Exceptionally High 2018 Equilibrium Line Altitude on Taku Glacier, Alaska

The Juneau Icefield Research Program (JIRP) has been examining the glaciers of the Juneau Icefield since 1946. The height of the transient snowline (TSL) at the end of the summer represents the annual equilibrium line altitude (ELA) for the glacier, where ablation equals accumulation. On Taku Glacier the ELA has been observed annually from 1946 to 2018. Since 1998 multiple annual observations of the TSL in satellite imagery identify both the migration rate of the TSL and ELA. The mean ELA has risen 85 ± 10 m from the 1946–1985 period to the 1986–2018 period. In 2018 the TSL was observed at: 900 m on 5 July; 975 m on 21 July; 1075 m on 30 July; 1400 m on 16 September; and 1425 m on 1 October. This is the first time since 1946 that the TSL has reached or exceeded 1250 m on Taku Glacier. The 500 m TSL rise from 5 July to 30 July, 8.0. md−1, is the fastest rate of rise observed. This combined with the observed balance gradient in this region yields an ablation rate of 40–43 mmd−1, nearly double the average ablation rate. On 22 July a snow pit was completed at 1405 m with 0.93 m w.e. (water equivalent), that subsequently lost all snow cover, prior to 16 September. This is one of eight snow pits completed in July providing field data to verify the ablation rate. The result of the record ELA and rapid ablation is the largest negative annual balance of Taku Glacier since records began in 1946.