The detection of long-term changes in the glacial firn line using L-Band SAR

The rapid shrinkage of glaciers in recent years is a result of global warming. Long-term, worldwide observations of glacier mass balance—excluding the Antarctic and Greenland ice sheets—based on satellite data indicate that estimated mass loss between 2000 and 2019 accounts for about 21 ± 3% of observed sea-level rise (Hugonnet et al., 2021). The retreat of glaciers is expected to have major environmental and social impacts; therefore, predicting future glacier responses to a changing climate is crucial for anticipating and mitigating these impacts (Bolibar et al., 2022).

One way to quantify glacier responses to climate change is by monitoring the equilibrium line altitude (ELA) (e.g., Zemp et al., 2007). The ELA is defined as the spatially averaged altitude on the glacier surface where the climatic mass balance is zero at a given time. Moreover, it represents the lowest boundary of climatic glacierization (Ohmura et al., 1992). Hence, analyzing changes over time in glacier ELA is important for predicting future glacier behavior. However, field-based ELA observations (the glaciological method) are limited to only a few hundred glaciers due to the considerable labor and time required. It is also possible to detect the ELA using optical satellite images (Rastner et al., 2019), but these observations are often restricted at the end of the snowmelt season by cloud cover or the polar night.

In contrast, synthetic aperture radar (SAR) is largely insensitive to weather conditions and can observe glaciers regardless of solar illumination or cloud cover. Additionally, glacier zones (such as firn, superimposed ice, and ice) could be distinguished using SAR images (Barzycka et al., 2023). The lower boundary of the firn area is referred to as the firn line. In temperate glaciers without superimposed ice, the altitude of the newly formed annual firn line can be considered equivalent to that year’s ELA. However, the firn line does not exhibit strong year-to-year variability because the previous year’s firn remains in place. Instead, multiple consecutive years of negative (or positive) mass balance will cause the firn line to retreat (or advance). Consequently, firn line variations tend to smooth out annual fluctuations, revealing long-term trends of ELA (König et al., 2002).

In this study, we investigated long-term changes in the firn line altitude (FLA) of two temperate glaciers (Taku glacier, Kesselwandferner) with extensive ELA observation records using a time series of L-band SAR images (JERS-1, ALOS, and ALOS-2). We then compared the SAR-derived FLA with ELA recorded in the long-term. The results indicate that the FLA is consistent with long-term ELA changes, suggesting that the SAR-derived FLA effectively captures the long-term trends in glacier ELA.