Christopher F. Larsen

Recent glacier mass changes in the Gulf of Alaska region from GRACE mascon solutions

The mass changes of the Gulf of Alaska (GoA) glaciers are computed from the Gravity Recovery and Climate Experiment (GRACE) inter-satellite range-rate data for the period April 2003–September 2007. Through the application of unique processing techniques and a surface mass concentration (mascon) parameterization, the mass variations in the GoA glacier regions have been estimated at high temporal (10 day) and spatial (2 × 2 arc-degrees) resolution. The mascon solutions are directly estimated from a reduction of the GRACE K-band inter-satellite range-rate data and, unlike previous GRACE solutions for the GoA glaciers, do not exhibit contamination by leakage from mass change occurring outside the region of interest. The mascon solutions reveal considerable temporal and spatial variation within the GoA glacier region, with the largest negative mass balances observed in the St Elias Mountains including the Yakutat and Glacier Bay regions. The most rapid losses occurred during the 2004 melt season due to record temperatures in Alaska during that year. The total mass balance of the GoA glacier region was −84 ± 5 Gt a −1 contributing 0.23 ± 0.01 mm a −1 to global sea-level rise from April 2003 through March 2007. Highlighting the large seasonal and interannual variability of the GoA glaciers, the rate determined over the period April 2003–March 2006 is −102 ± 5 Gt a −1 , which includes the anomalously high temperatures of 2004 and does not include the large 2007 winter balance-year snowfall. The mascon solutions agree well with regional patterns of glacier mass loss determined from aircraft altimetry and in situ measurements.

Recent and future warm extreme events and high-mountain slope stability

The number of large slope failures in some high-mountain regions such as the European Alps has increased during the past two to three decades. There is concern that recent climate change is driving this increase in slope failures, thus possibly further exacerbating the hazard in the future. Although the effects of a gradual temperature rise on glaciers and permafrost have been extensively studied, the impacts of short-term, unusually warm temperature increases on slope stability in high mountains remain largely unexplored. We describe several large slope failures in rock and ice in recent years in Alaska, New Zealand and the European Alps, and analyse weather patterns in the days and weeks before the failures. Although we did not find one general temperature pattern, all the failures were preceded by unusually warm periods; some happened immediately after temperatures suddenly dropped to freezing. We assessed the frequency of warm extremes in the future by analysing eight regional climate models from the recently completed European Union programme ENSEMBLES for the central Swiss Alps. The models show an increase in the higher frequency of high-temperature events for the period 2001–2050 compared with a 1951–2000 reference period. Warm events lasting 5, 10 and 30 days are projected to increase by about 1.5–4 times by 2050 and in some models by up to 10 times. Warm extremes can trigger large landslides in temperature-sensitive high mountains by enhancing the production of water by melt of snow and ice, and by rapid thaw. Although these processes reduce slope strength, they must be considered within the local geological, glaciological and topographic context of a slope.

Airborne surface profiling of glaciers : a case-study in Alaska

A relatively lightweight and simple airborne system for surface elevation profiling of glaciers in narrow mountain valleys has been developed and tested. The aircraft position is determined by kinematic global positioning system (GPS) methods. The distance to the glacier surface is determined with a laser ranger. The accuracy is about 0.3 m, sufficient to permit future changes to be observed over short time intervals. Long-term changes can be estimated by comparison of profiles with existing maps. Elevation profiles obtained in 1993-94 from three glaciers in central and south-central Alaska are compared with maps made about 1950. The resulting area-averaged, seasonally corrected thickness changes during the interval are: Gulkana Glacier (central Alaska Range) -11 m, Worthington Glacier (central Chugach Mountains) + 7 m, and Bear Lake Glacier (Kenai Mountains) -12 m. All three glaciers retreated during the interval of comparison. The estimated uncertainty in the average thickness change is ±5 m, which is mainly due to errors in the existing maps. Constraints on the accuracy of the maps are obtained by profiling in proglacial areas.

Measurement of temperature in a margin of Ice Stream B, Antarctica : implications for margin migration and lateral drag

Ice temperature was measured in and around the chaotically crevassed south margin of Ice Stream B, Antarctica, from 1992 to 1994. The temperatures at 30 m depth in the chaotic zone are about 12 K lower than in the adjacent uncrevassed ice, due to the ponding of cold winter air. At depths greater than 150 m, there is clear evidence of internal heating of the ice due to the large shear deformation rate in the marginal zone. Analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information. First, over the last half century the lateral shear stress averaged 2.0 x 10 5 Pa in the top third of the margin and, second, the margin moved outward at an average rate of 7.3 m a These values do not involve any assumptions about the flow law of ice. The uncertainties are roughly 20%. The value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides.

Coseismic slip distribution of the 2002 MW7.9 Denali fault earthquake, Alaska, determined from GPS measurements

[1] On 3 November 2002 an Mw7.9 earthquake occurred in central Alaska. The earthquake ruptured portions of the Susitna Glacier, Denali, and Totschunda faults. Inversion of the GPS-measured displacement field indicates that the event was dominated by a complex, right-lateral strike-slip rupture along the Denali fault. GPS sites closest to the epicenter show the effect of thrust motion on the Susitna Glacier fault. The preferred coseismic slip model, with M w 7.8, indicates relatively low slip on the western part of the rupture and high slip from about 60 km east of the hypocenter extending to the junction of the Denali and Totschunda faults. We find mostly shallow slip from the surface to 15 km depth, but the inversion suggests one large deep slip patch about 110 km east of the hypocenter. Our model predicts surface slip in good agreement with surface geological observations, where model resolution is good.

