Seasonal dynamic thinning at Helheim Glacier

AbstractWe investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using TanDEM-X interferometric digital elevation models (DEMs), bedrock GPS measurements, and ice velocity from feature-tracking. The DEMs exhibit seasonal surface elevation cycles at elevations up to 800 m.a.s.l. with amplitudes of up to 19 m, from a maximum in July to a minimum in October or November, concentrated on the fast-flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin. By modelling the detrended bedrock loading/unloading signal we estimate a mean density for the loss of 671±70 kgm−3 and calculate that total water equivalent volume loss from the active part of the glacier (surface flow speeds >1 m day−1) ranges from 0.5 km3 in 2011 to 1.6 km3 in 2013. A rough ice-flux divergence analysis shows that at lower elevations (<200 m) mass loss by dynamic thinning fully explains seasonal elevation changes. In addition, surface elevations decrease by a greater amount than field observations of surface ablation or surface-energy-balance modelling predict, emphasising the dynamic nature of the mass loss. We conclude, on the basis of ice-front position observations through the time series, that melt-induced acceleration is most likely the main driver of the seasonal dynamic thinning, as opposed to changes triggered by retreat

Brief communication: Thwaites Glacier cavity evolution

Between 2014 and 2017, ocean melt eroded a large cavity beneath and along the western margin of the fast-flowing core of Thwaites Glacier. Here we show that from2017 to the end of 2020 the cavity persisted but did not ex-pand. This behaviour, of melt concentrated at the groundingline within confined sub-shelf cavities, fits with prior observations and modelling studies. We also show that acceleration and thinning of Thwaites Glacier grounded ice continued, with an increase in speed of 400 m a−1and a thinning rate of at least 1.5 m a−1, between 2012 and 2020

Strong Ocean Melting Feedback During the Recent Retreat of Thwaites Glacier

Accelerating ice loss from Thwaites Glacier is contributing approximately 5% of global sea-level rise, and could add tens of centimeters to sea level over the coming centuries. We use an ocean model to calculate sub-ice melting for a succession of Digital Elevation Models of the main trunk of Thwaites Glacier from 2011 to 2022. The ice evolution during this period induces a strong geometrical feedback onto melting. Ice thinning and retreat provides a larger melting area, thicker and better-connected sub-ice water column, and steeper ice base. This leads to stronger sub-ice ocean currents, increasing melting by over 30% without any change in forcing from wider ocean conditions. This geometrical feedback over just 12 years is comparable to melting changes arising from plausible century-scale changes in ocean conditions and subglacial meltwater inflow. These findings imply that ocean-driven ice loss from Thwaites Glacier may only be weakly influenced by anthropogenic emissions mitigation

Seawater softening of suture zones inhibits fracture propagation in Antarctic ice shelves

Suture zones are abundant on Antarctic ice shelves and widely observed to impede fracture propagation, greatly enhancing ice-shelf stability. Using seismic and radar observations on the Larsen C Ice Shelf of the Antarctic Peninsula, we confirm that such zones are highly heterogeneous, consisting of multiple meteoric and marine ice bodies of diverse provenance fused together. Here we demonstrate that fracture detainment is predominantly controlled by enhanced seawater content in suture zones, rather than by enhanced temperature as previously thought. We show that interstitial seawater can reduce fracture-driving stress by orders of magnitude, promoting both viscous relaxation and the development of micro cracks, the incidence of which scales inversely with stress intensity. We show how simple analysis of viscous buckles in ice-penetrating radar data can quantify the seawater content of suture zones and their modification of the ice-shelf’s stress regime. By limiting fracture, enhancing stability and restraining continental ice discharge into the ocean, suture zones act as vital regulators of Antarctic mass balance

Flow-line model code for accumulation of ice along velocity-based trajectories

The flow-line model was designed to enable estimation of the age and surface origin for various ice bodies identified within hot-water drilled boreholes on Larsen C Ice Shelf. Surface fluxes are accumulated, converted to thicknesses, and advected down flow from a fixed number of selected points. The model requires input datasets of surface mass balance, surface velocity, vertical strain rates, ice-shelf thickness, and a vertical density profile. This model is part of a larger project. Input datasets such as density profiles and trajectory vectors are available separately. Resolution is dependent on the input datasets. Funding was provided by the NERC grant NE/L005409/1

Effect of angiotensin-converting enzyme inhibitor and angiotensin receptor blocker initiation on organ support-free days in patients hospitalized with COVID-19

