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The Paris Agreement aims to limit global mean warming in the twenty-first century to less than 2 degrees Celsius above preindustrial levels, and to promote further efforts to limit warming to 1.5 degrees Celsius1. The amount of greenhouse gas emissions in coming decades will be consequential for global mean sea level (GMSL) on century and longer timescales through a combination of ocean thermal expansion and loss of land ice2. The Antarctic Ice Sheet (AIS) is Earth’s largest land ice reservoir (equivalent to 57.9 metres of GMSL)3, and its ice loss is accelerating4. Extensive regions of the AIS are grounded below sea level and susceptible to dynamical instabilities5–8that are capable of producing very rapid retreat8. Yet the potential for the implementation of the Paris Agreement temperature targets to slow or stop the onset of these instabilities has not been directly tested with physics-based models. Here we use an observationally calibrated ice sheet–shelf model to show that with global warming limited to 2 degrees Celsius or less, Antarctic ice loss will continue at a pace similar to today’s throughout the twenty-first century. However, scenarios more consistent with current policies (allowing 3 degrees Celsius of warming) give an abrupt jump in the pace of Antarctic ice loss after around 2060, contributing about 0.5 centimetres GMSL rise per year by 2100—an order of magnitude faster than today4. More fossil-fuel-intensive scenarios9result in even greater acceleration. Ice-sheet retreat initiated by the thinning and loss of buttressing ice shelves continues for centuries, regardless of bedrock and sea-level feedback mechanisms10–12or geoengineered carbon dioxide reduction. These results demonstrate the possibility that rapid and unstoppable sea-level rise from Antarctica will be triggered if Paris Agreement targets are exceeded.

We develop regionally optimized Gravity Recovery and Climate Experiment (GRACE) solutions to evaluate the mass balance of the drainage basins of Amery Ice Shelf, East Antarctica, and Getz Ice Shelf, West Antarctica. We find that the Amery region is near balance, while the Getz region is rapidly losing mass. We compare the results with the mass budget method (MBM) combining ice discharge along the periphery with surface mass balance derived from three regional climate models: (1) Regional Atmospheric Climate Model (RACMO) 2.3p1 and (2) 2.3p2, and (3) Modèle Atmosphérique Régional 3.6.41. For Amery, MBM/RACMO2.3p1 agrees with GRACE, while MBM/RACMO2.3p2 and MBM/MAR3.6.41 suggest a positive mass balance. For Getz, all estimates agree with a mass loss and the GRACE results are robust to uncertainties in glacial isostatic adjustment derived from an ensemble 128,000 forward models. Over the period April 2002 to November 2015, the mass loss of the Getz drainage basin is 22.9 ± 10.9 Gt/yr with an acceleration of 1.6 ± 0.9 Gt/yr2. We use a regional optimization methodology for processing data from the Gravity Recovery and Climate Experiment (GRACE) to evaluate the ice mass change of the drainage basins of two major ice shelves in Antarctica and evaluate the performance of regional climate models (RCMs). The Getz Ice Shelf basin in West Antarctica has shown previous disagreements between various mass balance estimates and is influenced by heterogenous conditions that make it vulnerable and challenging to study. We find this region to be in a state of accelerating mass loss. Furthermore, all three examined RCMs are in good agreement with GRACE in this region. The Amery Ice Shelf in East Antarctica is the third largest Antarctic ice shelf with a basin that has enough ice to raise sea level by 7.8 m but has presented challenges in previous mass balance efforts. We find that the mass in this drainage basin is not changing significantly. Furthermore, only one out of the three examined RCMs agrees with GRACE observations in this region. These results suggest that the RCMs may need to be revisited in some regions of the ice sheet. The drainage of Getz Ice Shelf, West Antarctica, is rapidly losing mass, whereas the drainage of Amery Ice Shelf is near balanceThe GRACE mass balance agrees with the mass budget method with several regional climate models on Getz but only with RACMO2.3p1 on AmeryThe GRACE‐based methodology helps evaluate RCMs and increase confidence in mass balance estimates around Antarctica

