Holes are appearing in the Siberian soil, and that worries scientists

For three consecutive summers, the forests of northeastern Siberia burned with unprecedented intensity. In 2019, 2020 and 2021. “I’m amazed by it,” says Sander Veraverbeke, physical geographer at the Free University in Amsterdam. Since 2013 he has been researching wildfires in boreal forests. That started in Alaska and Canada. In 2016, when he was appointed at the VU, he shifted his focus to Siberia. “You would expect that after one or two years everything that can burn will burn out.” But another record year followed. It fascinates him, he says in his study. How is this possible, three years in a row, and so violently? At the same time it frightens him. “The picture of the fires has only become more disturbing.”

In recent decades, fires have become increasingly common in the boreal forests. And more intense. The fires are also advancing further north. The cause is climate change. A few weeks ago Veraverbeke published with colleagues an article in Science, in which they link the wildfires to two other phenomena: snow that melts early in the season and high air currents (the jet stream) that create warm and dry conditions. We see these air currents more and more often, explains Veraverbeke. “And the snow is starting to melt earlier and earlier in the season.” With a combination of early snow melt and those air currents, there is a greater chance of fires, their study shows.

How is it possible that the fires are advancing north?

“The Arctic is now warming three times as fast as the world average. Shrubs and trees can grow further and further north. And we also see the lightnings advancing to the north.”

And lightning is an important cause of fires in the far north?

“You need three things for a fire: oxygen, fuel and an ignition source. In the inhabited world, people are usually the source of ignition, consciously or unconsciously. But in the barely inhabited far north, that is lightning, we have discovered.”

Do you know the Batagaika crater in Yakutia? It’s a giant thaw hole

How is it possible that more lightning has started?

“Due to the rapid warming of the Arctic, you often have situations in which warm air rises quickly. You then get more so-called convective clouds. That can create storm clouds.”

What do boreal forests look like?

“In Central and Eastern Siberia they are dominated by the larch. It’s a special tree. In winter it loses its needles. Every spring he makes them again. And he has found a way in evolution to grow on permafrost. His roots grow like this…” Veraverbeke holds his hands in front of him and moves them apart. “At latitude 66 you can still find larch forest.”

And the bottom can also burn?

“The soil is very rich in carbon. You have a lot of peat here.”

The idea is that greenhouse gas emissions from those fires will increase. What is your image?

“We don’t know enough about that yet. The fires release CO₂ free. But the warming also stimulates the growth of shrubs and trees. In doing so, CO₂ captured. The soil also accounts for the largest share of greenhouse gas emissions, about 80 percent. In the event of a fire, 20, 30, 40 centimeters from the bottom just goes up in smoke.”

Hardly anything was known about fires in Siberia, even though the area is larger than Canada and Alaska combined

There is some evidence that the advancing fires are accelerating the thawing of the permafrost. This would then boost the decomposition of peat soil by bacteria and lead to accelerated emissions of the strong greenhouse gas methane, which is released during this decomposition.

“More research is needed there as well.” Veraverbeke gets up from the table, walks to his computer and looks up something. “Do you know the Batagaika crater in Yakutia? It’s a giant thaw hole, a thermobox. We know from thermal chambers that methane emissions are accelerating. A big question I have is whether we can expect more holes like this. It is plausible. Scientists are concerned about it. A postdoc of mine is now looking into this, based on satellite images. We are already seeing subsidence of centimeters in many places.”

When you were appointed at the VU in 2016, you started to focus on Siberia. Why?

“Canada, and certainly Alaska, were already very much studied. Hardly anything was known about fires in Siberia, even though the area is larger than Canada and Alaska combined. It was niche. I also thought I would have little chance of getting research funding if I submitted yet another proposal to study fires in Alaska.”

Veraverbeke received a Vidi grant (maximum 800,000 euros) from NWO in 2018. “Enough to do research in Siberia for 4 to 5 years,” he says. In 2020, a grant of more than 2 million euros from Brussels followed.

But then came corona.

“And then Russia started the war against Ukraine. We traveled to Siberia for the first time in 2019 and carried out measurements there for a month and a half. Since then we can no longer go to the area.”

Because I don’t see it happening that we can go to Russia for the time being, I want to focus on tropical forests

How do you solve that?

“Our research has two pillars, fieldwork and satellite data. We have strongly switched to the latter. Together with my PhD student Rebecca Scholten, I carefully studied satellite data from a few large fires in Canada and Alaska. In particular, we looked at their edges. We noticed that we saw fires flare up in the spring in places where they had apparently gone out in the winter. And they couldn’t have been caused by lightning, because those don’t come until later in the year. We wondered if those fires could survive the winter underground. Peat is known to smolder for a very long time. We emailed and video-called firefighters in Alaska about it. Did they know this phenomenon? Yes, they said. They’ve known it for a long time. But our publication about it, last year in Naturewas the first scientific article about those so-called zombie fires.”

How important are zombie fires?

“If you look at burned areas, their share is about 1 percent. They are an intriguing oddity.”

Will you continue to focus on boreal forests?

“Because I don’t see it coming to Russia for the time being, I want to focus on tropical forests. There is much more lightning than in the far north, but it also rains a lot. So lightnings won’t start as many fires. But they do hit. I want to see how it contributes to the death of trees.”