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

Recent studies have investigated trends and interannual variability in the potential intensity (PI) of tropical cyclones (TCs), but relatively few have examined TC PI seasonality or its controlling factors. Potential intensity is a function of environmental conditions that influence thermodynamic atmosphere-ocean disequilibrium and the TC thermodynamic efficiency-primarily sea surface temperatures and the TC outflow temperatures-and therefore varies spatially across ocean basins with different ambient conditions. This study analyzes the seasonal cycles of TC PI in each main development region using reanalysis data from 1980 to 2013. TC outflow in the western North Pacific (WNP) region is found above the tropopause throughout the seasonal cycle. Consequently, WNP TC PI is strongly influenced by the seasonal cycle of lower-stratospheric temperatures, which act to damp its seasonal variability and thereby permit powerful TCs any time during the year. In contrast, the other main development regions (such as the North Atlantic) exhibit outflow levels in the troposphere through much of the year, except during their peak seasons. Mathematical decomposition of the TC PI metric shows that outflow temperatures damp WNP TC PI seasonality through thermodynamic efficiency by a quarter to a third, whereas disequilibrium between SSTs and the troposphere drives 72%-85% of the seasonal amplitude in the other ocean basins. Strong linkages between disequilibrium and TC PI seasonality in these basins result in thermodynamic support for powerful TCs only during their peak seasons. Decomposition also shows that the stratospheric influence on outflow temperatures in the WNP delays the peak month of TC PI by a month.

Mitigation of anthropogenic greenhouse gases with short lifetimes (order of a year to decades) can contribute to limiting warming, but less attention has been paid to their impacts on longer-term sea-level rise. We show that short-lived greenhouse gases contribute to sealevel rise through thermal expansion (TSLR) over much longer time scales than their atmospheric lifetimes. For example, at least half of the TSLR due to increases in methane is expected to remain present for more than 200 y, even if anthropogenic emissions cease altogether, despite the 10-y atmospheric lifetime of this gas. Chlorofluorocarbons and hydrochlorofluorocarbons have already been phased out under the Montreal Protocol due to concerns about ozone depletion and provide an illustration of how emission reductions avoid multiple centuries of future TSLR. We examine the "world avoided" by the Montreal Protocol by showing that if these gases had instead been eliminated in 2050, additional TSLR of up to about 14 cm would be expected in the 21st century, with continuing contributions lasting more than 500 y. Emissions of the hydrofluorocarbon substitutes in the next half-century would also contribute to centuries of future TSLR. Consideration of the time scales of reversibility of TSLR due to short-lived substances provides insights into physical processes: sea-level rise is often assumed to follow air temperature, but this assumption holds only for TSLR when temperatures are increasing. We present a more complete formulation that is accurate even when atmospheric temperatures are stable or decreasing due to reductions in short-lived gases or net radiative forcing. climate change | sea-level rise | greenhouse gases | reversibility | Montreal ProtocolA tmospheric concentrations of a range of greenhouse gases (GHGs), including carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and halocarbons (HCs), have increased since the beginning of the industrial revolution and have been the main drivers of warming global air and ocean temperatures as well as rising sea levels (1). The goal of this paper is to assess whether the sea-level rise through thermal expansion (TSLR) induced by anthropogenic increases in GHGs that are short-lived (i.e., those with atmospheric residence times of years to decades) is reversible, and on what time scales (2, 3). We also discuss the TSLR due to long-lived gases (defined here as those with residence times of several centuries or longer) and show how the comparison of responses of TSLR to different gases elucidates the climate-system processes that control TSLR reversibility. Understanding how emissions of different gases each affect the Earth's climate over time is central to policy evaluation (e.g., when tradeoffs between mitigation options for different GHGs are considered).A fundamental goal of the United Nations Framework Convention on Climate Change (UNFCCC) and its Paris agreement is the stabilization of GHGs at a level that would avoid dangerous anthropogenic interference with the climate system. Future...

In 2012, Hurricane Sandy hit the East Coast of the United States, creating widespread coastal flooding and over $60 billion in reported economic damage. The potential influence of climate change on the storm itself has been debated, but sea level rise driven by anthropogenic climate change more clearly contributed to damages. To quantify this effect, here we simulate water levels and damage both as they occurred and as they would have occurred across a range of lower sea levels corresponding to different estimates of attributable sea level rise. We find that approximately $8.1B ($4.7B–$14.0B, 5th–95th percentiles) of Sandy’s damages are attributable to climate-mediated anthropogenic sea level rise, as is extension of the flood area to affect 71 (40–131) thousand additional people. The same general approach demonstrated here may be applied to impact assessments for other past and future coastal storms.