Browsing by Author "Pritchard, Matthew E."

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    Brief communication: Unabated wastage of the Juneau and Stikine icefields (southeast Alaska) in the early 21st century
    Berthier, Etienne; Larsen, Christopher; Durkin, William J.; Willis, Michael J.; Pritchard, Matthew E. (Copernicus, 2018-04-27)
    The large Juneau and Stikine icefields (Alaska) lost mass rapidly in the second part of the 20th century. Laser altimetry, gravimetry and field measurements suggest continuing mass loss in the early 21st century. However, two recent studies based on time series of Shuttle Radar Topographic Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation models (DEMs) indicate a slowdown in mass loss after 2000. Here, the ASTER-based geodetic mass balances are recalculated carefully avoiding the use of the SRTM DEM because of the unknown penetration depth of the C-band radar signal. We find strongly negative mass balances from 2000 to 2016 (-0.68 ± 0.15 m w.e. a-1 for the Juneau Icefield and -0.83 ± 0.12 m w.e. a-1 for the Stikine Icefield), in agreement with laser altimetry, confirming that mass losses are continuing at unabated rates for both icefields. The SRTM DEM should be avoided or used very cautiously to estimate glacier volume change, especially in the North Hemisphere and over timescales of less than ∼ 20 years.
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    Dynamic Changes at Yahtse Glacier, the Most Rapidly Advancing Tidewater Glacier in Alaska
    Durkin, William J.; Bartholomaus, Timothy C.; Willis, Michael J.; Pritchard, Matthew E. (Frontiers, 2017-03-03)
    Since 1990, Yahtse Glacier in southern Alaska has advanced at an average rate of ∼100 myear−1 despite a negative mass balance, widespreadthinning in its accumulation area, and a low accumulation-area ratio. To better understand the interannual and seasonal changes at Yahtse and the processes driving these changes, we construct velocity and ice surface elevation time series spanning the years 1985–2016 and 2000–2014, respectively, using satellite optical and synthetic aperture radar (SAR) observations. We find contrasting seasonal dynamics above and below a steep (up to 35% slope) icefall located approximately 6 km from the terminus. Above the icefall, velocities peak in May and reach their minima in October synchronous with the development of a small embayment at the calving terminus. The up-glacier minimum speeds, embayment, and plume of turbid water that emerges from the embayment are consistent with an efficient, channelized subglacial drainage system that lowers basal water pressures and leads to focused submarine melt in the calving embayment. However, velocities near the terminus are fastest in the winter, following terminus retreat, possibly off of a terminal moraine that results in decreased backstress. Between 1996 and 2016 the terminus decelerated by ∼40%at an average rate of ∼0.4 mday−1 year−1, transitioned from tensile to compressive longitudinal strain rates, and dynamically thickened at rates of 1-6 m year−1, which we hypothesize is in response to the development and advance of a terminal moraine. The described interannual changes decay significantly upstream of the icefall, indicating that the icefall may inhibit the upstream transmission of stress perturbations. We suggest that diminished stress transmission across the icefall could allow moraine-enabled terminus advance despite mass loss in Yahtse’s upper basin. Our work highlights the importance of glacier geometry in controlling tidewater glacier re-advance, particularly in a climate favoring increasing equilibrium line altitudes.
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    Stikine Icefield Mass Loss between 2000 and 2013/2014
    Melkonian, Andrew K.; Willis, Michael J.; Pritchard, Matthew E. (Frontiers, 2016-10-19)
    We calculate thinning rates (formula presented) at the 5800 km2 Stikine Icefield of southeast Alaska from stacked digital elevation models (DEMs) acquired between 2000 and 2013/2014, and glacier velocities between 1985 and 2014 from feature tracking on optical image pairs. We find a mass change rate of −3.3 ± 1.1 Gt yr−1 between 2000 and 2014, equivalent to an area-averaged elevation change rate of −0.57 ± 0.18m w.e. yr−1. In 2014, land-terminating glaciers are 50% of the Stikine Icefield’s glaciated area and contribute −0.9 ± 0.4 Gt yr−1 of mass change (27% of the total), while marine-terminating glaciers are only 30% of the total glaciated area, but contribute −1.5 ± 0.3 Gt yr−1 (or 45% of total mass change, with the remaining mass loss from lacustrine-terminating glaciers). We estimate the frontal ablation flux between 2000 and 2014 at the four largest marine-terminating glaciers on the Stikine Icefield (covering 90–95% of the marine-terminating glaciated area) using our glacier velocities and maps of fjord bathymetry to estimate terminus cross sections and glacier thicknesses. The combined 2014 frontal ablation flux of these four glaciers is 1.18 ±0.14 Gt yr−1, which may account for the difference in average mass loss between marine- and land-terminating glaciers on the Stikine Icefield. The Stikine and adjacent Juneau Icefields have very different mass loss contributions from marine-terminating glaciers (45% vs. effectively 0%), but both have area-averaged elevation change rates that are less negative than Alaska-wide estimates, which is surprising for these southernmost icefields in Alaska.