Changes in permafrost extent and active layer thickness in the Northern Hemisphere from 1969 to 2018

Understanding the evolutions of the permafrost extent and active layer thickness (ALT) in the Northern Hemisphere (NH) are critical for global carbon flux simulation, climate change prediction, and engineering risk assessment. The temporal change characteristics of the permafrost extent and ALT for the NH have not been studied. We used the Kudryavtsev method, integrating a 0.5° × 0.5° spatial resolution of air temperature, soil texture, snow depth, vegetation type, soil volume moisture content, and organic content to simulate the changes of permafrost extent and ALT in the NH from 1969 to 2018. The results indicated that permafrost extent decreased from 23.25 × 10⁶ km² (average from 1969 to 1973) to 21.64 × 10⁶ km² (average from 2014 to 2018), with a linear rate of -0.023 × 10⁶ km²/a. Siberia had the highest degradation rate of 0.014 × 10⁶ km²/a, followed by Alaska, Mongolian Plateau, Qinghai-Tibet Plateau, Northern Canada, and Greenland, with linear rates of -0.012 × 10⁶, -0.005 × 10⁶, -0.004 × 10⁶, -0.0014 × 10⁶, and - 0.0004× 10⁶ km²/a, respectively. The average ALT in the NH increased at a linear rate of 0.0086 m/a. Alaska and Mongolian Plateau had the highest thickening rate of 0.024 m/a, followed by Qinghai-Tibet Plateau, Siberia, Northern Canada, and Greenland, which had linear rates of 0.009, 0.008, 0.0072, and 0.003 m/a, respectively. The uncertainty of the results could be attributed to the inaccurate forcing data and limitations of the Kudryavtsev model.

Soil organic carbon stabilization in permafrost peatlands

In permafrost peatlands, the degradation of permafrost soil can raise soil temperature and alter moisture conditions, which increases the rate of loss of soil organic carbon (SOC). Here we selected three typical permafrost types that have very different active layer thicknesses but with soil originating from the same vegetation and which exist under comparable climatic conditions in the Da Xing'an mountain range: continuous permafrost, island permafrost, and island melting permafrost. To quantify the relative importance of control elements on SOC stabilization in these different permafrost types, we used correlation analysis to assess the relationship between organic carbon, physical and chemical properties and microorganisms, and explored the contribution of these factors to the accumulation of organic carbon. This study shows that the interaction between clay or silt, iron oxides and microorganisms have an important influence on the stability of organic carbon in permafrost peatlands.

Freezing/thawing index variations over the circum-Arctic from 1901 to 2015 and the permafrost extent

Due to sparse data and discontinuous time observations in the circum-Arctic region, freezing index and thawing index, as useful indicators, are widely used in permafrost distribution, climate changes and cold-region engineering analysis. However, previous researches on freezing/thawing index over this region were estimated based on mean monthly air temperature. In this paper, we analyzed the spatial and temporal variations of the freezing/thawing index over the circum-Arctic from 1901 to 2015 based on the daily datasets, besides monthly datasets. The results showed that freezing index had a downward changing trend and thawing index had an upward trend during 1901-2015. More important, the change trend in freezing/thawing index after 1988 was more significant than before. Furthermore, different freezing/thawing index based on daily datasets and the monthly datasets were assessed and compared according to daily data from 17 meteorological stations, comprehensive relative errors evaluation implied that freezing/thawing based on daily datasets was more accurate generally, although both of other datasets were available in calculating the freezing/thawing index. As the daily datasets are better in calculating annual freezing/thawing index, therefore, the permafrost extent was estimated by a climate-based predictive model combined with snow depth data from Canadian Meteorological Centre (CMC). Finally, considering that the published permafrost map of the circum-Arctic only shows the past permafrost distribution, but it cannot reflect the permafrost distribution after 2000 under the climate warming. Hence, we simulated the current (mean from 2000 to 2015) permafrost area which is 19.96 × 10⁶ km², and the results showed some discrepancies between published and simulated permafrost extent mainly located in isolated permafrost regions.