Abstract
An important feature of the Arctic is large spatial heterogeneity in active layer conditions, which is generally poorly represented by global models and can lead to large uncertainties in predicting regional ecosystem responses and climate feedbacks. In this study, we developed a spatially integrated modelling and analysis framework combining field observations, local scale (~ 50 m resolution) active layer thickness (ALT) and soil moisture maps derived from airborne low frequency (L+P-band) radar measurements, and global satellite environmental observations to investigate the ALT sensitivity to recent climate trends and landscape heterogeneity in Alaska. Modelled ALT results show good correspondence with in situ measurements in higher permafrost probability (PP ≥ 70%) areas (n = 33, R = 0.60, mean bias = 1.58 cm, RMSE = 20.32 cm), but with larger uncertainty in sporadic and discontinuous permafrost areas. The model results also reveal widespread ALT deepening since 2001, with smaller ALT increases in northern Alaska (mean trend = 0.32 ± 1.18 cm yr-1) and much larger increases (> 3 cm yr-1) across interior and southern Alaska. The positive ALT trend coincides with regional warming and a longer snow-free season (R = 0.60 ± 0.32). A spatially integrated analysis of the radar retrievals and model sensitivity simulations demonstrated that uncertainty in the spatial and vertical distribution of soil organic carbon (SOC) was the largest factor affecting modeled ALT accuracy, while soil moisture played a secondary role. Potential improvements in characterizing SOC heterogeneity, including better spatial sampling of soil conditions and advances in remote sensing of SOC and soil moisture, will enable more accurate predictions of active layer conditions and refinement of the modelling framework across a larger domain.
Highlights
At the boreal forest site, the model-simulated active layer thickness (ALT) (81 ± 15 cm) during the study period (2001–2015) was much larger than the ALT value reported at the tower site (∼ 43 cm, Nakai et al, 2013); the model-simulated ALT was close to the ALT (74±17 cm) calculated from the in situ soil temperature measurements during the observation period (2011–2013)
We developed a satellite-based modeling framework for permafrost active layer mapping at landscape scale (∼ 1 km) and applied it to the Alaskan domain
Local-scale (∼ 50 m resolution) maps of ALT and soil moisture (SM) derived from combined low-frequency (L + P-band) airborne radar remote sensing were used with in situ ground measurements to evaluate the model simulations
Summary
Regional warming in the northern high latitudes is occurring at roughly twice the global rate, leading to widespread permafrost degradation (Jorgenson et al, 2006; Romanovsky et al, 2010) and substantial changes in hydrologic and ecosystem processes, including earlier and potentially longer growing seasons (Kim et al, 2012), expansion of tundra shrub cover (Tape et al, 2006), changes in lake and wetland areas (Smith et al, 2005), and increasing thermokarst development (Liljedahl et al, 2016) and fire disturbances (Grosse et al, 2011). Thawing of permafrost can lead to widespread changes in the terrestrial water cycle, including alteration of water storage in surface reservoirs (including lakes, wetlands, and ponds) and the active layer (Walvoord et al, 2016). These hydrologic shifts will likely trigger profound changes to almost every aspect of the Arctic biophysical system. The accuracy of these methods is limited by the ability of sparse ground measurements representing landscape heterogeneity, and the resulting empirical models provide only limited insight and mechanistic understanding of underlying processes affecting active layer conditions. Large uncertainties remain in characterizing regional variability of subsurface soil organic carbon (SOC) content due to limited ground observations of this parameter in the Arctic region (Ping et al, 2008; Burnham and Sletten, 2010) and its effect on ground temperature evolution
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