Ocean-induced basal melting is responsible for much of
the Amundsen Sea Embayment ice loss in recent decades, but the total
magnitude and spatiotemporal evolution of this melt is poorly constrained.
...To address this problem, we generated a record of high-resolution digital
elevation models (DEMs) for Pine Island Glacier (PIG) using commercial
sub-meter satellite stereo imagery and integrated additional 2002–2015
DEM and altimetry data. We implemented a Lagrangian elevation change (Dh∕Dt)
framework to estimate ice shelf basal melt rates at 32–256 m resolution. We
describe this methodology and consider basal melt rates and elevation change
over the PIG ice shelf and lower catchment from 2008 to 2015. We document
the evolution of Eulerian elevation change (dh∕dt) and upstream propagation of
thinning signals following the end of rapid grounding line retreat around
2010. Mean full-shelf basal melt rates for the 2008–2015 period were
∼82–93 Gt yr−1, with ∼200–250 m yr−1 basal melt
rates within large channels near the grounding line, ∼10–30 m yr−1 over the main shelf, and ∼0–10 m yr−1 over the North shelf and
South shelf, with the notable exception of a small area with rates of
∼50–100 m yr−1 near the grounding line of a fast-flowing
tributary on the South shelf. The observed basal melt rates show excellent
agreement with, and provide context for, in situ basal melt-rate observations. We
also document the relative melt rates for kilometer-scale basal channels and keels
at different locations on the ice shelf and consider implications for ocean
circulation and heat content. These methods and results offer new indirect
observations of ice–ocean interaction and constraints on the processes
driving sub-shelf melting beneath vulnerable ice shelves in West Antarctica.
Glaciers in High Mountain Asia are an important freshwater resource for large populations living downstream who rely on runoff for hydropower, irrigation, and municipal use. Projections of glacier ...mass change and runoff therefore have important socio-economic impacts. In this study, we use a new dataset of geodetic mass balance observations of almost all glaciers in the region to calibrate the Python Glacier Evolution Model (PyGEM) using Bayesian inference. The new dataset enables the model to capture spatial variations in mass balance and the Bayesian inference enables the uncertainty associated with the model parameters to be quantified. Validation with historical mass balance observations shows the model performs well and the uncertainty is well captured. Projections of glacier mass change for 22 General Circulation Models (GCMs) and four Representative Concentration Pathways (RCPs) estimate that by the end of the century glaciers in High Mountain Asia will lose between 29 ± 12% (RCP 2.6) and 67 ± 10% (RCP 8.5) of their total mass relative to 2015. Considerable spatial and temporal variability exists between regions due to the climate forcing and glacier characteristics (hypsometry, ice thickness, elevation range). Projections of annual glacier runoff reveal most monsoon-fed river basins (Ganges, Brahmaputra) will hit a maximum (peak water) prior to 2050, while the Indus and other westerlies-fed river basins will likely hit peak water after 2050 due to significant contributions from excess glacier meltwater. Monsoon-fed watersheds are projected to experience large reductions in end-of-summer glacier runoff. Uncertainties in projections at regional scales are dominated by the uncertainty associated with the climate forcing, while at the individual glacier level, uncertainties associated with model parameters can be significant.
High-mountain Asia (HMA) constitutes the largest glacierized region outside of the Earth's polar regions. Although available observations are limited, long-term records indicate sustained HMA glacier ...mass loss since ~1850, with accelerated loss in recent decades. Recent satellite data capture the spatial variability of this mass loss, but spatial resolution is coarse and some estimates for regional and HMA-wide mass loss disagree. To address these issues, we generated 5,797 high-resolution digital elevation models (DEMs) from available sub-meter commercial stereo imagery (DigitalGlobe WorldView-1/2/3 and GeoEye-1) acquired over HMA glaciers from 2007 to 2018 (primarily 2013–2017). We also reprocessed 28,278 ASTER DEMs over HMA from 2000 to 2018. We combined these observations to generate robust elevation change trend maps and geodetic mass balance estimates for 99% of HMA glaciers between 2000 and 2018. We estimate total HMA glacier mass change of −19.0 ± 2.5 Gt yr−1 (−0.19 ± 0.03 m w.e. yr−1). We document the spatial pattern of HMA glacier mass change with unprecedented detail, and present aggregated estimates for HMA glacierized sub-regions and hydrologic basins. Our results offer improved estimates for the HMA contribution to global sea level rise in recent decades with total cumulative sea-level rise contribution of ~0.7 mm from exorheic basins between 2000 and 2018. We estimate that the range of excess glacier meltwater runoff due to negative glacier mass balance in each basin constitutes ~12–53% of the total basin-specific glacier meltwater runoff. These results can be used for calibration and validation of glacier mass balance models, satellite gravimetry observations, and hydrologic models needed for present and future water resource management.