Southeast Greenland has been a major participant in the ice sheet mass loss over the last several decades. Interpreting the evolution of glacier fronts requires information about their depth below sea level and ocean thermal forcing, which are incompletely known in the region. Here, we combine airborne gravity and multibeam echo sounding data from the National Aeronautics and Space Administration's Oceans Melting Greenland (OMG) mission with ocean probe and fishing boat depth data to reconstruct the bathymetry extending from the glacier margins to the edges of the continental shelf. We perform a three‐dimensional inversion of the gravity data over water and merge the solution with a mass conservation reconstruction of bed topography over land. In contrast with other parts of Greenland, we find few deep troughs connecting the glaciers to the sources of warm Atlantic Water, amidst a relatively uniform, shallow (350 m) continental shelf. The deep channels include the Kangerlugssuaq, Sermilik, Gyldenløve, and Tingmiarmiut Troughs. A multisensor approach resolves the bathymetry on the continental shelf and glacial fjords of southeast GreenlandFew deep or complex networks of troughs connect the glaciers to the sources of warm Atlantic Water; that is, access to ocean heat is limitedThe new bathymetry helps interpret the past and recent evolution of southeast Greenland glaciers and contribution to sea level rise

Time-resolved satellite gravimetry has revolutionized understanding of mass transport in the Earth system. Since 2002, the Gravity Recovery and Climate Experiment (GRACE) has enabled monitoring of the terrestrial water cycle, ice sheet and glacier mass balance, sea level change and ocean bottom pressure variations, as well as understanding responses to changes in the global climate system. Initially a pioneering experiment of geodesy, the time-variable observations have matured into reliable mass transport products, allowing assessment and forecast of a number of important climate trends, and improvements in service applications such as the United States Drought Monitor. With the successful launch of the GRACE Follow-On mission, a multi-decadal record of mass variability in the Earth system is within reach. The Gravity Recovery and Climate Experiment (GRACE) mission, launched in 2002, allows monitoring of changes in hydrology and the cryosphere with terrestrial and ocean applications. This Review Article focuses on its contribution to the detection and quantification of climate change signals.

The surface mass balance of the Greenland ice sheet critically depends on the intensity of ice melt/snowmelt in its ablation zone, but in situ data have been too limited to quantify the error of regional climate models. Here we use 23 years of NASA satellite and airborne laser altimetry from the Airborne Topographic Mapper; Land, Vegetation, and Ice Sensor; and Ice, Cloud, and land Elevation Satellite to generate time series of elevation change to compare with surface mass balance products from the Regional Atmospheric Climate Model and from the Modèle Atmosphérique Régional. For 1994–2016, the results agree at the 15–26% level, with the largest discrepancy in North Greenland. During the cold summer of 2015, the root‐mean‐square discrepancy is 40% in the north, 30% in the southwest, and 18–25% at low elevation. The difference drops to 23% in the southwest and 14% at low elevation during the 2016 warm summer. Snowelt/ice melt is a major component of the mass loss of the Greenland ice sheet and its contribution to global sea level rise. Here we evaluate the accuracy of regional atmospheric climate models at reconstructing ice melt/snowmelt in Greenland for the first time using 23 years of thousands of kilometers of laser altimetry data. Such study was not possible in the past due to a lack of long‐term, geographically diverse, in situ data. We find that the regional climate models track down the long‐term trend in snowmelt/ice melt with remarkable accuracy across Greenland and do very well in warm summers, but we caution that model performance degrades at the regional level, for example, in North Greenland. We assess the accuracy of reconstructed runoff from regional climate models, a dominant control on the Greenland ice sheet mass balanceTime series of mass changes from 23 years of altimetry and seasonal data indicate that the error in runoff is at the 20% levelAirborne laser altimetry provides orders of magnitude more data for regional climate model assessment than available in situ