IMPORTANCE Overactivation of the renin-angiotensin system (RAS) may contribute to poor clinical outcomes in patients with COVID-19. Objective To determine whether angiotensin-converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB) initiation improves outcomes in patients hospitalized for COVID-19. DESIGN, SETTING, AND PARTICIPANTS In an ongoing, adaptive platform randomized clinical trial, 721 critically ill and 58 non–critically ill hospitalized adults were randomized to receive an RAS inhibitor or control between March 16, 2021, and February 25, 2022, at 69 sites in 7 countries (final follow-up on June 1, 2022). INTERVENTIONS Patients were randomized to receive open-label initiation of an ACE inhibitor (n = 257), ARB (n = 248), ARB in combination with DMX-200 (a chemokine receptor-2 inhibitor; n = 10), or no RAS inhibitor (control; n = 264) for up to 10 days. MAIN OUTCOMES AND MEASURES The primary outcome was organ support–free days, a composite of hospital survival and days alive without cardiovascular or respiratory organ support through 21 days. The primary analysis was a bayesian cumulative logistic model. Odds ratios (ORs) greater than 1 represent improved outcomes. RESULTS On February 25, 2022, enrollment was discontinued due to safety concerns. Among 679 critically ill patients with available primary outcome data, the median age was 56 years and 239 participants (35.2%) were women. Median (IQR) organ support–free days among critically ill patients was 10 (–1 to 16) in the ACE inhibitor group (n = 231), 8 (–1 to 17) in the ARB group (n = 217), and 12 (0 to 17) in the control group (n = 231) (median adjusted odds ratios of 0.77 [95% bayesian credible interval, 0.58-1.06] for improvement for ACE inhibitor and 0.76 [95% credible interval, 0.56-1.05] for ARB compared with control). The posterior probabilities that ACE inhibitors and ARBs worsened organ support–free days compared with control were 94.9% and 95.4%, respectively. Hospital survival occurred in 166 of 231 critically ill participants (71.9%) in the ACE inhibitor group, 152 of 217 (70.0%) in the ARB group, and 182 of 231 (78.8%) in the control group (posterior probabilities that ACE inhibitor and ARB worsened hospital survival compared with control were 95.3% and 98.1%, respectively). CONCLUSIONS AND RELEVANCE In this trial, among critically ill adults with COVID-19, initiation of an ACE inhibitor or ARB did not improve, and likely worsened, clinical outcomes. TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT0273570

Stable dynamics in a Greenland tidewater glacier over 26 years despite reported thinning

Daugaard Jensen Gletscher, Greenland, is a large tidewater glacier terminating in the northwest corner of the Scoresby Sund fjords. We present a time series of surface flow speeds spanning 1985-2010 based on feature tracking of satellite images. The time series confirms that flow speeds remained stable and reveals a persistent summer acceleration of up to 10 over the lower 10km of the glacier. The front of the 6 km floating tongue fluctuates by little more than the average size of calved icebergs, 1 km. While we are unable to detect any imbalance between ice discharge and surface mass balance within our error estimates, observations suggest that the region is losing mass and experiencing decreases in surface elevation. We conclude that as flow speeds and surface mass balance have remained steady since 1985, the shift from balance to imbalance, leading to elevation decrease, must have occurred prior to this date. As for other stable Greenland tidewater glaciers, the seasonal melt cycle is the dominant influence on flow velocity variation but, if the apparent current thinning rates continue, there is potential for the grounding line to retreat, for calving rates to increase and for the glacier to accelerate. © 2012 Publishing Technology

Rapid fragmentation of Thwaites Eastern Ice Shelf

Funding was provided by the National Science Foundation (NSF: grant nos. 1738896 and 1929991) and Natural Environment Research Council (NERC: grant no. NE/S006605/1). Rupert Gladstone is supported by Academy of Finland (grant no. 322430), Thomas Zwinger by grant no. 322978.Ice shelves play a key role in the dynamics of marine ice sheets by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice-sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last 5 years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and a release of tabular bergs on both the eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear, upstream of the main pinning point, developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedback between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished over time as the pinning point shrunk, accumulation of damage has ensured that the ice in the shear zone remained the weakest link in the system. Experiments with the BISICLES ice-sheet model indicate that additional damage to or unpinning of TEIS is unlikely to trigger significantly increased ice loss from WAIS, but the calving response to the loss of TEIS remains highly uncertain. It is widely recognised that ice-shelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice-shelf margins or grounding points. Our results indicate a third mechanism, backstress triggered failure, that can occur if and when an ice shelf is no longer able to withstand stress imposed by pinning points. In most circumstances, pinning points are essential for ice-shelf stability, but as ice shelves thin and weaken, the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.Publisher PDFPeer reviewe