GMSL change at 2100 and 2500, respectively.

 

Observational evidence indicates that the West Antarctic Ice Sheet (WAIS) is losing mass at an accelerating rate. Impacts to global climate resulting from changing ocean circulation patterns due to increased freshwater runoff from Antarctica in the future could have significant implications for global heat transport, but to-date this topic has not been investigated using complex numerical models with realistic freshwater forcing. Here, we present results from a high resolution fully coupled ocean-atmosphere model (CESM 1.2) forced with runoff from Antarctica prescribed from a high resolution regional ice sheet-ice shelf model. Results from the regional simulations indicate a potential freshwater contribution from Antarctica of up to 1 m equivalent sea level rise by the end of the century under RCP 8.5 indicating that a substantial input of freshwater into the Southern Ocean is possible. Our high resolution global simulations were performed under IPCC future climate scenarios RCP 4.5 and 8.5. We will present results showing the impact of WAIS collapse on global ocean circulation, sea ice, air temperature, and salinity in order to assess the potential for abrupt climate change triggered by WAIS collapse.