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Intensifying fire regimes in the High North: Climatic drivers and consequences


The boreal region encompasses approximately 30% of the world’s forests and the arctic-boreal region stores about 35% of the world’s soil carbon. Climate warming is occurring more rapidly in these northern high latitudes than in the rest of the world. These warmer temperatures are causing the number of fires to rise. Media reports on the 2019 fire season in the arctic-boreal region reported (some of) the worst fire seasons on record in Alaska, Siberia and within the Arctic Circle. We use a 23-year satellite record from the Global Fire Emissions Database to assess the 2019 fire emissions in reference to prior years. Fire emissions within the Arctic Circle in 2019 by far exceeded any prior year. This record was largely driven by forest fires in Arctic Northeast Siberia, and not so much by tundra fires as was often reported in the media. The 2019 fire season in Siberia ranked as the third largest since 1997, while in Alaska it was the fourth largest.
This presentation further highlights three aspects of a changing arctic-boreal fire regime: 1) warming-induced lightning ignitions as driver of increased fire activity, 2) overwintering fires as an emerging phenomenon, and 3) current progress on refining carbon emissions from boreal forest fires.
First, we show that warming-induced increases in lightning ignitions were a major driver of two recent large fire years in boreal North America, in the Northwest Territories in 2014, and in Alaska in 2015. We further append upon these case studies, by combining lightning observations from the Optical Transient Detector in combination with climate proxies for lightning from reanalysis to model contemporary and future lightning in the high latitudes. Results indicate a lightning-richer future for arctic-boreal regions, suggesting consequent increases in lightning fires.
Second, we found that the legacy of large fire years extends into the subsequent fire season through the phenomenon of overwintering fires. Overwintering fires are fires that seemingly stop at the end of the fire season when rain and snow arrive, yet these fires smolder in organic soils throughout winter to eventually reemerge in the subsequent spring under weather conditions that favor fire spread. We analyzed the spatiotemporal occurrence of overwintering fires in relation to climatic and environmental variables and conclude that we may expect more holdover fires in the future.
Third, as part of NASA’s Arctic-Boreal Vulnerability Experiment, field data of carbon combustion by fire were synthesized over 417 plots in boreal North America. We compiled the best available bottom-up, i.e. fuels and drainage, and top-down fire weather data for these sites. We found that bottom-up drivers explain more variability in carbon combustion than fire weather. Large scale fire emissions model often implement fire weather as a prime driver, yet we demonstrate the importance of fine-scale variability in fuel and drainage condition. Further, prior research has shown that fire regimes are different between the boreal continents: high severity stand-replacing fires dominate in North America, whereas low severity surface fires are more frequent in Eurasia. Significant progress has been made in quantifying carbon combustion in North American ecosystems, yet few measurements have been made in Eurasian ecosystems. This presentation shows a preliminary view on our team’s field campaign in Siberia during the summer of 2019.
This presentation highlights multiple aspects of changing fire regimes in the northern high latitudes, which may have strong impacts on ecosystem structure, carbon balance, and water and energy cycles in these regions.