This paper accompanies a map that shows the extent of permafrost in the Northern Hemisphere between 25 and 17 thousand years ago. The map is based upon existing archival data, common throughout the Northern Hemisphere, that include ice-wedge pseudomorphs, sand wedges and large cryoturbations. Where possible, a distinction is made between areas with continuous permafrost and areas where permafrost is either spatially discontinuous or sporadic. The associated mean annual palaeo-temperatures that are inferred on the basis of present-day analogues increase understanding of the possible changes in permafrost extent that might accompany current global warming trends. Areas with relict permafrost and areas that were formerly exposed due to lower sea level (submarine permafrost) are also mapped. Mapping is mostly limited to lowland regions (areas approximately <1000 m a.s.l.). Striking features that appear from the map are (i) the narrow permafrost zone in North America, which contrasts with the broader LPM permafrost zone in Eurasia (that may be related to different snow thickness or vegetation cover), (ii) the zonal extent of former LPM permafrost (that may reflect sea-ice distribution), which contrasts with the present-day pattern of permafrost extent (especially in Eurasia) and (iii) the relatively narrow zones of LPM discontinuous permafrost (that may indicate strong temperature gradients).

Northeastern China has the second largest expanse of permafrost in China, primarily known as Xing'an-Baikal permafrost. Located on the southeastern edges of the Eurasian cryolithozone, the permafrost is thermally unstable and ecologically sensitive to external changes. The combined impacts of climatic, environmental, and anthropogenic changes cause 3-dimensional degradation of the permafrost. To predict these changes on the southern limit and ground temperature of permafrost in Northeastern China, an equivalent latitude model (ELM) for the mean annual ground surface temperature (MAGSTs) was proposed, and further improved to take into account of the influences of vegetation and snow-cover based on observational data and using the SHAW model. Using the finite element method and assuming a climate warming rate of 0.048°C a 1 , the ELM was combined with the unsteady-state heat conduction model to simulate permafrost temperatures at present, and to predict those after 50 and 100 a. The results indicate that at present, sporadic permafrost occurs in the zones with MAGSTs of 1.5°C or colder, and there would still be a significant presence of permafrost in the zones with the present MAGSTs of 0.5°C or colder after 50 a, and in those of 0.5°C or colder after 100 a. Furthermore, the total areal extent of permafrost would decrease from 2.57×10 5 km 2 at present to 1.84×10 5 km 2 after 50 a and to 1.29×10 5 km 2 after 100 a, i.e., a reduction of 28.4% and 49.8% in the permafrost area, respectively. Also the permafrost would degrade more substantially in the east than in the west. Regional warming and thinning of permafrost would also occur. The area of stable permafrost (mean annual ground temperature, or MAGT≤1.0°C) would decrease from present 1.07×10 5 to 8.8×10 4 km 2 after 50 a, and further decrease to 5.6×10 4 km 2 after 100 a. As a result, the unstable permafrost and seasonally frozen ground would expand, and the southern limit of permafrost would shift significantly northwards. The changes in the permafrost environment may adversely affect on ecological environments and engineering infrastructures in cold regions. Avoidance of unnecessary anthropogenic changes in permafrost conditions is a practical approach to protect the permafrost environment. permafrost, Northeastern China, climate change, equivalent latitude model (ELM), predictionCitation:

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The formation and evolution of permafrost in China during the last 20 ka were reconstructed on the basis of large amount of paleo-permafrost remains and paleo-periglacial evidence, as well as paleoglacial landforms, paleo-flora and paleofauna records. The results indicate that, during the local Last Glacial Maximum (LLGM) or local Last Permafrost Maximum (LLPMax), the extent of permafrost of China reached 5.310 6 km 2 5.410 6 km 2 , or thrice that of today, but permafrost shrank to only 0.8010 6 km 2 0.8510 6 km 2 , or 50% that of present, during the local Holocene Megathermal Period (LHMP), or the local Last Permafrost Minimum (LLPMin). On the basis of the dating of periglacial remains and their distributive features, the extent of permafrost in China was delineated for the two periods of LLGM (LLPMax) and LHMP (LLPMin), and the evolution of permafrost in China was divided into seven periods as follows: 1) LLGM in Late Pleistocene (ca. 20,000  13,000-10,800 a BP) with extensive evidence for the presence of intensive permafrost expansion for outlining its LLPMax extent; 2) A period of dramatically changing climate during the early Holocene (10,800  8,500-7,000 a BP) when permafrost remained relatively stable but with a general trend of shrinking areal extent; 3) The LHMP in the Mid-Holocene (8,500-7,000  4,000-3,000 a BP) when permafrost degraded intensively and extensively, and shrank to the LLPMin; 4) Neoglaciation during the late Holocene (4,000-3,000  1,000 a BP, when permafrost again expanded; 5) Medieval Warming Period (MWP) in the late Holocene (1,000  500 a BP) when permafrost was in a relative decline; 6) Little Ice Age (LIA) in the late Holocene (500  100 a BP), when permafrost relatively expanded, and; 7) Recent warming (during the 20 th century), when permafrost continuously degraded and still is degrading. The paleo-climate, geography and paleopermafrost extents and other features were reconstructed for each of these seven periods.

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Forest fires have significantly impacted the permafrost environment, and many research programs looking at this have been undertaken at higher latitudes. However, their impacts have not yet been systematically studied and evaluated in the northern part of northeast China at mid‐latitudes. This study simultaneously measured ecological and geocryological changes at various sites in the boreal forest at different stages after forest fires (chronosequence approach) in the northern Da Xing'anling (Hinggan) Mountains, Northeast China. We obtained results through field investigations, monitoring and observations, remote sensing interpretations, and laboratory tests. The results show that forest fires have resulted in a decreased Normalized Difference Vegetation Index (NDVI) and soil moisture contents in the active layer, increased active layer thickness (ALT) and ground temperatures, and the release of a large amount of C and N from the soils in the active layer and at shallow depths of permafrost. NDVI and species biodiversity have gradually increased in the years since forest fires. However, the vegetation has not fully recovered to the climax community structures and functions of the boreal forest ecosystems. For example, ground temperatures, ALT, and soil C and N contents have been slowly recovering in the 30 years after the forest fires, but they have not yet been restored to pre‐fire levels. This study provides important scientific bases for assessment of the impacts of forest fires on the boreal forest ecosystems in permafrost regions, environmental restoration and management, and changes in the carbon stock of soils at shallow (<3 m) depths in the Da Xingan'ling Mountains in northeast China.

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