# Lightning-Ignited Wildfires beyond the Polar Circle
## 4 Discussion
### Changes in Wildfire Dynamics for the Arctic  
In the second decade of the 21 st century, wildfire occurrence strongly (>3.0 times) increased in Eurasia and twofold in the entire Arctic, which is mostly attributed to the wildfires in East Siberia (Figure 2b). This data is consistent with the pattern of increased fire in the entire Siberia since the 1990s [55] and in Alaska and Canada [23; 56; 57]. The highest density of wildfires (i.e., fires number per km\({}^{2}\)) was observed in East Siberia (0.10/km\({}^{2}\)), with low values in European Russia, West Siberia, and Scandinavia (ca. 0.01/km\({}^{2}\)) with moderate values (ca. 0.03/km\({}^{2}\)) in North America (Figure 3). Meanwhile, for North America, fire Arctic trends are non-significant (Figure 2a), which should be attributed to the differences in the \"fuel load\" in East Siberia, e.g., the proportion of tundra, shrubs, and trees vegetation (Figure 2b) as well as the differences in the fuel readiness for lightning-caused ignitions. Additionally, relatively short time series may be responsible for the low coefficients of determination and the non-significant trends in North America. Low fire density in Scandinavia, European Russia, West Siberia, and Far East Arctic referred to the Atlantic and Pacific Oceans' influence and a higher level of firefighting in Scandinavia and, partly, in European Russia. Low fire activity in the West Siberia plain is also explained by the presence of numerous lakes, rivers, and as well as marshlands. On the contrary, East Siberian Arctic included vast territories of plateaus, hills, and mountains. The seasonal distribution of wildfires in the extreme years changes from typical for the Arctic unimodal pattern to the bimodal pattern, which is typical for lower latitudes [55]. Thus, in extremely dry years, the longer fire season is accompanied by burning peaks both at the beginning and at the end of the fire season. The increased fire season in West Siberia (plus ca. 40 days) correlated to the air temperature, which, in turn, increased the flammability of fuel. The decreased duration of the fire season in the Far East sector (minus ca. 20 days) was correlated to increased soil moisture, resulting in a decrease in fuel flammability. This suggests that the increase in wildfire frequency is likely due to the combination of a greater land area potentially subjected to wildfire due to dryer ground cover, a longer fire season, and increased lightning frequency as a source of ignition. In the Arctic, the wildfire boundary is migrating northward; it is also determined mostly by migration in East Siberia, where that boundary reaches the Arctic Ocean coast. The mean boundary shift correlated with air temperature, whereas extreme shifts synchronized with air temperature anomalies (heat waves). Earlier fire boundary advance was noted on the Yamal Peninsula of Western Siberia [15]. This result shows that the relationship of fire to climate is based not just on continuous climate trends but also on heat waves, the spikes in air temperature. The northward migration of the fires' northern boundary in Siberia is expected to result in an increased number of fires and, consequently, it subjects a greater proportion of Arctic tundra and forest-tundra to wildfire. The northward migration of the fires' northern boundary in Siberia is expected to result in an increased number of fires and, consequently, it subjects a greater proportion of Arctic tundra and forest-tundra to wildfires. Consequently, that migration increases the contribution of greenhouse gases from Arctic soils that have accumulated carbon throughout the Holocene, particularly in turf deposits in the wetlands of Western Siberia and Northern America, which has been described as a threat to shifting the status of the Arctic from a sink to a source of carbon [56; 57; 58]. The increased frequency of fires will likely accelerate the northward migration of the taiga-tundra boundary [18].