Vegetation fires become the concern worldwide due to their substantial impacts on climate and environment, and in particular in the circum-Arctic. Assessing vegetation fires and associated emissions and causes can improve understanding of fire regime and provide helpful information for vegetation fires solution. In this study, satellitebased vegetation fires and emissions during 2001-2020 were investigated and contributions of different types of fires were analyzed. Furthermore, climate anomalies related to extreme vegetation fires were explored. The main results showed that the region south of the Arctic circle (50 degrees N-67 degrees N) experienced a greater number of vegetation fires compared to the Arctic (north of 67 degrees N). During 2001-2020, interannual variability of vegetation fires between 50 degrees N and 67 degrees N appeared to be decreasing while emissions (including carbon, dry matter, PM2.5, and BC) appeared to be increasing overall, which were contributed by the increasing summer boreal forest fires in this region largely. In the Arctic, vegetation fires and emissions increased in recent years distinctly, and those were dominated by the summer forest fires. Spatially, large increases of vegetation fires were located in the eastern Siberia and northern North America while large decreases were located in the northwestern Eurasia mainly. Additionally, in the Arctic, the unprecedented vegetation fires were observed in the eastern Siberia and Alaska in 2019 and in the eastern Siberia in 2020, which could be attributed to high pressure, high near-surface temperature, and low air moisture anomalies. Meanwhile, obvious anticyclonic anomalies in Alaska in 2019 and in the eastern Siberia in 2020 and cyclonic anomalies in the western Siberia in 2019, also played an important role on fire occurrences making drier conditions.

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 x 10(6) km(2), and the results showed some discrepancies between published and simulated permafrost extent mainly located in isolated permafrost regions. (c) 2019 Elsevier B.V. All rights reserved.