# Lightning-Ignited Wildfires beyond the Polar Circle
## 1 Introduction  
Throughout Holocene, wildfire is a major natural factor that shapes boreal forest communities' structure and successions [1; 2; 3]. Fire has long been the primary disturbance agent for vast taiga landscapes. However, wildfires are a recent driver for tundra ecosystem dynamics. Since the onset of the current warming, the burning rate has increased throughout the boreal biome, including, alongside forested areas, the Arctic tundra [4; 5]. It has been known that the fire regime is determined by fuel availability, fuel flammability, and a fire ignition source. Fuel in the Arctic tundra and forest-tundra is available as open forests, woody shrubs, moss, lichens and herbaceous species, and peat deposits. Current warming may increase fuel flammability by moisture decrease. The dominant source of fire ignition (>90%) at high latitudes is lightning strikes [6]. Meanwhile, lightning activity is non-linearly dependent on temperature [7]. [PERSON] et al. [8] showed that global cloud-to-ground lightning activity would increase by up to 21.3% for the RCP85 projection at the end of the century. Although the international burned area will be slightly affected by these changes, significant burned area increases (over 100%) are expected in high-latitude regions. Recent studies [9] showed that warming will lead to a global decrease in lightning. However, that decrease will be observed geographically at low latitudes, whereas lightning activity will increase in the Arctic. Data obtained by World Wide Lightning Location Network (WWLLN) indicated an increase in lightning frequency during the recent decade [10]. Atmospheric warming leads to an increase in lightning frequency in the Arctic [11]. Consequently, it has the potential to further increase the burning rate throughout the Arctic, including the Greenland tundra [3; 12; 13; 14]. The fire return interval in the northern areas has shortened from that reported earlier [15; 16]. Arctic fires coupled with warming tend to transform tundra into shrulands [17] and potentially accelerate the northward expansion of trees [18; 19; 20]. However, our knowledge of the warming-driven lightning influence on the fire in the Arctic is still poor [5]. Throughout the Arctic, especially West Siberia and Canada, forest-tundra and tundra ecoregion largely consists of lowland taga forests and wetlands that have accumulated carbon (C) throughout the Holocene. Increased frequency and extent of fire have released long-stored carbon into the atmosphere and threaten to transform the high latitude territory from a C sink to a source of C [21; 22]. Increased C in the atmosphere can fuel a positive feedback loop, increasing warming and fire frequency [23]. Meanwhile, increased CO\({}_{2}\) provides a C-fertilization with consequent increasing C fixation and primary productivity at high latitudes [24; 25; 26; 27; 28]. Thus, analysis of tree ring chronologies linked increased radial growth for subarctic forests in Canada and Siberia with atmospheric warming [29; 30; 31]. CO\({}_{2}\) fertilization and warming may potentially offset C loss from increased wildfire. Due to the geographic scale and remoteness of Arctic regions, wildfire analysis is primarily based on satellite data with a resolution of 250-1000 m (e.g., MODIS, Vegetation, or VIIRS instruments) [32; 33; 34]. Detailed fire influence and post-fire vegetation succession analysis is based on medium (10-30 m) and high (1-2 m) resolution instruments (Landsat, Sentinel, or QuickBird [35; 36; 37; 38]. MODIS data also provide coordinates of fire ignitions (\"thermal points\" or \"hotspots\") [39]. Here, we report on the fire occurrence dynamics in the 21 st century within the circum-polar Arctic. We test the hypotheses that (1) warming-driven lightning frequency increase has caused a frequency increase in fires, and (2) the northern limit of fire occurrence has migrated northward to the Arctic Ocean.