In 2023, the Canada wildfires that incinerated more than 17 million hectares (42 million acres) of boreal forest were so hot they melted the paint on approaching fire trucks and smoldered underground all winter. That heat created vast columns of rising air, carrying dust, volatile organic compounds, and huge quantities of a simple particle with the potential to exacerbate climate change: black carbon.
Commonly known as soot, black carbon is a type of pollution formed by the incomplete combustion of fossil fuels or biomass such as trees. It’s a risk to human health, having been linked to respiratory and cardiovascular diseases. It’s also a potent short-term warming agent. Black carbon absorbs copious heat from the sun and, when it coats a layer of ice or snow, reduces its ability to reflect solar energy back into space.
The choking wildfire smoke that enveloped New York and other cities far to the south two years ago was notable for where else it traveled. Propelled by intense heat high into the atmosphere and carried by fierce winds, it made it to Greenland and beyond.
How much of the black carbon in that smoke reached the Arctic is a pressing question for climate scientists, who fear the aerosol could become a major contributor of further heating to what’s already the fastest-warming place on Earth.
Halfway into Canada’s 2025 fire season, this year’s blazes are likely to end up the second worst in three decades, after the all-time record set in 2023 — and finding answers has gained urgency.
“The hotter the fire is, the higher it can be lifted,” said Sarah Smith, an atmospheric physicist and Ph.D. candidate at Columbia University who’s studying the 2023 wildfires to determine how they, and future fires, may affect the Greenland ice sheet. “We are still learning just how high these black carbon air masses can be injected into the atmosphere and what their effects are.”
For a while, levels of black carbon in Greenland seemed to be dropping, not rising. Ice core samples dating back to the 1700s show semi-regular deposits associated with forest fires upwind in North America. In the early 20th century, levels began to rise steadily as demand for heating oil and coal surged in Canada, the US and Western Europe.
But subsequent measures to improve air quality, such as adding scrubbers to coal plants, along with a shift to cleaner fuels made the problem seem less pressing, especially when compared to other humanmade drivers of climate change.
“I wouldn’t say we lost interest, but there was some evidence that there was a declining trend of black carbon in the Arctic,” said Mark Flanner, a scientist at the University of Michigan who studies interactions between the cryosphere and the climate.
The possibility of more frequent and severe northern wildfires changes that, Flanner said: “2023 was eye-opening.”
The dynamics of black carbon in the Arctic environment aren’t simple and they still aren’t fully understood by researchers. Scientists don’t know whether the aerosol primarily increases or decreases cloud cover. Different types of clouds can either trap heat or reflect sunlight, resulting in net warming or cooling. When black carbon hangs in the atmosphere, its potency may depend on its altitude: Lower down, it seems to be more warming. And although it interferes with the planet’s ability to reflect sunlight, its impact may be minimal when covered with a dusting of fresh snow.
The challenge is untangling these processes from a myriad of other overlapping changes in a region that acts as a climate shield for the rest of the planet.
To figure out how black carbon from a new breed of mega-wildfires fits into the mix, Smith started by looking at aerosol optical depth (AOD), a measure of light blockage in the atmosphere. That allowed her to gauge how much solar radiation was absorbed by smoke columns from the 2023 wildfires. Measurements showed record-breaking AOD as far away as Western Europe, she said, and smoke plumes so thick over the northeastern US and North Atlantic that, on average for the month of June, two-thirds of sunlight would have been attenuated.
This summer, her team hopes to expand their understanding by pulling hourly readings from the Purple Air network of ground sensors across Greenland (including a sensor that was installed by Flanner in Kangerlussuaq, in the west, in 2019) to determine if particulate matter near the ground spiked in June 2023 as the wildfire smoke moved over Greenland. If it did — rather than being rained out over the ocean during transit — the next step is connecting the dots to see if the black carbon led to changes in the ice sheet. (Yet another unknown is whether the substance became less dark through chemical reactions during its journey, and therefore potentially less damaging.)
Up to 85% of sunlight that hits a white surface is reflected, and black carbon can significantly reduce that when it lands on snow or ice, causing the surface to absorb solar radiation instead. Not only does that add to the planet’s heat tally in and of itself, but that surface warmth can also melt the surrounding area, creating a larger patch of darker water, which then captures more heat, and so on.
“You could just have a really amped-up greenhouse effect right over the ice sheet, and that can sometimes have a very, very big impact on ice-sheet melting,” Smith said.
Vicious Warming Cycles
The main sources of black carbon in the Arctic are thought to be flaring at Russian oil and gas facilities, the combustion of shipping fuel and forest fires.
Flaring, which occurs when refineries burn unwanted flammable gases, is projected to become the largest source of black carbon in Arctic countries by 2030. Marine traffic in the region is also set to rise with longer ice-free seasons and new trading routes. A 2024 ban on the use of heavy fuel oil in Arctic waters will reduce black carbon emissions per vessel, but there will inevitably be more ships.
Arctic “greening” — the spread of trees further north as temperatures rise — has coincided with increased wildfires due to hotter, drier conditions. Those fires can themselves exacerbate global warming in a vicious cycle, especially if they thaw carbon-rich permafrost or burn through stores of carbon in peat.
Such feedback loops are what worry scientists the most because they’ve already made the rise in Arctic temperatures an accelerating, rather than linear, phenomenon.
“The warmth in the Arctic, and the rate at which things are changing, is taking us beyond the boundaries of anything we’ve seen in thousands of years,” said Drew Shindell, a professor of Earth science at Duke University, who chaired a 2011 United Nations assessment on the impact of black carbon in the troposphere. That report concluded that about 0.5C of Arctic warming could be offset by reducing black carbon sources in coming decades.
Within the Arctic, reducing flaring at Russian oil and gas facilities would significantly lower black carbon emissions, Shindell said, and limiting shipping traffic would help avoid future increases. Emissions in Europe are more likely to travel to the Arctic than to Asia or mid-latitude North America, so controlling black-carbon-heavy fuels there, like diesel and residential coal and biofuels, also would also help. Slashing fossil fuel use generally would help slow global warming, which might keep wildfire conditions from getting worse.
The research on black carbon needs to be updated as more becomes known about the aerosol, and that makes tracking wildfire smoke even more important. Canadian wildfires this season have already burned 4.25 million hectares, with four of the last five years trending above the 20-year average.
For a while, it looked like black carbon emissions in the Arctic might have peaked. Now, Shindell says, “there’s good reason to think they might go in the opposite direction.”
Top photograph: A gas flare burns at the central processing plant for oil and gas in the Salym oilfields near Surgut, Russia. Photo credit: Alexander Zemlianichenko Jr./Bloomberg
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