Changing glaciers in the Brooks Range and western Chugach Mountains, Alaska: mass loss, runoff increase, and supraglacial volcanic tephra coverage

Abstract

Glaciers in Alaska cover over ~87,000 km² (~ 6 % of the state) with most glaciers thinning and retreating at an increasing rate. The thinning and retreating of glaciers worldwide can have an immediate socio-economic implication in addition to the longer-term glacier meltwater contribution to sea level rise. This dissertation investigated Alaskan glaciers in the Brooks Range for mass loss and area reductions over the period 1970-2001 (Chapter 2), historic mass balance and runoff for Eklutna Glacier, located in western Chugach Mountains, using a temperature index model over 1984-2019 period (Chapter 3), and the persistence of tephra from a volcanic eruption of Mt. Spurr in 1992 on seven western Chugach Mountain glaciers (Chapter 4). Glaciers in the Brooks Range in Arctic Alaska (> 68° N) are important indicators of climate change and provide information on long-term climate variations in an area that has few high elevation meteorological stations. Digital elevation models (DEMs) reconstructed from topographic maps were differenced from an interferometric synthetic aperture radar DEM to calculate the volume and mass changes of 107 glaciers (42 km²). Over the period 1970-2001, total ice volume loss was 0.69 ± 0.06 km³ corresponding to a mean (area-weighted) specific mass balance rate of -0.54 ± 0.05 m w.e. a⁻¹ (± uncertainty). The arithmetic mean of all glaciers' specific mass balance rates was -0.47 ± 0.27 m w.e. a⁻¹ (± 1 std. dev.). A subsample of 36 glaciers found a 26 ± 16 % mean area reduction over ~35 years. Alaska's largest city, Anchorage, is critically dependent upon the melt water of Eklutna Glacier (29 km²) for both drinking water and hydropower generation; however, the glacier is rapidly retreating. We used a temperature index model to reconstruct the glacier's mass balance for the period 1985-2019 and quantify the impacts of glacier change on runoff. Eklutna Glacier experienced a significant annual mean surface mass balance negative trend (-0.38 m w.e. Decade⁻¹). Mean annual cumulative melt increased by 24 % between the 1985-93 and 2011-19 period. Additionally, the day of the year when 95% of annual melt has occurred was eight days later in the later time period than in the earlier period, demonstrating a prolongation of the melt season. The modeled mean annual discharge increased at a rate of 0.2 m decade⁻¹. This indicates that peak water, i.e. the year when annual discharge starts decreasing as the glacier becomes smaller, has not been reached. The past increases in runoff quantity and melt season length provide opportunities for water resource managers that must be balanced against future decreased runoff as the glacier continues to shrink. Volcanic eruptions deposit volcanic tephra on glaciers in Alaska, modifying surface albedo and glacier melt. We mapped the distribution of tephra originating from the eruption of Mt. Spurr in 1992 using aerial photos and satellite imagery on seven glaciers located approximately 180 km east of the volcano in western Chugach Mountains in southcentral Alaska. The glaciers were completely covered with ≥ 500 g m⁻² tephra immediately after the event. Tephra deposits are still visible on all glaciers 26 years after the eruption. Using LandSat 8 surface reflectance bands, we quantified percentages of tephra glacier coverage. Results suggest an increasing tephra extent on five of the seven investigated glaciers over 2013-2018 period explained by firn line retreat. The mean percent increase for all glaciers was 4% with Troublesome Glacier showing greatest increase (~ 7 %) and Finch Glacier showing a slight decrease (~ 1 %). This long- term tephra persistence on glacier surfaces most likely enhanced melt although the precise effect remains unknown.