Flow velocities of Alaskan glaciers

Our poor understanding of tidewater glacier dynamics remains the primary source of uncertainty in sea level rise projections. On the ice sheets, mass lost from tidewater calving exceeds the amount lost from surface melting. In Alaska, the magnitude of calving mass loss remains unconstrained, yet immense calving losses have been observed. With 20% of the global new-water sea level rise coming from Alaska, partitioning of mass loss sources in Alaska is needed to improve sea level rise projections. Here we present the first regionally comprehensive map of glacier flow velocities in Central Alaska. These data reveal that the majority of the regional downstream flux is constrained to only a few coastal glaciers. We find regional calving losses are 17.1 Gt a(-1), which is equivalent to 36% of the total annual mass change throughout Central Alaska.

Surface melt dominates Alaska glacier mass balance

Mountain glaciers comprise a small and widely distributed fraction of the worlds terrestrial ice, yet their rapid losses presently drive a large percentage of the cryospheres contribution to sea level rise. Regional mass balance assessments are challenging over large glacier populations due to remote and rugged geography, variable response of individual glaciers to climate change, and episodic calving losses from tidewater glaciers. In Alaska, we use airborne altimetry from 116 glaciers to estimate a regional mass balance of −75 ± 11 Gt yr−1 (1994–2013). Our glacier sample is spatially well distributed, yet pervasive variability in mass balances obscures geospatial and climatic relationships. However, for the first time, these data allow the partitioning of regional mass balance by glacier type. We find that tidewater glaciers are losing mass at substantially slower rates than other glaciers in Alaska and collectively contribute to only 6% of the regional mass loss.

Validation of high-resolution GRACE mascon estimates of glacier mass changes in the St Elias Mountains, Alaska, USA, using aircraft laser altimetry

We acquired center-line surface elevations from glaciers in the St Elias Mountains of Alaska/northwestern Canada using aircraft laser altimetry during 2000-05, and compared these with repeat measurements acquired in 2007. The resulting elevation changes were used to estimate the mass balance of 32 900 km 2 of glaciers in the St Elias Mountains during September 2003 to August 2007, yielding a value of -21.2±3.8Gta -1 , equivalent to an area-averaged mass balance of -0.64±0.12 m a -1 water equivalent (w.e.). High-resolution (2 arc-degrees spatial and 10 day temporal) Gravity Recovery and Climate Experiment (GRACE) mass-balance estimates during this time period were scaled to glaciers of the St Elias Mountains, yielding a value of -20.6 ± 3.0 Gt a -1 , or an area-averaged mass balance of -0.63 ± 0.09 m a -1 w.e. The difference in balance estimates (altimetry minus GRACE) was -0.6 ± 4.8 Gta -1 , well within the estimated errors. Differences likely resulted from uncertainties in subgrid sampling of the GRACE mass concentration (mascon) solutions, and from errors in assigning an appropriate near-surface density in the altimetry estimates. The good correspondence between GRACE and aircraft altimetry data suggests that high-resolution GRACE mascon solutions can be used to accurately assess mass-balance trends of mountain glacier regions that are undergoing large changes.

Rapid thinning of lake-calving Yakutat Glacier and the collapse of the Yakutat Icefield, southeast Alaska, USA

Both lake-calving Yakutat Glacier (337 km 2 ), Alaska, USA, and its parent icefield (810 km 2 ) are experiencing strong thinning, and under current climate conditions will eventually disappear. Comparison of digital elevation models shows that Yakutat Glacier thinned at area-averaged rates of 4.76 � 0.06 m w.e. a -1 (2000-07) and 3.66 � 0.03 m w.e. a -1 (2007-10). Simultaneously, adjacent Yakutat Icefield land-terminating glaciers thinned at lower but still substantial rates (3.79 and 2.94 m w.e. a -1 respectively for the same time periods), indicating lake-calving dynamics helps drive increased mass loss. Yakutat Glacier terminates into Harlequin Lake and for over a decade sustained a � 3 km long floating tongue, which started to disintegrate into large tabular icebergs in 2010. Such floating tongues are rarely seen on temperate tidewater glaciers. We hypothesize that this difference is likely due to the lack of submarine melting in the case of lake-calving glaciers. Floating-tongue ice losses were evaluated in terms of overall mass balance and contribution to sea-level rise. The post-Little Ice Age collapse of Yakutat Icefield was driven in part by tidewater calving retreats of adjacent glaciers, the lake-calving retreat of Yakutat Glacier, a warming climate and by the positive feedback mechanisms through surface lowering.

Low‐frequency radar sounding of temperate ice masses in Southern Alaska

We present the Warm Ice Sounding Explorer (WISE), a low-frequency (2.5 MHz) radar for the sounding of temperate ice. WISE deployment in southern Alaska in 2008 and 2012 provides comprehensive measurements of glacier thickness, reveals deep valleys beneath glaciers and the full extent of zones grounded below sea level. The east branch of Columbia Glacier is deeper that its main branch and remains below sea level 20 km farther inland. Ice is 1000 m deep on Tazlina Glacier. On Bering glacier, two sills separate three deep bed depressions (>1200 m) that coincide with the dynamic balance lines during surges. The piedmont lobe of Malaspina Glacier and the lower reaches of Hubbard Glacier are entirely grounded below sea level 40 and 10 km, respectively, from their termini. Knowledge of ice thickness in these regions helps better understand their glacier dynamics, mass balance, and impact on sea level. Key Points Low frequency radar sounding works extremely well in Alaska Ice is much thicker than suggested by limited surveys Data reveals full extent of areas grounded below sea level ©2013. American Geophysical Union. All Rights Reserved.