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Early snowmelt and northward jet stream setting Siberia ablaze

3 November 2022
Earth and climate scientists from Vrije Universiteit (VU) Amsterdam united forces to disentangle the combined effects of diminishing snow cover and a changing jet stream on recent Siberian fire extremes.

The study was published today in the scientific journal Science.

With a total of about 200,000 km2 of forest and tundra burned - equaling nearly five times the size of the Netherlands - the years 2019, 2020 and 2021 were the largest fire years in northern Siberian larch forests since 2001. VU researchers found that these fires were facilitated by an earlier snowmelt together with a northward displacement of the polar jet stream. By linking atmospheric dynamics with fire sciences, the study is the first to show that the intensification of fire activity in the high North is sensitive not only to rising temperatures alone, but also to more complex interactions in the climate system.

The study was motivated by observations of record-breaking heat in central and eastern Siberia for three years in a row leading to severe fire seasons. “We were curious to explore how unprecedented these events were”, explains Rebecca Scholten, PhD student at the department of Earth Sciences of VU and lead author of the study. “We also wanted to find out what causes such extreme fire activity in this region. Is it due to short-term weather extremes or are long-term, maybe seasonal, processes of atmospheric circulation playing a role?” Investigating weeks with extreme fire activity, the team found that the combined effect of both snowmelt timing and specific jet stream states, can give rise to large fire seasons.

The Arctic front jet
“We saw a jet stream pattern emerging that we recently also identified as an important driver of European heat waves in an earlier study”, says co-author Dim Coumou, Professor of Climate Extremes and Societal Risk at VU and the Royal Netherlands Meteorological Institute (KNMI). This so-called double jet pattern is characterized by a northward displaced polar jet, or Arctic front jet, together with a strong subtropical jet. It has previously been linked to blocking of high pressure systems and hence heat waves in Europe and other mid-latitude regions. “What is fascinating about atmospheric dynamics studies is how extremes in different parts of the world are connected and can be explained by common drivers and mechanisms”, adds co-author Fei Luo, PhD candidate at VU’s Institute for Environmental Studies (IVM).

In addition, the researchers found early snowmelt onsets to be an important driver for a northward move of fires. “When the snow melts earlier, there is a longer time of fuel drying that makes vegetation and litter more susceptible to fire”, explains Sander Veraverbeke, Associate Professor in Climate and Ecosystems Change at VU and last author of the study. Extreme lightning activity also plays a major role in northern fire activity. Heatwaves build up convective energy in the atmosphere culminating in lightning-rich thunderstorms. “In the remote Arctic, human ignitions are scarce. Lightning thus brings ignitions to places which have seen very little fire before.”

Permafrost
Worryingly, these extreme fire seasons in northern high latitudes may also further accelerate climate warming. “Arctic fires burn in carbon-rich ecosystems and as such emit large amounts of greenhouse gases”, says Veraverbeke. “On top of that, the vast majority of these fires burn in permafrost landscapes which safeguard tremendous carbon stocks.” How permafrost soils will react to the combination of higher temperatures and more wildfires is still largely unknown, and this is currently an active research area of the team.

The study found that climate change may be the cause behind the elevated fire activity in Siberia. “We found that on average snowmelt in eastern Siberia is now starting nearly a week earlier than 40 years ago”, says Scholten. “Simultaneously, the frequency of Arctic front jets over Eurasia has more than tripled." These drastic accelerations of compound drivers of fire ignition and spread have increased the likelihood of extreme fire years.

Illustration: The extreme fire years of 2019, 2020 and 2021 were co-influenced by early snow melt followed by the occurrence of an Arctic front jet. Copyright: Stefan Witte Fotografie.