Reconstructing historical climate change from deep ground temperature measurements in cold regions is often complicated by the presence of permafrost. Existing methods are typically unable to account for latent heat effects due to the freezing and thawing of the active layer. In this work, we propose a novel method for reconstructing historical ground surface temperatures (GST) from borehole temperature measurements that accounts for seasonal thawing and refreezing of the active layer. Our method couples a recently developed fast numerical modeling scheme for two-phase heat transport in permafrost soils with an ensemble-based method for approximate Bayesian inference. We evaluate our method on two synthetic test cases covering both cold and warm permafrost conditions as well as using real data from a 100m deep borehole on Sardakh Island in northeastern Siberia. Our analysis of the Sardakh Island borehole data confirms previous findings that ground surface temperatures in the region have likely risen by 5 to 9°C between the pre-industrial period of 1750–1855 and 2012. We also show that latent heat effects due to seasonal freeze-thaw have a substantial impact on the resulting reconstructed surface temperatures. We find that neglecting the thermal dynamics of the active layer can result in biases of roughly -1 to -1.5°C in cold conditions (i.e. mean annual ground temperature below -5°C) and as much as -2 to -3°C in warmer conditions where substantial active layer thickening (>200cm) has occurred. Our results highlight the importance of considering seasonal freeze-thaw in GST reconstructions from permafrost boreholes.

Permafrost degradation on the Tibetan Plateau is well-documented and expected to continue throughout this century. However, the impact of thawing permafrost on the distribution, composition, and resilience of vegetation communities in this region is not well understood. In this study, we combined a transient numerical permafrost model with machine learning algorithms to project the near-future thermal state of permafrost and vegetation (represented by the Normalized Difference Vegetation Index [NDVI]) changes under two contrasting climate pathways (Shared Socioeconomic Pathway 1–2.6 [SSP1–2.6] and SSP5–8.5). The contribution of climatic and terrestrial variables to vegetation evolution was quantified using ridge regression. By 2100, permafrost areas were expected to decrease by 21±4%, and 55±2% under the SSP1–2.6 and SSP5–8.5 scenarios, respectively, relative to the baseline period (2000–2018). Under the SSP1–2.6 scenarios, the mean annual ground temperature and active layer thickness were projected to fluctuate stably, while under the SSP5–8.5 scenarios, a significant increasing trend was anticipated. Satellite-based observations indicated an increasing trend of NDVI within the permafrost areas from 2000 to 2018 (0.01 per decade), mainly attributed to climatic factors. In the future, vegetation greenness was expected to possibly remain stable under SSP1–2.6 scenarios, whereas a rising trend was likely noted under SSP5–8.5 scenarios during 2019–2050, mainly controlled by the surface air temperature and liquid water content at the root zone during the growing season. Our modeling work provides a potential approach for investigating future vegetation changes and offers more possibilities to improve understanding of the interaction between soil-vegetation-atmosphere in cold regions.

Due to an imbalance of incoming and outgoing radiation at the top of the atmosphere, excess heat has been accumulating in Earth’s climate system in recent decades, driving global warming and climatic changes. To date it has not been quantified how much of this excess heat is being used for the melting of ground ice in the terrestrial permafrost region. Here, we diagnose changes in sensible and latent heat contents in the northern terrestrial permafrost region from ensemble simulations of a numerical permafrost model. We find that about 3.9 (+1.5|-1.7) ZJ of heat, of which 1.7 (+1.4|-1.5) ZJ (45%) were used to melt ground ice, were taken up by permafrost from 1980 to 2018. This suggests that permafrost is a persistent heat sink similar in magnitude to other components of the cryosphere that requires an explicit consideration in assessments of the Earth’s energy imbalance.