The Rise and Fall of Alaska and Yukon Glaciers Detected by TOPEX/Poseidon and Jason‐2 Altimeters Using a Novel Glacier‐Threshold Method.

Radar altimetry has been used to monitor sea level changes and ice sheet elevation changes for decades. Over mountain glaciers, radar altimetry has limited applications due to contaminated waveforms caused by complex glacier surfaces and steep terrains. In this study, we develop a glacier‐threshold method (GTM) to determine glacier elevation changes in Alaska and Yukon. The GTM retracks waveforms, reduces terrain effect and detects invalid elevation observations from the TOPEX/Poseidon (T/P) and Jason‐2 (J2) altimeters, resulting in an average usable rate of 35% from original altimeter‐measured heights. The selected measurements are used to construct time series of glacier elevation changes over 1993–2002 (T/P) and 2008–2016 (J2) at 47 sites. A crossover analysis, validation by airborne laser altimetry observations, and comparisons with recent studies confirm the estimated glacier elevation changes. Our findings suggest that the thinning of glaciers in Alaska and Yukon has been ubiquitous in recent years. The site near Walsh Glacier has the highest thinning rate of −5.71 ± 0.09 m/yr, followed by Chitina Glacier at −4.51 ± 0.21 m/yr. Only 17% of sites show glacier thickening due to surges and mass accumulations. Using a sophisticated data processing algorithm like the GTM, we show that altimeter data from the repeat TOPEX‐Jason missions can be used to monitor long‐term glacier elevation changes at inaccessible spots, creating an additional value for altimeter missions originally purposed for monitoring long‐term sea level change. Plain Language Summary: Satellite radar altimetry is an important observation technology to determine changes in surface elevations. However, altimeter measurement accuracy is very sensitive to terrain complexity, especially over mountain glaciers. Here, we present a glacier‐threshold method (GTM) to investigate the glacier elevation changes in Alaska and Yukon, using TOPEX/Poseidon (T/P) and Jason‐2 (J2) altimeter measurements. Internal and external measurement assessments show that the GTM enables us to retrieve precise changes in glacier elevations and improve the usable rate of original radar elevation observations. During the recent study period, the vast majority of Alaska and Yukon glaciers' elevations are rapidly declining. But in fewer glacier accumulation areas and over glaciers with active surface processes, the glacier elevations are rising. Environmental factors analysis shows that Pacific decadal oscillation, temperature, precipitation, surges, altitudes and locations of glaciers are responsible for the asynchronously spatiotemporal glacier elevation changes. All these findings from the repeat‐track radar altimetry and the GTM help to improve our knowledge about Alaska and Yukon glacier processes. Key Points: Glacier‐threshold method retrieves Alaska and Yukon glacier elevation changes using approximately 35% of repeated altimeter measurementsAlaska and Yukon have recently experienced notable glacier thinning, with the highest rate of −5.71 ± 0.09 m/yr near Walsh GlacierPacific decadal oscillation, surges, altitudes, and glacier locations affect Alaska and Yukon glacier elevation change pattern [ABSTRACT FROM AUTHOR]