Abstract
The soil active layer in boreal forests is sensitive to climate warming. Climate-induced changes in the active layer may greatly affect the global carbon budget and planetary climatic system by releasing large quantities of greenhouse gases that currently are stored in permafrost. Ground surface temperature is an immediate driver of active layer thickness (ALT) dynamics. In this study, we mapped ALT distribution in Chinese boreal larch forests from 2000 to 2015 by integrating remote sensing data with the Stefan equation. We then examined the changes of the ALT in response to changes in ground surface temperature and identified drivers of the spatio-temporal patterns of ALT. Active layer thickness varied from 1.18 to 1.3 m in the study area. Areas of nonforested land and low elevation or with increased air temperature had a relatively high ALT, whereas ALT was lower at relatively high elevation and with decreased air temperatures. Interannual variations of ALT had no obvious trend, however, and the ALT changed at a rate of only −0.01 and 0.01 m year−1. In a mega-fire patch of 79,000 ha burned in 2003, ΔALT (ALTi − ALT2002, where 2003 ≤ i ≤ 2015) was significantly higher than in the unburned area, with the influence of the wildfire persisting 10 years. Under the high emission scenario (RCP8.5), an increase of 2.6–4.8 °C in mean air temperature would increase ALT into 1.46–1.55 m by 2100, which in turn would produce a significant positive feedback to climate warming.
Highlights
As a crucial component of the permafrost system, the soil active layer is subjected to freezing and thawing on an annual basis and plays an important role in regulating the energy, water, and carbon cycles [1,2,3]
To analyze the related uncertainty, we found that active layer thickness (ALT) would increase about 9.5% if thermal conductivity increased by 20%; an increase of 20% in soil volumetric latent heat of fusion would result in a decrease in ALT by 8.8%
We modelled ALT in the future, the influence of winter air temperature and snow were not taken into account
Summary
As a crucial component of the permafrost system, the soil active layer is subjected to freezing and thawing on an annual basis and plays an important role in regulating the energy, water, and carbon cycles [1,2,3]. An increase in the active layer thickness (ALT) is regarded as an initial response of permafrost to global warming [4,5]. Thickening of the active layer releases carbon dioxide and methane stored in the upper layer of permafrost, which further contributes to warming the global climate [6]. In the context of global change, the thickness and distribution of the active layer may be significantly influenced by interactions among climate, topography, land cover, and land use at various spatial scales [7]. Knowledge of active layer dynamics is crucial to determining ecosystem stability and its societal impacts at high-latitude locations of the northern hemisphere [4], making it a significant topic in cryosphere research
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