Abstract
Abstract. Seasonally variable thermal conductivity in active layers is one important factor that controls the thermal state of permafrost. The common assumption is that this conductivity is considerably lower in the thawed than in the frozen state, λt/λf < 1. Using a 9-year dataset from the Qinghai–Tibet Plateau (QTP) in conjunction with the GEOtop model, we demonstrate that the ratio λt/λf may approach or even exceed 1. This can happen in thick (> 1.5 m) active layers with strong seasonal total water content changes in the regions with summer-monsoon-dominated precipitation pattern. The conductivity ratio can be further increased by typical soil architectures that may lead to a dry interlayer. The unique pattern of soil hydraulic and thermal dynamics in the active layer can be one important contributor for the rapid permafrost warming at the study site. These findings suggest that, given the increase in air temperature and precipitation, soil hydraulic properties, particularly soil architecture in those thick active layers must be properly taken into account in permafrost models.
Highlights
Permafrost warming has been widely observed in the Arctic and sub-Arctic as well as in midlatitude alpine regions like the Alps and the Tibetan Plateau (Romanovsky et al, 2013; Harris et al, 2009; Cheng and Wu, 2007)
This study presented an interesting case which showed the effects of stratified active layers with a high ratio of seasonal thermal conductivity, λt/λf ≥ 1.0, on permafrost warming on the Qinghai– Tibet Plateau (QTP)
Apart from the climate drivers and the unusual high geothermal flux from the bottom, a high ratio of seasonal thermal conductivity in the stratified active layer is instrumental in regulating the interaction between climate and permafrost
Summary
Permafrost warming has been widely observed in the Arctic and sub-Arctic as well as in midlatitude alpine regions like the Alps and the Tibetan Plateau (Romanovsky et al, 2013; Harris et al, 2009; Cheng and Wu, 2007). Permafrost temperature can differ greatly in the same region due to local factors like topography, soil properties and vegetation. Responses of permafrost controlled by these local factors to climate change is expected to differ. The permafrost along the Qinghai–Xizang (Tibet) railway experienced a mean warming rate of 0.02 ◦C yr−1 at a depth of 6.0 m over the period from 2006 to 2010, and the highest warming rate even reached 0.08 ◦C yr−1 in the Fenghuo Mts. area (Wu et al, 2012). Given the same change in climate variables, these local factors still cause the underlying permafrost to develop differently. One recent study in the central QTP shows that permafrost at 10 sites experienced highly differing warming rates over the period of 2002–2014 (Wu et al, 2015). Compared to the rate of 0.03 ◦C yr−1 for the past 5 decades over the QTP
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