With global warming, the carbon pool in the degradation zone of permafrost around the Arctic will gradually be disturbed and may enter the atmosphere in the form of released methane gas, becoming an important factor of environmental change in permafrost areas. We selected the northwestern section of the Xiao Xing'an Mountains in China as the study area, located in the degradation zone on the southern margin of the permafrost region in Eurasia, and set up multiple study monitoring areas equipped with methane concentration sensors, air temperature sensors, pore water pressure sensors and soil temperature sensors for long-term monitoring of data changes using the high-density electrical method, ground penetrating radar and on-site drilling to survey the distribution of frozen soil and geological conditions in the study area, combined with remote sensing images of Sentinel-2 L1C and unmanned aerial vehicle photographs and three-dimensional image reconstruction, analysis of fire activities and related geological environmental factors. The results show that since 2004, the permafrost thickness of the marsh wetland in the study area has gradually reduced and the degradation rate obviously accelerated; the organic matter and methane hydrate (metastable methane hydrate and stable methane hydrate) stored in the permafrost under the marsh wetland are gradually entering the atmosphere in the form of methane gas. Methane emissions show seasonal changes, and the annual methane emissions can be divided into three main stages, including a high-concentration short-term emission stage (March to May), a higher-concentration long-term stable emission stage (June to August) and a higher-concentration short-term emission stage (September to November); there is a certain correlation between the change in atmospheric methane concentration and the change in atmospheric pressure and pore water pressure. From March to May every year (high-concentration short-term emission stage), with snow melting, the air humidity reaches an annual low value, and the surface methane concentration reaches an annual high value. The high concentration of methane gas entering the surface in this stage is expected to increase the risk of wildfire in the permafrost degradation area in two ways (increasing the regional air temperature and self-combustion), which may be an important factor that leads to a seasonal wildfire frequency difference in the permafrost zone of Northeast China and Southeast Siberia, with the peak in spring and autumn and the monthly maximum in spring. The increase in the frequency of wildfires is projected to further generate positive feedback on climate change by affecting soil microorganisms and soil structure. Southeastern Siberia and northeastern China, which are on the southern boundary of the permafrost region of Eurasia, need to be targeted to establish fire warning and management mechanisms to effectively reduce the risk of wildfires.

We report the 87Sr optical lattice clock at NIM with a clock laser referenced to a 30 cm ULE cavity. Several improvements, such as the atomic temperature and density, the lattice laser frequency stabilization, the fiber noise cancellation, etc, have been made since its first evaluation in 2015. Its systematic frequency shifts are carefully evaluated with a total relative uncertainty of 2.9 × 10−17. The measured absolute frequency is 429 228 004 229 873.07(0.13) Hz with a relative uncertainty of 3.1 × 10−16, with reference to the ensemble of primary and secondary frequency standards published in the Circular T bulletin by BIPM through a satellite link.

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A random pursuit strategy was further developed to minimize the uncertainty of atomic clock prediction errors by applying a new weighting method in its predictor ensemble. It was applied to experimental data from a hydrogen maser maintained at the timekeeping laboratory of the National Institute of Metrology in China. Compared to the method in our previous work, the new strategy moderately reduces the prediction uncertainty of the hydrogen maser. It could be a useful tool for some cases of clock prediction or other aspects.