# Lightning-Ignited Wildfires beyond the Polar Circle
## 2 Materials and Methods
### Fire Occurrence Determination  
We calculated the fire occurrence dynamics, fire season duration, and the northern limit of fire occurrence. The dynamics of the northern limit of fire occurrence were calculated based on the mean of three maximum values for each year within the 2001-2022 period.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Value** & **Name** & **Description** \\ \hline
1 & Evergreen Needleleaf Forests &
\begin{tabular}{c} Dominated by evergreen conifer trees \\ (canopy \textgreater{} 2 m). Tree cover \textgreater{} 60\% \\ Dominated by deciduous needle leaf (larch) trees \\ (canopy \textgreater{} 2 m). Tree cover \textgreater{} 60\% \\ \end{tabular} \\
3 & Deciduous Needleleaf Forests &
\begin{tabular}{c} Dominated by deciduous broadleaf trees \\ (canopy \textgreater{} 2 m). Tree cover \textgreater{} 60\% \\ Dominated by neither deciduous nor evergreen \\ (40–60\% of each) tree types (canopy \textgreater{} 2 m). Tree cover \textgreater{} 60\% \\ \end{tabular} \\
4 & Deciduous & Dominated by deciduous broadleaf trees \\ Broadleaf Forests &
\begin{tabular}{c} (canopy \textgreater{} 2 m). Tree cover \textgreater{} 60\% \\ Dominated by neither deciduous nor evergreen \\ (40–60\% of each) tree types (canopy \textgreater{} 2 m). Tree cover \textgreater{} 60\% \\ \end{tabular} \\
5 & Mixed Forests &
\begin{tabular}{c} Dominated by woody preeminals (1–2 m height) \\ > 60\% cover \\ Dominated by woody preeminals (1–2 m height) \\ > 60\% cover \\ \end{tabular} \\
7 & Open Shrublands & Dominated by woody preeminals (1–2 m height) \\
8 & Woody Savannas & Tree cover 30–60\% cover \\
9 & Savannas & Tree cover 10–30\% (canopy \textgreater{} 2 m) \\
10 & Grasslands & Dominated by herbaceous annuals (\textless{}2 m) \\
11 & Permanent & Permanently inundated lands with 30–60\% \\ Wetlands & water cover and \textgreater{}10\% vegetated cover \\ \hline \hline \end{tabular}
\end{table}
Table 1: Vegetation cover types included in the analysis (according to the MCD12Q1 Land Cover Type).  
Figure 1: Geography (**a**) and vegetation (**b**) of the Arctic sectors. (**a**) Fire sites are indicated by red dots. Abbreviations: Scand—Scandinavia, EurBus—European Russia, WestGib—Western Siberia, EastGib—Eastern Siberia, FarEast—Far East, NorthAm—North America. (**b**) A generalized vegetation map of the Arctic.  
I.e., we computed the mean of the three maximum values for each year, and then this mean value was assigned to each year. Fire occurrence was calculated based on the analysis of thermal points (\"hotspots\") acquired by Terra/Aqua/MODIS. The data (\(N\) = 375,083 hotspots with spatial resolution 1 \(\times\) 1 km) were obtained from the MODIS Collection 6 database package MCD14 DL [41]. The false hotspots were filtered by following procedures.  
(1) Within package MCD14 DL, hotspots from an active volcano, other static land sources, and offshore were removed. That package contained hotspots that presumed vegetation fire.  
(2) We additionally removed hotspots located out of vegetation cover based on the MODIS Land Cover Type (MCD12Q1) [40].  
(3) Finally, we excluded hotspots caused by settlements and industrial infrastructures located within vegetation cover. For this purpose, we used expert-based analysis of (a) topographic maps and (b) high-resolution satellite scenes from Google Earth ([[https://earth.google.com/web](https://earth.google.com/web)]([https://earth.google.com/web](https://earth.google.com/web)) (accessed on 27 March 2023)), Bing Maps ([[https://www.bing.com/maps](https://www.bing.com/maps)]([https://www.bing.com/maps](https://www.bing.com/maps)) (accessed on 27 March 2023)), and Yandex Maps ([[https://yandex.ru/maps](https://yandex.ru/maps)]([https://yandex.ru/maps](https://yandex.ru/maps)) (accessed on 27 March 2023)). On-ground resolution of these satellite scenes enabled the detection of those objects. After that filtering, the final number of hotspots was \(N\) = 325,352.