We adapted the automated, open source NASA Ames Stereo Pipeline (ASP) to generate digital elevation models (DEMs) and orthoimages from very-high-resolution (VHR) commercial imagery of the Earth. ...These modifications include support for rigorous and rational polynomial coefficient (RPC) sensor models, sensor geometry correction, bundle adjustment, point cloud co-registration, and significant improvements to the ASP code base. We outline a processing workflow for ∼0.5m ground sample distance (GSD) DigitalGlobe WorldView-1 and WorldView-2 along-track stereo image data, with an overview of ASP capabilities, an evaluation of ASP correlator options, benchmark test results, and two case studies of DEM accuracy. Output DEM products are posted at ∼2m with direct geolocation accuracy of <5.0m CE90/LE90. An automated iterative closest-point (ICP) co-registration tool reduces absolute vertical and horizontal error to <0.5m where appropriate ground-control data are available, with observed standard deviation of ∼0.1–0.5m for overlapping, co-registered DEMs (n=14,17). While ASP can be used to process individual stereo pairs on a local workstation, the methods presented here were developed for large-scale batch processing in a high-performance computing environment. We are leveraging these resources to produce dense time series and regional mosaics for the Earth’s polar regions.
The speed of Greenland's fastest glacier, Jakobshavn Isbrae, has varied substantially since its speedup in the late 1990s. Here we present observations of surface velocity, mélange rigidity, and ...surface elevation to examine its behaviour over the last decade. Consistent with earlier results, we find a pronounced cycle of summer speedup and thinning followed by winter slowdown and thickening. There were extended periods of rigid mélange in the winters of 2016-17 and 2017-18, concurrent with terminus advances ~6 km farther than in the several winters prior. These terminus advances to shallower depths caused slowdowns, leading to substantial thickening, as has been noted elsewhere. The extended periods of rigid mélange coincide well with a period of cooler waters in Disko Bay. Thus, along with the relative timing of the seasonal slowdown, our results suggest that the ocean's dominant influence on Jakobshavn Isbrae is through its effect on winter mélange rigidity, rather than summer submarine melting. The elevation time series also reveals that in summers when the area upstream of the terminus approaches flotation, large surface depressions can form, which eventually become the detachment points for major calving events. It appears that as elevations near flotation, basal crevasses can form, which initiates a necking process that forms the depressions. The elevation data also show that steep cliffs often evolve into short floating extensions, rather than collapsing catastrophically due to brittle failure. Finally, summer 2019 speeds were slightly faster than the prior two summers, leaving it unclear whether the slowdown is ending.
We produced a 6 year time series of differential tidal displacement for Pine Island Ice Shelf, Antarctica, using speckle‐tracking methods applied to fine‐resolution TerraSAR‐X data. These results ...reveal that the main grounding line has maintained a relatively steady position over the last 6 years, following the speedup that terminated in ~2009. In the middle of the shelf, there are grounded spots that migrate downstream over the 6 year record. Examination of high‐resolution digital elevation models reveals that these grounded spots form where deep keels (thickness anomalies) advect over an approximately flow‐parallel bathymetric high, maintaining intermittent contact with the bed. These data sets also reveal several subsidence and uplift events associated with subglacial lake drainages in the fast‐flowing region above the grounding line. Although these drainages approximately double the rate of subglacial water flow over periods of a few weeks, they have no discernible effect on horizontal flow speed.
