Permafrost warming near the northern limit of permafrost on the Qinghai–Tibetan Plateau during the period from 2005 to 2017: A case study in the Xidatan area

Permafrost that exists near the boundary of the permafrost zone is generally more sensitive to climate change. By analyzing ground temperatures observed from two 30‐m‐deep boreholes, a case study was conducted to present some characteristics of recent permafrost warming in the Xidatan area, near the northern limit of the permafrost zone on the Qinghai–Tibetan Plateau. The rate of permafrost degradation from top to bottom in the area was far less than that from bottom to top. Local conditions produced spatial differences in permafrost characteristics, and thus the site covered by alpine meadow had a thinner active layer and lower rate of change than the site with desert steppe. With permafrost warming, the depths of zero annual amplitude at the two sites showed significant decreasing trends, suggesting that the warming could change the proportion of unfrozen water and ice in permafrost, and then lead to a decrease in the mean thermal diffusivity of formation. Mean annual permafrost temperatures at depth of zero annual amplitude of the two boreholes were respectively −0.4°C and −0.7°C, indicating that high‐temperature permafrost is widely distributed in the study area. The lower temperature permafrost had a higher warming rate and a higher upward shift rate of the permafrost base. The pattern of permafrost degradation near the limit of permafrost was characterized by nonuniform speed and staged development.