Lightning Is the Leading Cause of Fires in Boreal Forests

The vast coniferous forests of the northern hemisphere are under threat from the sky.

Researchers have documented that lightning – not human activity – is the dominant cause of wildfires in these boreal forests.

These lightning strikes will only become more frequent as climate change progresses, the researchers warn in their new paper, published in Nature Geoscience.

A bolt of lightning

The international research team used machine learning technology to predict the dominant source of wildfire ignitions – human or ”natural” lightning ignitions – in all world regions. Reference data from seven world regions were used to optimize the predictions from the algorithm.

The researchers – from the University of East Anglia, Vrije Universiteit Amsterdam, the University of Leeds, Jiangsu Academy of Agricultural Sciences in China and BeZero Carbon Ltd. – say it’s the first study to attribute fire ignition sources globally. 

While the study did have a global focus, the researchers observed that one area of the world had a particular problem with wildfires: northern boreal forests. The data results showed that 77% of the burned areas in these forests are related to lightning ignitions. Tropical forest fires, on the other hand, were much more likely to be started by human activity.

Boreal forests are characterized by large swathes of coniferous trees like pines, fir and spruce. These woods can weather harsh winters and are principally found in the northern reaches of eight countries: Canada, Russia, Finland, Norway, Sweden, China, Japan, and the US state of Alaska. Combined, these vast forests are considered by some scientists to be the world’s largest biome.

Boreal (also known as taiga or extratropical forests) are globally significant because they store vast quantities of carbon in vegetation and permafrost soils, carbon that can be released if the forest burns down.

However, this biome is under increasing threat. After adapting their data with climate models, the research team observed that lightning frequency increased by 11–31% per degree of global warming over intact boreal forests.

The team’s earlier work found that boreal forests are also becoming dryer and more flammable as global warming takes effect. Combined, this flammability and the increasing rate of lightning strikes mean boreal forests are becoming worryingly vulnerable, say the researchers, at a time when they’re more vital than ever.

“Extratropical forests are globally important because they lock up dense stores of carbon in vegetation and soils, helping to keep CO₂ out of the atmosphere and moderate global warming,” says Dr. Matthew Jones, a research fellow at the University of East Anglia and co-author of the study.

“However, when fires occur in these regions, they emit more CO₂ per unit area than virtually anywhere else on Earth.”

“Our research highlights that extratropical forests are vulnerable to the combined effects of a warmer, drier climate and a heightened likelihood of ignitions by lightning strikes,” Jones added.

“Future increases in lightning ignitions threaten to destabilize vast carbon stores in extratropical forests, particularly as weather conditions become warmer, drier and overall more fire-prone in these regions.” 

To protect the boreal forests (and those who live within and near them), net zero efforts that wean societies off fossil fuels must be prioritized, according to Jones and his colleagues.

“Our work has shown that the risk of lightning ignitions increases substantially as the planet warms, meaning that every tenth of a degree of warming that we can avoid will translate directly into a reduced risk of wildfire,” he said.

“Curbing emissions of greenhouse gases from fossil fuel use and land use change is critical to avoiding the worst additional risks of wildfire in many regions, but especially in the boreal forests where fires are so sensitive to warming.” 

This article is a rework of a press release issued by the University of Leeds. Material has been edited for length and content.

Reference: Janssen TAJ, Jones MW, Finney D, et al. Extratropical forests increasingly at risk of lightning fires. Nat. Geo. 2023. doi: 10.1038/s41561-023-01322-z