Ubiquitous acceleration in Greenland Ice Sheet calving from 1985 to 2022.

Nearly every glacier in Greenland has thinned or retreated over the past few decades1-4, leading to glacier acceleration, increased rates of sea-level rise and climate impacts around the globe5-9. To understand how calving-front retreat has affected the ice-mass balance of Greenland, we combine 236,328 manually derived and AI-derived observations of glacier terminus positions collected from 1985 to 2022 and generate a 120-m-resolution mask defining the ice-sheet extent every month for nearly four decades. Here we show that, since 1985, the Greenland Ice Sheet (GrIS) has lost 5,091 ± 72 km2 of area, corresponding to 1,034 ± 120 Gt of ice lost to retreat. Our results indicate that, by neglecting calving-front retreat, current consensus estimates of ice-sheet mass balance4,9 have underestimated recent mass loss from Greenland by as much as 20%. The mass loss we report has had minimal direct impact on global sea level but is sufficient to affect ocean circulation and the distribution of heat energy around the globe10-12. On seasonal timescales, Greenland loses 193 ± 25 km2 (63 ± 6 Gt) of ice to retreat each year from a maximum extent in May to a minimum between September and October. We find that multidecadal retreat is highly correlated with the magnitude of seasonal advance and retreat of each glacier, meaning that terminus-position variability on seasonal timescales can serve as an indicator of glacier sensitivity to longer-term climate change.

Annual mass budget of Antarctic ice shelves from 1997 to 2021.

Antarctic ice shelves moderate the contribution of the Antarctic Ice Sheet to global sea level rise; however, ice shelf health remains poorly constrained. Here, we present the annual mass budget of all Antarctic ice shelves from 1997 to 2021. Out of 162 ice shelves, 71 lost mass, 29 gained mass, and 62 did not change mass significantly. Of the shelves that lost mass, 68 had statistically significant negative mass trends, 48 lost more than 30% of their initial mass, and basal melting was the dominant contributor to that mass loss at a majority (68%). At many ice shelves, mass losses due to basal melting or iceberg calving were significantly positively correlated with grounding line discharge anomalies; however, the strength and form of this relationship varied substantially between ice shelves. Our results illustrate the utility of partitioning high-resolution ice shelf mass balance observations into its components to quantify the contributors to ice shelf mass change and the response of grounded ice.

Antarctic calving loss rivals ice-shelf thinning.

Antarctica's ice shelves help to control the flow of glacial ice as it drains into the ocean, meaning that the rate of global sea-level rise is subject to the structural integrity of these fragile, floating extensions of the ice sheet1-3. Until now, data limitations have made it difficult to monitor the growth and retreat cycles of ice shelves on a large scale, and the full impact of recent calving-front changes on ice-shelf buttressing has not been understood. Here, by combining data from multiple optical and radar satellite sensors, we generate pan-Antarctic, spatially continuous coastlines at roughly annual resolution since 1997. We show that from 1997 to 2021, Antarctica experienced a net loss of 36,701 ± 1,465 square kilometres (1.9 per cent) of ice-shelf area that cannot be fully regained before the next series of major calving events, which are likely to occur in the next decade. Mass loss associated with ice-front retreat (5,874 ± 396 gigatonnes) has been approximately equal to mass change owing to ice-shelf thinning over the past quarter of a century (6,113 ± 452 gigatonnes), meaning that the total mass loss is nearly double that which could be measured by altimetry-based surveys alone. We model the impacts of Antarctica's recent coastline evolution in the absence of additional feedbacks, and find that calving and thinning have produced equivalent reductions in ice-shelf buttressing since 2007, and that further retreat could produce increasingly significant sea-level rise in the future.

Basal channels drive active surface hydrology and transverse ice shelf fracture.

Ice shelves control sea-level rise through frictional resistance, which slows the seaward flow of grounded glacial ice. Evidence from around Antarctica indicates that ice shelves are thinning and weakening, primarily driven by warm ocean water entering into the shelf cavities. We have identified a mechanism for ice shelf destabilization where basal channels underneath the shelves cause ice thinning that drives fracture perpendicular to flow. These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures. We report direct evidence that a major 2016 calving event at Nansen Ice Shelf in the Ross Sea was the result of fracture driven by such channelized thinning and demonstrate that similar basal channel-driven transverse fractures occur elsewhere in Greenland and Antarctica. In the event of increased basal and surface melt resulting from rising ocean and air temperatures, ice shelves will become increasingly vulnerable to these tandem effects of basal channel destabilization.

Wind causes Totten Ice Shelf melt and acceleration.

Totten Glacier in East Antarctica has the potential to raise global sea level by at least 3.5 m, but its sensitivity to climate change has not been well understood. The glacier is coupled to the ocean by the Totten Ice Shelf, which has exhibited variable speed, thickness, and grounding line position in recent years. To understand the drivers of this interannual variability, we compare ice velocity to oceanic wind stress and find a consistent pattern of ice-shelf acceleration 19 months after upwelling anomalies occur at the continental shelf break nearby. The sensitivity to climate forcing we observe is a response to wind-driven redistribution of oceanic heat and is independent of large-scale warming of the atmosphere or ocean. Our results establish a link between the stability of Totten Glacier and upwelling near the East Antarctic coast, where surface winds are projected to intensify over the next century as a result of increasing atmospheric greenhouse gas concentrations.

Laboratory investigation of a passive acoustic method for measurement of underwater gas seep ebullition.

Passive acoustic techniques are of interest as a low-power means of quantifying underwater point-source gas ebullition. Toward the development of systems for logging natural seep activity, laboratory experiments were performed that exploited the bubble's Minnaert natural frequency for the measurement of gas flow from a model seep. Results show agreement among acoustic, optical, and gas trap ebullition measurements over the range of emission rates from 0 to 10 bubbles per second. A mathematical model is proposed to account for the real gas behavior of bubbles which cannot be approximated as ideal, such as methane at marine depths exceeding 30 m.