Totten and Moscow University glaciers, in the marine‐based sector of East Antarctica, contain enough ice to raise sea level by 5 m. Obtaining precise measurements of their mass balance is challenging owing to large area of the basins and the small mass balance signal compared to West Antarctic glaciers. Here we employ a locally optimized processing of Gravity Recovery and Climate Experiment (GRACE) harmonics to evaluate their mass balance at the sub‐basin scale and compare the results with mass budget method (MBM) estimates using regional atmospheric climate model version 2.3 (RACMO2.3) or Modèle Atmosphérique Régional version 3.6.4 (MAR3.6.4). The sub‐basin mass loss estimate for April 2002 to November 2015 is 14.8 ± 4.3 Gt/yr, which is weakly affected by glacial isostatic adjustment uncertainties (±1.4 Gt/yr). This result agrees with MBM/RACMO2.3 (15.8 ± 2.0 Gt/yr), whereas MBM/MAR3.6.4 underestimates the loss (6.6 ± 1.6 Gt/yr). For the entire drainage, the mass loss for April 2002 to August 2016 is 18.5 ± 6.6 Gt/yr, or 15 ± 4% of its ice flux. These results provide unequivocal evidence for mass loss in this East Antarctic sector. Totten and Moscow University glaciers in East Antarctica drain a marine‐based sector that holds an ice volume equivalent to several meters of global sea level rise. Recent observations of warm water intrusion on the continental shelf suggest that the glaciers may be changing in response to ocean warming. Understanding this glacier evolution is therefore of global significance. Measurements are difficult to obtain in this region due to the shear size of the basins and small rate of mass loss compared to other parts of Antarctica. To resolve this problem and evaluate independent estimates, we present a new methodology to process Gravity Recovery and Climate Experiment satellite gravity data that is optimized at the regional scale for these basins. Our results are in excellent agreement with independent estimates and provide unequivocal evidence that these glaciers have been losing mass rapidly for the past 15 years. We also compare different reconstructions of surface mass balance and determine that one model significantly underestimates the mass loss in this sector. A similar approach would be applicable to other parts of Antarctica to resolve residual uncertainties in its mass budget and contribution to sea level rise. Regionally optimized GRACE mascons at sub‐basin scale reveal mass loss of East Antarctic glaciers with multimeter sea level rise potentialMultiple lines of evidence from GRACE and mass budget method indicate significant mass loss from Totten and Moscow University glaciersSub‐basin mass balance assessments from GRACE help evaluate and compare regional atmospheric climate models

Mass changes of ice sheets, glaciers and ice caps, land water hydrology, atmosphere, and ocean cause a nonuniform sea level rise due to the self‐attraction and loading effects called sea level fingerprints (SLF). SLF have been previously derived from a combination of modeled and observed mass fluxes from the continents into the ocean. Here we derive improved SLF from time series of time variable gravity data from the Gravity Recovery and Climate Experiment (GRACE) mission for April 2002 to October 2014. We evaluate the GRACE‐derived SLF using ocean bottom pressure (OBP) data from stations in the tropics, where OBP errors are the lowest. We detect the annual phase of the SLF in the OBP signal and separate it unambiguously from the barystatic sea level (BSL) at two stations. At the basin scale, the SLF explain a larger fraction of the variance in steric‐corrected altimetry than the BSL, which has implications for evaluating mass transport between ocean basins. As ice sheets and glaciers and ice caps melt into the ocean, the pattern of regional sea level rise is nonuniform and tracked via the sea level fingerprints. Here we provide the first observational evidence that the SLF calculated from satellite observations match the record from ocean stations that measure mass changes over time, i.e., the Gravity Recovery and Climate Experiment‐based satellite technique correctly captures the distribution of freshwater fluxes to the ocean and the signal is large enough to be detected by ocean in situ observations in the tropics. The results are critical to improve regional projections of sea level rise and its impact on coastlines and human systems. Detection of sea level fingerprints (SLF) derived from GRACE gravity data using ocean bottom pressure data in the tropicsComparison of SLF and steric‐corrected altimetry reveals that the SLF have a significant impact on the estimation of ocean mass transportStudy provides a road map for improved detection of SLF

We compare four independent estimates of the mass balance of the Amundsen Sea Embayment of West Antarctica, an area experiencing rapid retreat and mass loss to the sea. We use ICESat and Operation IceBridge laser altimetry, Envisat radar altimetry, GRACE time‐variable gravity, RACMO2.3 surface mass balance, ice velocity from imaging radars, and ice thickness from radar sounders. The four methods agree in terms of mass loss and acceleration in loss at the regional scale. Over 1992–2013, the mass loss is 83 ± 5 Gt/yr with an acceleration of 6.1 ± 0.7 Gt/yr2. During the common period 2003–2009, the mass loss is 84 ± 10 Gt/yr with an acceleration of 16.3 ± 5.6 Gt/yr2, nearly 3 times the acceleration over 1992–2013. Over 2003–2011, the mass loss is 102 ± 10 Gt/yr with an acceleration of 15.7 ± 4.0 Gt/yr2. The results reconcile independent mass balance estimates in a setting dominated by change in ice dynamics with significant variability in surface mass balance. Four independent techniques of mass balance agree in the Amundsen Sea EmbaymentMost of the loss from that sector occurred in the last 10 yearsProduce regional mass balance record cross validated by four techniques

