The Paris Agreement aims to limit global mean warming in the 21 st century to less than 2 ºC above preindustrial levels, and to promote further efforts to limit the warming to 1.5 ºC. Here, we use an observationally calibrated ice sheet-shelf model including ductile and brittle processes that can initiate dynamic instabilities, to test Antarctica's response to future climate scenarios representing Paris Agreement aspirations versus more fossil-fuel intensive emissions scenarios. We find that global mean warming above 2 ºC substantially increases the risk of triggering rapid ice-sheet retreat, initiated by the thinning and loss of Antarctic ice shelves. A scenario consistent with current policies and allowing +3 ºC of warming by 2100 causes an abrupt jump in the pace of ice loss after ~2060, equivalent to ~0.5 cm sea level rise per year. Once initiated, rapid Antarctic ice loss continues for centuries, regardless of bedrock/sea level feedbacks or geoengineered carbon dioxide reduction (CDR). These results demonstrate the possibility that unstoppable, catastrophic sea level rise from Antarctica will be triggered if Paris Agreement temperature targets are exceeded. Greenland is currently losing ice at a faster pace than Antarctica 1,2 , but Antarctica contains almost eight (7.74) times more ice above floatation, equivalent to 58 m of global mean sea level (GMSL) 3 .The Antarctic Ice Sheet (AIS) is fundamentally different from the Greenland Ice Sheet, because most of its margin terminates directly in the surrounding ocean, with massive ice shelves (floating extensions of glacial ice) providing resistance (buttressing) to the seaward flow of the grounded ice upstream 4 . About a third of the AIS rests on bedrock hundreds to thousands of meters below sea level 3 and in places where subglacial bedrock slopes downward away from the ocean (reversesloped), the ice margin is susceptible to dynamical instabilities; the Marine Ice-Sheet Instability (MISI) 5,6 and possibly a Marine Ice-Cliff Instability (MICI) 7,8 that can drive rapid retreat. The West Antarctic Ice Sheet (WAIS), with the potential to cause ~5 m of sea level rise 3 , is particularly

Meltwater and ice discharge from a retreating Antarctic Ice Sheet could have important impacts on future global climate. Here, we report on multi-century (present–2250) climate simulations performed using a coupled numerical model integrated under future greenhouse-gas emission scenarios IPCC RCP4.5 and RCP8.5, with meltwater and ice discharge provided by a dynamic-thermodynamic ice sheet model. Accounting for Antarctic discharge raises subsurface ocean temperatures by >1°C at the ice margin relative to simulations ignoring discharge. In contrast, expanded sea ice and 2° to 10°C cooler surface air and surface ocean temperatures in the Southern Ocean delay the increase of projected global mean anthropogenic warming through 2250. In addition, the projected loss of Arctic winter sea ice and weakening of the Atlantic Meridional Overturning Circulation are delayed by several decades. Our results demonstrate a need to accurately account for meltwater input from ice sheets in order to make confident climate predictions.

Coupled global climate models (GCMs) generally fail to reproduce the observed sea‐surface temperature (SST) trend pattern since the 1980s. The model‐observation discrepancies may arise in part from the lack of realistic Antarctic ice‐sheet meltwater input in GCMs. Here we employ two sets of CESM1‐CAM5 simulations forced by anomalous Antarctic meltwater fluxes over 1980–2013 and through the 21st century. Both show a reduced global warming rate and an SST trend pattern that better resembles observations. The meltwater drives surface cooling in the Southern Ocean and the tropical southeast Pacific, in turn increasing low‐cloud cover and driving radiative feedbacks to become more stabilizing (corresponding to a lower effective climate sensitivity). These feedback changes can contribute as substantially as ocean heat uptake efficiency changes in reducing the global warming rate. Accurately projecting historical and future warming thus requires improved representation of Antarctic meltwater and its impacts.

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.

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 surface temperature (GMST) 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 GMST warming by 2100. This paper reviews and synthesizes 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 (AIS) under a changing climate: 1) 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 2) future ice sheet melt will result in a negative feedback on GMST, thus delaying temperature rise. When considering these impacts in conjunction, justice concerns associated with the Paris Agreement are exacerbated. This case study demonstrates that mitigation policies should consider climate impacts in addition to GMST, particularly sea level rise.

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Coupled global climate models (GCMs) generally fail to reproduce the observed sea-surface temperature (SST) trend pattern since the 1980s. The model-observation discrepancies may arise in part from the lack of realistic Antarctic ice-sheet meltwater imbalance in GCMs. Here we employ two sets of CESM1-CAM5 simulations forced by anomalous Antarctic meltwater fluxes over 1980-2013 and into the 21st century. Both show a reduced global warming rate and an SST trend pattern that better resembles observations. The meltwater drives surface cooling in the Southern Ocean and the tropical southeast Pacific, in turn increasing low-cloud cover and driving radiative feedbacks to become more stabilizing (corresponding to a lower effective climate sensitivity). These feedback changes contribute more than ocean heat uptake efficiency changes in reducing the global warming rate. Accurately projecting historical and future warming thus requires improved representation of Antarctic meltwater and its impacts in models.

No abstract