Carbon release through boreal fires could considerably accelerate Arctic warming; however, boreal fire occurrence mechanisms and dynamics remain largely unknown. Here, we analyze fire activity and relevant large-scale atmospheric conditions over southeastern Siberia, which has the largest burned area fraction in the circumboreal and high-level carbon emissions due to high-density peatlands. It is found that the annual burned area increased when a positive Arctic Oscillation (AO) takes place in early months of the year, despite peak fire season occurring 1 to 2 months later. A local high-pressure system linked to the AO drives a high-temperature anomaly in late winter, causing premature snowmelt. This causes earlier ground surface exposure and drier ground in spring due to enhanced evaporation, promoting fire spreading. Recently, southeastern Siberia has experienced warming and snow retreat; therefore, southeastern Siberia requires appropriate fire management strategies to prevent massive carbon release and accelerated global warming.

( A ) Mean burned area fraction (% year −1 ) over mid- and high latitudes in the Northern Hemisphere. Hatched areas indicate permafrost regions. The black box indicates the study area in southeastern Siberia (100°–150°E, 45°–55°N). ( B ) Monthly burned area (Mha month −1 ) in southeastern Siberia for 1997–2016 in each year (gray), mean (thick black), composite for February to March AO index > 0.5 SD cases (red), and AO < −0.5 SD cases (blue). ( C ) Mean burned area according to February to March AO index (orange) and 850-hPa geopotential height anomaly over southeastern Siberia (red). Bins on the x axis indicate <20%, <40%, <60%, <80%, and <100% rank ranges.

Fires in the northern circumpolar region show a distinct spatial distribution that consists of two major regions in Central Asia and southeastern Siberia. The Global Fire Emissions Database version 4.1 with small fires (GFED4.1s) gives an observed burned fraction of more than 10% year −1 over the past 20 years in this region ( Fig. 1A ) ( 7 ). Central Asia has an extensive burned area, but this is mostly from agricultural fires surrounding the Black Sea ( 8 ). In contrast, southeastern Siberia (100°–150°E, 45°–55°N), which contains dense boreal forests and peatlands in the permafrost zone, has had substantial forest fires. Although climate phenomena such as the El Niño–Southern Oscillation (ENSO) and Arctic Oscillation (AO) have been reported as affecting fire activity in several key regions through driving atmospheric processes ( 9 – 13 ), there is still a considerable lack of understanding of fire activity variability, especially in permafrost areas, despite their importance for global climate.

Arctic permafrost has received attention as a potential global warming amplifier. Permafrost zone carbon stocks are estimated to be more than double the atmospheric carbon pool (~750 PgC) ( 1 ). The large quantities of carbon stored in frozen soils can be released into the atmosphere through ongoing degradation of permafrost resulting from recent Arctic warming ( 2 , 3 ). In addition to carbon release from thawing permafrost, over the past two decades, boreal fires have released substantial amounts of carbon in boreal North America (60 TgC year −1 ) and Asia (124 TgC year −1 ) ( 4 ). Carbon release by boreal fires can accelerate global and Arctic warming and play a role in positive feedback between accumulation of atmospheric carbon and Arctic warming ( 5 , 6 ).

RESULTS AND DISCUSSION

Seasonal and interannual fire activity over southeastern Siberia Boreal fires usually occur in the summer because fires at high latitudes are temperature limited over the rest of the year (14–16). Central and eastern Siberian fires north of 55°N reach a maximum in June and July, but southeastern Siberian fire activity shows a monthly maximum in the spring (fig. S1) (15, 16). Seasonal fire variation in southeastern Siberia peaks in both spring and autumn, but the spring peak is approximately four times greater than the autumn peak (Fig. 1B). Area burned during April and May amounts to 62% of the annual total, whereas that during September to November amounts to only 16%. Unlike northern Siberia, southeastern Siberia has noticeable summer precipitation affected by the East Asian monsoon, which suppresses summertime fire activity (fig. S2). In addition to seasonality, southeastern Siberian fire activity also has considerable year-to-year fluctuations (Fig. 1B and fig. S3). Year-to-year fire activity has been found to be closely related to the AO index in late winter (Table 1), which is the predominant Northern Hemisphere atmospheric circulation pattern. The correlation coefficient between southeastern Siberia’s annual total burned area and the February to March averaged AO index is 0.53, which is significant at the 95% confidence level. The AO is characterized by a dipole pressure pattern with one sign in the Arctic and the opposite sign in mid-latitudes (17). The AO is known to have enormous influence on Eurasian climate variability, affecting surface temperature, rainfall, snowfall, storm activity, and even vegetation activity (18–20). Mean burned area amounts categorized by the AO show generally greater fire activity in the higher rank bin (Fig. 1C). In particular, it is evident that the strong fire activity hardly occurs under the negative phase of the AO, and the fire activity in April is even smaller than the activity in May (Fig. 1B). Although summer AO variability is known to drive boreal fire activity simultaneously (9, 12), we have found notable lagged linkage between the winter AO and spring fire activity, with strong implication on fire prediction. Table 1 Correlation matrix between burned area and climatic variables. * and ** indicate significance at the 95 and 99% confidence level based on Student’s t test, respectively. View this table:

