# Lightning-Ignited Wildfires beyond the Polar Circle
## 4 Discussion  
### Lightning vs. Wildfire in the Arctic  
Lightning is the primary source of wildfire ignition at high latitudes which is especially efficient during so-called dry thunderstorms that yield no precipitation [6; 52]. The results obtained indicate that lightning strikes influence wildfire ignition significantly varied throughout different Arctic sectors. The highest lightning-explained variation in fire occurrence (>40%) was observed in the East Siberian Arctic sector, whereas in the other Eurasian sectors as well as in North America, lightning input is non-significant. For East Siberia, those values exceed the previous estimates, which are attributed to the use in the analysis  
Figure 13: (**a**) Post-fire GPP trends within fire locations (\(N\) = 81,342). GPP trends are increasing within ca. 100% of fire locations (\(p\) < 0.05). (**b**) For the entire continental Arctic, the proportions of positive, negative, and non-significant GPP trends are 23.5%, 0.6%, and 75.9%, respectively (\(p\) < 0.05).  
of environmental variables taken within the location of a given wildfire [5]. A rather strong influence of lightning at the scale of Russia, Eurasia and the entire Arctic (ca. 40%, Table 2) was explained by the input of Eastern Siberia. The rest variance of wildfires is determined mostly by moisture parameters (Table 2). Soil moisture and terrestrial water content are the primary determinants of wildfire occurrence. Thus, in West Siberia, wildfire occurrence is mostly determined by moisture since that territory is over-moistened. Air temperature, as well as precipitation, influence fire occurrence indirectly by affecting fuel flammability. Meanwhile, the warming-driven moisture decreases in North America and East Siberia on the background precipitation stagnation (Figures 8c,d and 51b) are potentially increasing the likelihood that a given lightning strike will result in a wildfire.  
Moreover, warming itself is increasing lightning frequency within the circumboreal Arctic [8; 9; 14]. Research on lightning strikes projects that an increase in air temperature by 1 \({}^{\circ}\)C leads to an increase in lightning by about 12% [12]. For the Northern American Arctic, lightning ignition was estimated to increase by 90-230% by the end of this century [14]. Increased lightning-caused wildfires were also reported for forests in North America [13]. In addition to the combustion of aboveground vegetation, wildfires may smolder and overwinter in the carbon-rich wetlands soils and emerge in the spring as described for the Northern American and Siberian Arctic [53; 54].  
With continued warming, the role of lightning as a source of wildfire ignition is likely to increase both because of a direct increase in lightning frequency and because of the increased availability of dry fuel. Data published by World Wide Lightning Location Network (WWLLN) showed an increase in lightning frequency during the last decade [11]. The exponential dependence of fire number on the lightning frequency is worth noting (Figure 5). In addition, recent modeling [9] predicted that warming would lead to a global decrease in lightning, geographically that will be observed at low latitudes, whereas in the Arctic, lightning activity will increase. Together with lightning increases, negative trends of the significant fire drivers, terrestrial water, and soil moisture content are currently observed (Figure 9). In combination with increased lightning frequency, it could generate a synergy influence on the burning rate in the Arctic.