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We use satellite and airborne altimetry to estimate annual mass changes of the Greenland Ice Sheet. We estimate ice loss corresponding to a sea‐level rise of 6.9 ± 0.4 mm from April 2011 to April 2020, with a highest annual ice loss rate of 1.4 mm/yr sea‐level equivalent from April 2019 to April 2020. On a regional scale, our annual mass loss timeseries reveals 10–15 m/yr dynamic thickening at the terminus of Jakobshavn Isbræ from April 2016 to April 2018, followed by a return to dynamic thinning. We observe contrasting patterns of mass loss acceleration in different basins across the ice sheet and suggest that these spatiotemporal trends could be useful for calibrating and validating prognostic ice sheet models. In addition to resolving the spatial and temporal fingerprint of Greenland's recent ice loss, these mass loss grids are key for partitioning contemporary elastic vertical land motion from longer‐term glacial isostatic adjustment (GIA) trends at GPS stations around the ice sheet. Our ice‐loss product results in a significantly different GIA interpretation from a previous ice‐loss product. Plain Language Summary: Greenland ice loss has accelerated over the last three decades. The Greenland Ice Sheet is currently one of the largest contributors to global sea‐level rise. We combine airborne and satellite altimetry measurements to make annual digital elevation models of the ice sheet during 2011–2020. Over this period, we find that the ice sheet lost an ice volume corresponding to 6.9 ± 0.4 mm of global sea‐level equivalent. The peak loss year was April 2019 to April 2020, when the ice sheet lost 1.4 mm of global sea‐level equivalent. This peak loss rate is equivalent to losing 15,850 tonnes of ice per second for 12 months. We also find that the acceleration of ice loss differs across different ice‐sheet sectors. We suggest that these regional ice loss trends may be a good target for the ice‐sheet models used to project future ice loss. Key Points: The Greenland Ice Sheet contributed 6.9 ± 0.4 mm to sea‐level from April 2011 to April 2020Satellite altimetry suggests a peak annual ice loss of 498 ± 45 Gt from April 2019 to April 2020The terminus of Jakobshavn Isbræ is once again dynamically thinning, following a period of dynamic thickening during 2016–2018 [ABSTRACT FROM AUTHOR]

Accurately projecting mass loss from ice sheets is of critical societal importance. However, despite recent improvements in ice sheet models, our analysis of a recent effort to project ice sheet contribution to future sea level suggests that few models reproduce historical mass loss accurately and that they appear much too confident in the spread of predicted outcomes. The inability of models to reproduce historical observations raises concerns about the models' skill at projecting mass loss. Here we suggest that uncertainties in the future sea level contribution from Greenland and Antarctica may well be significantly higher than reported in that study. We propose a roadmap to enable a more realistic accounting of uncertainties associated with such forecasts and a formal process by which observations of mass change should be used to refine projections of mass change. Finally, we note that tremendous government investment and planning affecting tens to hundreds of millions of people is founded on the work of just a few tens of scientists. To achieve the goal of credible projections of ice sheet contribution to sea level, we strongly believe that investment in research must be commensurate with the scale of the challenge. [ABSTRACT FROM AUTHOR]

We present a novel method to estimate dynamic ice loss of Greenland's three largest outlet glaciers: Jakobshavn Isbræ, Kangerlussuaq Glacier, and Helheim Glacier. We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure elastic displacements of the solid Earth caused by dynamic thinning near the glacier terminus. When we compare our results with discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS time series we are able to predict speedup/slowdown of Jakobshavn Isbræ by up to 10.4 months. For Kangerlussuaq Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months). Our methodology and results could be important for studies that attempt to model and understand mechanisms controlling short‐term dynamic fluctuations of outlet glaciers in Greenland. Plain Language Summary: A wide range of sensors and methods have been used to study the changes of the Greenland Ice Sheet, including satellite gravimetry, altimetry, and the input‐output method. Here, we present a novel fourth method to estimate dynamic ice loss of Greenland's three largest outlet glaciers: Jakobshavn Isbræ, Kangerlussuaq Glacier, and Helheim Glacier. We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure rise of land masses caused by ongoing ice mass loss near the glacier terminus. When we compare our results with ice discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic induced thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS uplift time series we are able to predict ice flow speedup/slowdown of Jakobshavn Isbræ by up to 10 months. For Kangerlussuaq Glacier and Helheim Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months) and 0.03 ± 0.16 years, respectively. Our methodology and results could be important for studies that attempt to model and understand mechanisms controlling short‐term dynamic fluctuations of outlet glaciers in Greenland. Key Points: A novel method to estimate dynamic ice loss of Greenland's three largest outlet glaciers, Jakobshavn, Kangerlussuaq, and Helheim glacierDynamic thinning/thickening occurs 0.87 ± 0.07 years before speedup/slowdown at Jakobshavn IsbræA similar time lag between change in uplift rate and flow speed change allows us to predict future ice discharge from past uplift [ABSTRACT FROM AUTHOR]

Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland's largest icebergs and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial radar interferometric observations from August 2012 to characterize the events that led to record-high flow. We find that the highest speeds occurred in response to a small calving retreat, while several larger calving events produced negligible changes in glacier speed. This non-linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position. Our observations indicate that a glacier's response to calving is a consequence of two competing feedbacks: (1) an increase in strain rates that leads to dynamic thinning and faster flow, thereby promoting destabilization, and (2) an increase in flow rates that advects thick ice toward the terminus and promotes restabilization. The competition between these feedbacks depends on temporal and spatial variations in the glacier's proximity to flotation. This study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability, and further suggests the latter may be limiting the current retreat due to the thick ice that occupies Jakobshavn Isbræ's retrograde bed. [ABSTRACT FROM AUTHOR]

Outlet glaciers undergo rapid spatial and temporal changes in flow velocity during calving events. Observing such changes requires both high temporal and high spatial resolution methods, something now possible with terrestrial radar interferometry. While a single such radar provides line-of-sight velocity, two radars define both components of the horizontal flow field. To assess the feasibility of obtaining the two-dimensional (2-D) flow field, we deployed two terrestrial radar interferometers at Jakobshavn Isbrae, a major outlet glacier on Greenland's west coast, in the summer of 2012. Here, we develop and demonstrate a method to combine the line-of-sight velocity data from two synchronized radars to produce a 2-D velocity field from a single (3 min) interferogram. Results are compared with the more traditional feature-tracking data obtained from the same radar, averaged over a longer period. We demonstrate the potential and limitations of this new dual-radar approach for obtaining high spatial and temporal resolution 2-D velocity fields at outlet glaciers. [ABSTRACT FROM PUBLISHER]

The recent retreat and speedup of outlet glaciers, as well as enhanced surface melting around the ice sheet margin, have increased Greenland's contribution to sea level rise to 0.6 ± 0.1 mm yr-1 and its discharge of freshwater into the North Atlantic. The widespread, near-synchronous glacier retreat, and its coincidence with a period of oceanic and atmospheric warming, suggests a common climate driver. Evidence points to the marine margins of these glaciers as the region from which changes propagated inland. Yet, the forcings and mechanisms behind these dynamic responses are poorly understood and are either missing or crudely parameterized in climate and ice sheet models. Resulting projected sea level rise contributions from Greenland by 2100 remain highly uncertain. This paper summarizes the current state of knowledge and highlights key physical aspects of Greenland's coupled ice sheet-ocean-atmosphere system. Three research thrusts are identified to yield fundamental insights into ice sheet, ocean, sea ice, and atmosphere interactions, their role in Earth's climate system, and probable trajectories of future changes: 1) focused process studies addressing critical glacier, ocean, atmosphere, and coupled dynamics; 2) sustained observations at key sites; and 3) inclusion of relevant dynamics in Earth system models. Understanding the dynamic response of Greenland's glaciers to climate forcing constitutes both a scientific and technological frontier, given the challenges of obtaining the appropriate measurements from the glaciers' marine termini and the complexity of the dynamics involved, including the coupling of the ocean, atmosphere, glacier, and sea ice systems. Interdisciplinary and international cooperation are crucial to making progress on this novel and complex problem. [ABSTRACT FROM AUTHOR]

The article presents a case study on the surface melt and ice motion activities in several timescales on Jakoshavn Isbræ in Greenland. The researchers conclude the speed ups and extra slowdowns may be attributed to the meltwater or sudden influx of superglacial lakes drainage water. An overview of the methodology employed is also discussed.

The article explains a framework for iceberg-calving models ideal for any calving margin. Considering steady-state profiles of ice shelves is one way to estimate variations in ice-shelf thickness gradient. Thickness and velocity at the grounding line tend to strongly influence the geometry of the inner shelf while balance rate and shear stresses on the shelf margins determine the geometry of the outer shelf. The foundations of the framework include mass continuity and the idea that terminus thickness is larger after a calving event prior to the event. Also highlighted is the finding of the steady-state analysis on the key role of ice thickness, thickness gradient, strain rates and melting of the terminus in calving rate.

The article presents a study which analyzed 1985 aerial photos of the dramatic thinning of the Jakobshavn Isbræ glacier in West Greenland focusing on the amount of its ice loss. NASA Airborne Topographic Mapper (ATM) flights were used to assess volume changes to the ice surface between 1985 and 1997. Uncertainty estimates for the 1985 and 2007 digital elevation models (DEMs) were also assessed to identify elevation differences among ground-truth land points to the ice margins. It was found that ocean forcing and dynamic thinning causes the ice loss from the glacier since 1997.

The article discusses a study which examines the formation of icebergs that shows thick layers of temperate ice at the base of Jakobshavn Isbrae, Greenland. It states the glacier flows fast but has no seasonal velocity changes even if it has a large volume of surface-derived meltwater that enters the ice stream. It notes that icebergs show blue ice during and after calving and the presence of thick layer of temperature ice indicates high ice-deformations. The study show the prospect of using rotated icebergs to examine the vertical structure of the glacier.