By linking atmospheric dynamics with fire sciences, this 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.Earth and climate scientists from VU Amsterdam published their research in Science.
Record high temperatures
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 candidate 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 Amsterdam 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 Amsterdam's Institute for Environmental Studies (IVM).
Less snow and more lightning
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 Amsterdam 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.”
Impact on 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.
Caused by climate change?
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.
