The VU team estimated that the burned area from zombie fires accounted for 3.2 percent of the burned in Yakutia, eastern Siberia, between 2012 and 2020. In the large fire year of 2020, zombie fires accounted for 7.5 percent of the burned area. The study was published in the scientific journal Environmental Research Letters.
Zombie fires, lightning and human-induced fires
The study quantified the role of zombie fires, lightning and human-induced fires by overlaying fire, lightning and infrastructure data. “We derived daily burned area maps from satellite data and retrieved fire starts from it”, explains Wenxuan Xu, a visiting PhD student at VU Amsterdam from Nanjing University and first author of the paper, “By doing so, we can attribute burned area to different causes: zombie fires, and lightning and human-induced fires.”
The researchers estimated that between 2012 and 2020, 3.2 percent of the burned area stems from overwintering zombie fires, while lightning and human-induced fires accounted for 31.4 percent and 51 percent of the burned area (the cause of 14.4 of the burned area remained unknown). “The 3.2 percent of burned area from zombie fires may seem little, yet this fraction is higher than the earlier estimates from Canada and Alaska” says Xu, “and the large burned area from zombie fires in 2020 is especially notable.”
Hot and dry spring in 2020
That more zombie fires were to be expected in 2020 did not come as a surprise to Rebecca Scholten, PhD student at VU and co-author of the study. “In our earlier study we showed that zombie fires in Alaska and Canada were particularly common after hot summers with exceptionally large burned area”, explains Scholten. Indeed, in 2019, anomalously large areas burned in eastern Siberia, which have spurred the large number of overwintering fires that the team observed in 2020. But there was more to 2020. “Because of the compound effect of a drought and heatwave during the spring of 2020, snow melted earlier, and more zombie fires survived the winter and flared up again”, Scholten says. The team also calculated that the zombie fires that re-emerged in 2020 were about 9 times larger than zombie fires from between 2012 and 2019. Hot and dry conditions in spring, after a large fire year, thus promoted the survival and growth of zombie fires.
Here to stay
The research team also found that zombie fires re-emerge earlier than lightning fires, and in other parts of the landscape than most human-induced fires. “Zombie fires are really different from lightning and human-induced fires, they behave differently” explains Sander Veraverbeke, Associate Professor in Climate & Ecosystems Change and senior author of the paper. “Current fire models do not account for zombie fires and we need to include zombie fires in these models when we make predictions of future fire activity in the Arctic” Veraverbeke continued, “because zombie fires are here to stay and are likely to occur more often with climate warming”.
The work was part of Wenxuan Xu’s research stay at VU Amsterdam, which was sponsored by the China Scholarship Council. The VU authors were supported by a Dutch Research Council Vidi grant awarded to Sander Veraverbeke, and a Netherlands Earth System Science Centre contribution awarded to Guido van der Werf. The project would not have been possible without lightning data from Vaisala’s global lightning detection network. An earlier video animation explaining the phenomenon of zombie fires can be watched here.