Large-scale atmospheric circulation related to fire activity Figure 2 shows low-level atmospheric circulation relative to total annual burned area in southeastern Siberia. Although April and May are peak southeastern Siberian fire activity months as shown in Fig. 1B, atmospheric circulation anomalies during those months are relatively weaker than late winter (February to March) anomalies. However, in late winter, there are distinctive negative geopotential height anomalies in the Arctic, but positive anomalies over Siberia (Fig. 2, A and B). The pressure gradient with the geopotential height pattern leads to southwesterly winds occurring in Siberia, which are accompanied by atmospheric warm advection that contributes to substantial positive temperature anomalies in Siberia during late winter. This large-scale atmospheric circulation and temperature response is similar to the AO pattern during the positive phase, with low pressure in the Arctic and high pressure in mid-latitudes (fig. S4). Strictly speaking, the fire activity–related high-pressure pattern extends further into southeastern Siberia than the typical AO pattern. This suggests that the AO provides preferable conditions for strong fire activity (i.e., high-temperature anomalies), but the positive pressure anomaly extending westward from the North Pacific to southeastern Siberia explains more southeastern Siberian fire activity variability. Fig. 2 Atmospheric circulation related to fire activity in southeastern Siberia. Regression coefficients of temperature (shading), 850-hPa geopotential height (contour; 100-gpm interval), and 850-hPa wind (vector) for February (A), March (B), April (C), and May (D) on normalized yearly burned area in southeastern Siberia (boxed area). The climatological 0°C line for 2-m temperature is shown as a thick yellow line. Wind vectors are displayed only in regions significant at the 95% confidence level based on Student’s t test. As shown in Fig. 2, burned area in southeastern Siberia is closely related to local geopotential height anomalies on an interannual time scale. We define the local geopotential height index by averaging geopotential height anomalies for February and March over an area (110°–140°E, 45°–55°N) covering most of southeastern Siberia except for the eastern coastal region and the ocean. The geopotential height index highly correlates with fire activity, with a correlation coefficient of 0.80 that is significant at the 99% confidence level (fig. S5). Figure 1C shows that the local geopotential height anomaly, rather than the AO index, is linearly proportional to fire activity. When the geopotential height is extremely high (more than 20%), the mean burned area is 1.9 times greater than the climatological value. We found that atmospheric circulations are much stronger before the fire-active season (February to March) than in the fire-active season (April to May), as shown in Fig. 2. The AO and local geopotential height indices for February and March show significant correlations with burned area in southeastern Siberia, but not for April and May (Table 1). Local temperature and geopotential height both show positive anomalies in April, which is when southeastern Siberian burned area is at its maximum; however, these anomalies are relatively weaker than those in late winter (February to March) and are also nonsignificant (Fig. 2C). In addition, substantial land-surface cooling is observed in May over Siberia (Fig. 2D), whereas strong warming precedes strong fire activity. It is conceived that fire-induced aerosols might block solar radiation, resulting in surface cooling, suggesting that fire is largely controlled by climate factors while, in turn, considerably affecting the climate system on a seasonal time scale (21). An aerosol optical depth at 550 nm over northeastern Siberia (120°–180°E, 55°–70°N), where the cooling is distinctively observed, shows a significant relationship with the burned area in southeastern Siberia in May, indicating that aerosols from the southeastern Siberian fire are transported northward (table S1).

Role of earlier snowmelt on fire activity In late winter, anticyclonic circulation accompanies anomalous southwesterlies, leading to surface warming due to warm advection over southeastern Siberia. Surface warming alters local snow conditions, leading to an earlier start of the fire season, and is thus a preferable condition for strong fire activity. However, as shown in Fig. 2, despite notable positive temperature anomalies, southeastern Siberia has a subzero climatological temperature in late winter (Fig. 2, A and B). In addition, climatological snow cover exceeds 50% in this region, which suppresses winter fire activity (Fig. 3A). Although this region experiences notable positive temperature anomalies from February onward, February snow variability is not sensitive to temperature anomalies because the climatological temperature is too low to induce snowmelt (fig. S2). In contrast, we found a significant negative relationship between March to April snow cover and total annual fire activity, as positive temperature anomalies related to a positive AO in February and March drive early snowmelt in March and April with a time lag of 1 to 2 months (Fig. 3, B and C, and fig. S6) (18, 19). This is consistent with results from a snow water equivalent dataset (fig. S7). Accumulated positive temperature anomalies in late winter lead to earlier melting in snow cover’s seasonal evolution. Once snow cover is reduced, a positive snow-albedo feedback accelerates surface warming and snowmelt (fig. S8). Thus, significant negative snowmelt is observed in March and April as a result (Fig. 3, B and C). Earlier snowmelt leads to faster exposure of the ground surface and litter, which, in turn, allows favorable conditions for fire spreading because this region consists mostly of larch (Larix gmelinii) forests with a high amount of litter that can act as fire fuel (22). This seasonal response of snow cover may explain the lagged relationship between fire activity, which has its maximum in April and May, and late winter atmospheric circulation anomalies. Fig. 3 Snow cover variation related to fire activity over southeastern Siberia. Climatological monthly snow cover (shading) and statistical confidence (dots) based on correlation coefficient between yearly burned area in southeastern Siberia (boxed area) and monthly snow cover anomalies for February (A), March (B), April (C), and May (D) based on Student’s t test.