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Lightning is the leading cause of wildfires in boreal forests

Lightning has been identified as the leading cause of wildfires in boreal forests, a critical area for carbon storage, and is expected to become more frequent with climate change. This is the alarming conclusion of a new study led by Dr. Matthew Jones from the University of East Anglia’s Tyndall Centre for Climate Change Research, alongside international collaborators.

Using machine learning, the team predicted wildfire ignitions globally, identifying whether they were caused by humans or natural lightning. This research marks the first time fire ignition sources have been attributed on a global scale.

Important insights

The findings revealed that 77% of burned areas in intact extratropical forests were caused by lightning. This contrasts sharply with tropical regions, where fires are predominantly human-caused. 

Intact extratropical forests, largely undisturbed and sparsely populated, are mainly located in the remote boreal forests of the northern hemisphere.

Lightning frequency

Further insights were gained using climate models to predict changes in lightning frequency as the planet warms. 

The study found an 11 to 31% increase in lightning frequency per degree of global warming over these forests, indicating a heightened risk of wildfires due to climate change.

Lightning fires

Lightning fires tend to be larger, more intense, and more confined to remote areas during periods of extreme fuel dryness compared to human-caused fires. 

The team’s previous work showed that conditions conducive to fires are becoming more frequent and intense as the climate warms, leading to increased forest flammability.

Extratropical forests 

Extratropical forests are globally significant, storing vast amounts of carbon in vegetation and permafrost soils. 

Remarkably, although they cover only about 1% of Earth’s land surface, these forests contribute over 8% of global CO2 emissions from fires. Fires in these regions could amplify greenhouse gas emissions from permafrost thaw by 30% by the end of the century, following a moderate emissions scenario.

Carbon stores

“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,” said Dr. Jones. “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.”

“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.” 

Extreme events

The research gains relevance in the context of Canada’s record-breaking fire season in 2023, where emissions were over four times the average of the past two decades. 

Dr. Thomas Janssen from VU, the study’s lead author, noted that while the research didn’t specifically address Canada’s fire season, it helps understand such extreme events.

“While our research did not focus specifically on this year’s extreme fire season in Canada, it does help us to understand this year’s events. Extreme fire seasons in boreal forests, like the one we saw in Canada this year, will be more likely in warmer climates due to hotter, drier weather and more lighting strikes,” said Dr. Janssen.

“The fires in Canada this year closely follow record-breaking fire seasons in the Siberian boreal forests in 2020 and 2021.” 

Reinforcing feedback

The researchers warn that greenhouse gas emissions from fires can contribute to rising concentrations of carbon in the atmosphere and drive additional warming, further exaggerating the likelihood of fires and other adverse impacts of climate change in future.   

“Increased greenhouse gas emissions from wildfires reinforces the problem of climate change, with more fires occurring as the climate warms and more greenhouse gases being emitted by fires,” said Professor Sander Veraverbeke.

“This ‘reinforcing feedback’ is particularly important in boreal forests, most of which are underlain by carbon-rich permafrost soils that take many hundreds of years to form if they are lost to fire.”  

Study implications

“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,” said Dr. Jones. 

“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.” 

The study is published in the journal Nature Geoscience.

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