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

[1] Atmospheric circulation in a Snowball Earth is critical for determining cloud behavior, heat export from the tropics, regions of bare ice, and sea glacier flow. These processes strongly affect Snowball Earth deglaciation and the ability of oases to support photosynthetic marine life throughout a Snowball Earth. Here we establish robust aspects of the Snowball Earth atmospheric circulation by running six general circulation models with consistent Snowball Earth boundary conditions. The models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation. The strength of the Snowball Hadley circulation is roughly double modern at low CO 2 and greatly increases as CO 2 is increased. We force a 1-D axisymmetric sea glacier model with general circulation model (GCM) output and show that, neglecting zonal asymmetry, sea glaciers would limit ice thickness variations to O(10%). Global mean ice thickness in the 1-D sea glacier model is well-approximated by a 0-D ice thickness model with global mean surface temperature as the upper boundary condition. We then show that a thin-ice Snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the GCMs if we use ice optical properties that favor the thin-ice solution. Finally, we examine Snowball oases for life using analytical models forced by the GCM output and find that conditions become more favorable for oases as the Snowball warms, so that the most critical time for the survival of life would be near the beginning of a Snowball Earth episode.

Heat stress harms human health, agriculture, the economy, and the environment more broadly. Exposure to heat stress is increasing with rising global temperatures. While most studies assessing future heat stress have focused on surface air temperature, compound extremes of heat and humidity are key drivers of heat stress. Here, we use atmospheric reanalysis data and a large initial-condition ensemble of global climate model simulations to evaluate future changes in daily compound heat-humidity extremes as a function of increasing global-mean surface air temperature (GSAT). The changing frequency of heat-humidity extremes, measured using wet bulb globe temperature (WBGT), is strongly related to GSAT and, conditional upon GSAT, nearly independent of forcing pathway. The historical~1 • C of GSAT increase above preindustrial levels has already increased the population annually exposed to at least one day with WBGT exceeding 33 • C (the reference safety value for humans at rest per the ISO-7243 standard) from 97 million to 275 million. Maintaining the current population distribution, this exposure is projected to increase to 508 million with 1.5 • C of warming, 789 million with 2.0 • C of warming, and 1.22 billion with 3.0 • C of warming (similar to late-century warming projected based on current mitigation policies).

This study aims to understand the relative roles of external forcing versus internal climate variability in causing the observed Barents Sea winter sea ice extent (SIE) decline since 1979. We identify major discrepancies in the spatial patterns of winter Northern Hemisphere sea ice concentration trends over the satellite period between observations and CMIP5 multi-model mean externally forced response. The CMIP5 externally forced decline in Barents Sea winter SIE is much weaker than that observed. Across CMIP5 ensemble members, March Barents Sea SIE trends have little correlation with global mean surface air temperature trends, but are strongly anti-correlated with trends in Atlantic heat transport across the Barents Sea Opening (BSO). Further comparison with control simulations from coupled climate models suggests that enhanced Atlantic heat transport across the BSO associated with regional internal variability may have played a leading role in the observed decline in winter Barents Sea SIE since 1979.