Key Points
Keels in the Pine Island Ice Shelf cause intermittent grounding
Current grounding position is stable and likely will remain so for the new few decades
Subglacial lake drainage has little influence of fast flow
Snow cover affects a diverse array of physical, ecological, and societal systems. As such, the development of optical remote sensing techniques to measure snow-covered area (SCA) has enabled progress ...in a wide variety of research domains. However, in many cases, the spatial and temporal resolutions of currently available remotely sensed SCA products are insufficient to capture SCA evolution at spatial and temporal resolutions relevant to the study of fine-scale spatially heterogeneous phenomena. We developed a convolutional neural network-based method to identify snow covered area using the ~3 m, 4-band PlanetScope optical satellite image dataset with ~daily, near-global coverage. By comparing our model performance to snow extent derived from high-resolution airborne lidar differential depth measurements and satellite platforms in two North American sites (Sierra Nevada, CA, USA and Rocky Mountains, CO, USA), we show that these emerging image archives have great potential to accurately observe snow-covered area at high spatial and temporal resolutions despite limited radiometric bandwidth and band placement. We achieve average snow classification F-Scores of 0.73 in our training basin and 0.67 in a climatically-distinct out-of-sample basin, suggesting opportunities for model transferability. We also evaluate the performance of these data in forested regions, suggesting avenues for further research. The unparalleled spatial and temporal coverage of CubeSat imagery offers an excellent opportunity for satellite remote sensing of snow, with real implications for ecological and water resource applications.
•Daily, 3-m snow covered area derived from Planet Labs Inc. “PlanetScope” data.•Model-based neural network approach enabled by airborne lidar.•Derived snow covered area compares well to Sentinel-2 and Landsat-8.•Good performance in both California and Colorado, USA, suggesting transferability.•Limited performance around tree canopies.
Abstract
The response of glaciers to climate change has major implications for sea-level change and water resources around the globe. Large-scale glacier evolution models are used to project glacier ...runoff and mass loss, but are constrained by limited observations, which result in models being over-parameterized. Recent systematic geodetic mass-balance observations provide an opportunity to improve the calibration of glacier evolution models. In this study, we develop a calibration scheme for a glacier evolution model using a Bayesian inverse model and geodetic mass-balance observations, which enable us to quantify model parameter uncertainty. The Bayesian model is applied to each glacier in High Mountain Asia using Markov chain Monte Carlo methods. After 10,000 steps, the chains generate a sufficient number of independent samples to estimate the properties of the model parameters from the joint posterior distribution. Their spatial distribution shows a clear orographic effect indicating the resolution of climate data is too coarse to resolve temperature and precipitation at high altitudes. Given the glacier evolution model is over-parameterized, particular attention is given to identifiability and the need for future work to integrate additional observations in order to better constrain the plausible sets of model parameters.
Supraglacial debris affects glacier mass balance as a thin layer enhances surface melting, while a thick layer reduces it. While many glaciers are debris‐covered, global glacier models do not account ...for debris because its thickness is unknown. We provide the first globally distributed debris thickness estimates using a novel approach combining sub‐debris melt and surface temperature inversion methods. Results are evaluated against observations from 22 glaciers. We find the median global debris thickness is ∼0.15 ± 0.06 m. In all regions, the net effect of accounting for debris is a reduction in sub‐debris melt, on average, by 37%, which can impact regional mass balance by up to 0.40 m water equivalent (w.e.) yr‐1. We also find recent observations of similar thinning rates over debris‐covered and clean ice glacier tongues is primarily due to differences in ice dynamics. Our results demonstrate the importance of accounting for debris in glacier modeling efforts.
Plain Language Summary
Many glaciers around the world have a layer of debris (boulders, rocks, and sand) covering the underlying ice over much of the glacier surface, yet global glacier models do not account for debris because the debris thickness is unknown. Here we provide the first estimates of debris thickness for debris‐covered glaciers globally and show the debris substantially reduces regional glacier mass loss. We also find that recent observations that debris‐covered and clean ice glaciers are thinning at similar speeds is primarily due to differences in how glaciers flow. Our results fundamentally advance our ability to account for debris in glacier reconstructions, landscape evolution models, hazard assessments, and glacier projections of glacier runoff and their contribution to sea‐level rise.
Key Points
We produce the first distributed global debris thickness estimates
Accounting for debris significantly reduces regional glacier mass loss
The similar thinning rates of debris‐covered and clean ice glaciers in High Mountain Asia is primarily caused by differences in ice dynamics
We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered ...a few decades ago by increased ocean‐induced thinning, which may have initiated marine ice sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold‐ocean period, the evolving glacier‐bed/ice shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin‐wide thinning signal did not change. Thus, as predicted by theory, once marine ice sheet instability is underway, a single transient high‐amplitude ocean cooling has only a relatively minor effect on ice flow. The long‐term effects of ocean temperature variability on ice flow, however, are not yet known.
Key Points
Pine Island Glacier speed is correlated with ocean temperature
Grounded ice speed slowed by only ~1% despite ~60% drop in ocean heat content
Ice speed recovered after the cold‐ocean anomaly ended
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