Remember all those stories you read about how global warming was increasing the number of forest fires in Alaska, and those forest fires burning deep into soils were, in turn, accelerating warming by adding more carbon dioxide (CO₂) to the atmosphere?
“Spike in Alaska wildfires is worsening global warming, US says,” the Guardian headlined in 2016.
“The increasing frequency of big fire seasons (in Alaska) is also a heavy contributor to carbon dioxide emissions, creating a positive feedback loop wherein global warming begets more warming,” Audubon magazine reported in 2019.
This was conventional wisdom.
“…The burning of trees, dead biomass and soil sends huge pulses of carbon to the atmosphere,” wrote Carly Phillips, the Kendall Fellow for Protecting Carbon in Alaska’s Boreal Forests with the Climate & Energy program at the Union of Concerned Scientists at that time.”In carbon-rich areas like boreal forests, arctic tundra, and peatlands, the impact of fire on climate change is further amplified.”
Only now come indications that at least when it comes to the boreal forests of Alaska, burning them down might not be so bad. Over the span of decades, it is even possible those fires might help reduce carbon emissions and slow climate change.
“…The deciduous trees replacing burned spruce forests more than make up for that loss, storing more carbon and accumulating it four times faster over a 100-year fire interval,” Phys.Org, a website for science, reported in summary. “The study, led by a team of researchers at the Center for Ecosystem Science and Society at Northern Arizona University (NAU), suggests that these faster-growing, less flammable deciduous forests may act as a stabilizing ‘firebreak’ against escalating fire patterns and nutrient loss in the region.”
Science is complicated. Open-minded scientists who go looking for data to determine how the world around us works don’t always find what they expect.
When Michelle Mack from NAU joined researchers from the University of Alaska Fairbanks, Auburn University, the University of Florida and the University of Saskatchewan tracking Alaska black spruce forests going up in flames in 2004, she was pretty sure of what she’d find.
“In 2005, I thought that there was no way these forests could recover the carbon they lost in this fire,” the biology professor confessed to the NAU News. “The literature is full of papers suggesting deeper, more severe fires burn more carbon than can be replaced before the next fire.”
That isn’t, however, what the Alaska research found.
“….Not only did we see (new) deciduous trees make up for those losses, they did so rapidly,” said Mack, who ended up as the lead author on a peer-reviewed study published in “Science” yesterday.
The paper would indicate the carbon freed in these fires is offset by the carbon sucked up by new, leafy, green vegetation in the regrowth that follows. The study suggests it is possible the net effect is that more carbon is taken out of the atmosphere over the course of time than the fires add during their short, flaming existence.
“Severe burning of organic soils shifted tree dominance from slow-growing black spruce to fast-growing deciduous broadleaf trees, resulting in a net increase in carbon storage by a factor of five over the disturbance cycle,” the scientists wrote. “Reduced fire activity in future deciduous-dominated boreal forests could increase the tenure of this carbon on the landscape, thereby mitigating the feedback to climate warming.”
Not only that, a warmer Alaska might be making it more likely that the fires that consume the state’s boreal spruce forests do so in such a way so as to encourage the growth of CO₂ sucking deciduous trees.
“In boreal forests,” the team of scientists concluded, “climate warming is shifting the wildfire disturbance regime to more frequent fires that burn more deeply into organic soils….we found that shifts in dominant plant species catalyzed by severe fire compensated for greater combustion of soil carbon over decadal time scales.”
The fate of Alaska’s boreal forests is part of a much bigger debate about the function, or not, of “positive feedback loops” in the carbon cycle.
Positive feedback loops are those that fuel reactions rather than stabilize them.
“A positive feedback loop increases the effect of the change and produces instability, as the National Oceanic and Atmospheric Administration (NOAA) describes this process. “In climate change, a feedback loop is something that speeds up or slows down a warming trend. A positive feedback accelerates a temperature rise, whereas a negative feedback slows it down.
The thinking before the latest study was that the Alaska fires created a positive feedback loop to accelerate warming. The new study indicates the negative feedback loop that follows fire could alter everything.
But it’s not that simple either because all plants eventually die, and “after plants die, they decay, releasing the carbon to the atmosphere,” the Earth Observatory of the National Aeronautics and Space Agency (NASA) notes.
Big plants or small plants, 5,000-year-old bristlecone pines or short-lived bamboo, they all die.
“The faster a plant grows, the more carbon dioxide (CO₂) it will use up per second. (And) by that measure, bamboo might be the best at sucking up CO₂. However, fast-growing plants tend not to live long and when a plant dies, all the carbon in the plant is broken down by insects, fungi and microbes and released as CO₂ again,” write Luis Villazon at Science Focus.
“So the plants that are considered the most adept at removing CO₂ from the atmosphere and locking it up are the longest-living ones, with the most mass – hardwood trees. It’s all temporary though. Eventually every plant returns all the carbon dioxide it uses back to the atmosphere.”
And, of course, land plants aren’t the only plants on the planet. There is more plant life in water, which covers about 71 percent of the planet’s than on land, and the ocean uptake of CO₂ by plants, which use CO₂ as the food of photosynthesis no matter where they grow, removes CO₂ from the atmosphere.
Plus the water itself works like a sponge for CO₂.
“Oceanographers started out wanting to know if the ocean was keeping up with the amount of carbon dioxide people are putting into the atmosphere,” Holli Riebeek of NASA’s Earth Observatory has observed. “Instead, they found that people aren’t the only players changing the ocean carbon cycle.
“Over decades, natural cycles in weather and ocean currents alter the rate at which the ocean soaks up and vents carbon dioxide. What’s more, scientists are beginning to find evidence that human-induced changes in the atmosphere also change the rate at which the ocean takes up carbon. In other words, it turns out that the world is not a simple place.”
And that might be an understatement.