Several studies have been carried out on the tropopause, stratopause, and mesopause (collectively termed as ‘pauses’) independently; however, all the pauses have not been studied together. We present global distribution of altitudes and temperatures of these pauses observed with long-term space borne high-resolution measurements of Global Positioning System (GPS) Radio Occultation (RO) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) aboard Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite. Here we study the commonality and differences observed in the variability of all the pauses. We also examined how good other datasets will represent these features among (and in between) different satellite measurements, re-analysis, and model data. Hemispheric differences observed in all the pauses are also reported. In addition, we show that asymmetries between northern and southern hemispheres continue up to the mesopause. We analyze inter and intra-seasonal variations and long-term trends of these pauses at different latitudes. Finally, a new reference temperature profile is shown from the ground to 110 km for tropical, mid-latitudes, and polar latitudes for both northern and southern hemispheres.

Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea‐level observing system, the knowledge of regional sea‐level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea‐level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea‐level change. Here we review the individual processes which lead to sea‐level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea‐level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea‐level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea‐level change. This review paper addresses three important questions: (1) What do we currently know about the processes contributing to sea level change? (2) What observations do we use to gain this knowledge? and (3) Where are these gaps in our knowledge and the need for further improvement in our understanding of the drivers of regional sea level? By answering these specific questions in a focused manner, this paper should be a useful resource for other scientists, sea‐level stakeholders, and a broader audience of those interested in sea level and our changing climate. An overview of the current state of understanding of the processes that cause regional sea‐level change is providedAreas where the lack of understanding or gaps in knowledge inhibit the ability to assess future sea‐level change are discussedThe role of the expanded sea‐level observation network in improving our understanding of sea‐level change is highlighted

We examine data continuity between the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (FO) missions over Greenland and Antarctica using independent data from the mass budget method, which calculates the difference between ice sheet surface mass balance and ice discharge at the periphery. For both ice sheets, we find consistent GRACE/GRACE‐FO time series across the data gap, at the continental and regional scales, and the data gap is confidently filled with mass budget method data. In Greenland, the GRACE‐FO data reveal an exceptional summer loss of 600 Gt in 2019 following two cold summers. In Antarctica, ongoing high mass losses in the Amundsen Sea Embayment of West Antarctica, the Antarctic Peninsula, and Wilkes Land in East Antarctica cumulate to 2130, 560, and 370 Gt, respectively, since 2002. A cumulative mass gain of 980 Gt in Queen Maud Land since 2009, however, led to a pause in the acceleration in mass loss from Antarctica after 2016. We demonstrate data continuity of the GRACE and GRACE‐FO missions over Greenland and Antarctica using independent dataGRACE‐FO data capture a record‐high summer loss (600 Gt) in Greenland in 2019Mass gain in Queen Maud Land mitigate high losses in the Amundsen Sea, Peninsula, and Wilkes Land to pause the acceleration in mass loss

There is considerable controversy regarding the long‐term strength of continents (Te). While some authors obtain both low and high Teestimates from the Bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength, others obtain estimates of only <25 km using the free‐air admittance and suggest that the mantle is weak. At the root of this controversy is how accurately Tecan be recovered from coherence and admittance. We investigate this question by using synthetic topography and gravity anomaly data for which Teis known. We show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free‐air admittance. We reformulate the admittance method and show that it can recover synthetic Teestimates similar to those recovered using coherence. In light of these results, we estimate Tein Fennoscandia and obtain similar results using both techniques. Teis 20–40 km in the Caledonides, 40–60 km in the Swedish Svecofennides, 40–60 km in the Kola peninsula, and 70–100 km in southern Karelia and Svecofennian central Finland. Independent rheological modeling, using a xenolith‐controlled geotherm, predicts similar high Tein central Finland. Because Teexceeds crustal thickness in this area, the mantle must contribute significantly to the total strength. Tein Fennoscandia increases with tectonic age, seismic lithosphere thickness, and decreasing heat flow, and low Tecorrelates with frequent seismicity. However, in Proterozoic and Archean lithosphere the relationship of Teto age is ambiguous, suggesting that compositional variations may influence the strength of continents.