Abstract
The existence of supraglacial lakes influences debris-covered glaciers in two ways. The absorption of solar radiation in the water leads to a higher ice ablation, and water draining through the glacier to its bed leads to a higher velocity. Rising air temperatures and changes in precipitation patterns provoke an increase in the supraglacial lakes in number and total area. However, the seasonal evolution of supraglacial lakes and thus their potential for influencing mass balance and ice dynamics have not yet been sufficiently analyzed. We present a summertime series of supraglacial lake evolution on Baltoro Glacier in the Karakoram from 2016 to 2020. The dense time series is enabled by a multi-sensor and multi-temporal approach based on optical (Sentinel-2 and PlanetScope) and Synthetic Aperture Radar (SAR; Sentinel-1 and TerraSAR-X) remote sensing data. The mapping of the seasonal lake evolution uses a semi-automatic approach, which includes a random forest classifier applied separately to each sensor. A combination of linear regression and the Hausdorff distance is used to harmonize between SAR- and optical-derived lake areas, producing consistent and internally robust time series dynamics. Seasonal variations in the lake area are linked with the Standardized Precipitation Index (SPI) and Standardized Temperature Index (STI) based on air temperature and precipitation data derived from the climate reanalysis dataset ERA5-Land. The largest aggregated lake area was found in 2018 with 5.783 km2, followed by 2019 with 4.703 km2, and 2020 with 4.606 km2. The years 2016 and 2017 showed the smallest areas with 3.606 and 3.653 km2, respectively. Our data suggest that warmer spring seasons (April–May) with higher precipitation rates lead to increased formation of supraglacial lakes. The time series decomposition shows a linear increase in the lake area of 11.12 ± 9.57% per year. Although the five-year observation period is too short to derive a significant trend, the tendency for a possible increase in the supraglacial lake area is in line with the pronounced positive anomalies of the SPI and STI during the observation period.
Highlights
Glaciers with an extensive debris cover respond in a more complex way to changes in climate than those that are debrisfree
Larger lakes comprised a larger percentage of the total lake area through time
To study the variability of supraglacial lakes on the Baltoro Glacier, we developed a semi-automatic approach based on multi-sensor and multi-temporal summertime series from 2016 to 2020 acquired by the optical sensors Sentinel-2 and PlanetScope and the Synthetic Aperture Radar (SAR) sensors Sentinel-1 and TerraSAR-X
Summary
Glaciers with an extensive debris cover respond in a more complex way to changes in climate than those that are debrisfree. The glacier response depends on debris thickness and its spatial distribution (Benn et al, 2012). A thin debris cover of only a few centimeters leads to enhanced ablation compared to clean ice due to increased absorption of solar radiation (Ostrem, 1959; Nicholson and Benn, 2006). A debris cover with greater thickness has an insulating effect on the energy transfer to the glacier ice from atmospheric energy sources and reduces ice ablation. With respect to mass balance calculations, various properties of debris cover need to be considered, such as thickness, slope, aspect, and lithology (Mihalcea et al, 2008). Mass balance models can include such debris-dependent surface properties, but due to a lack of empirical data, it is difficult to readily include their impact on glacier melt rates. Previous studies have indicated that lakes could be responsible for 1/8 of total ice loss in the Langtang Valley, Nepal (Miles et al, 2018; Miles et al, 2020)
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