Dig just about anywhere at far northern latitudes, and you’re bound to hit permafrost.
But this frozen soil is thawing as Arctic temperatures rise, and the carbon within it is escaping into the atmosphere in the form of carbon dioxide, a major greenhouse gas. Researchers have now experimentally studied how sunlight triggers carbon dioxide production from permafrost carbon that’s been flushed to lakes and rivers, a process long ignored in climate models.
Current estimates of global warming from permafrost carbon feedback are biased low, the team concluded.
No Longer Locked Up
Permafrost has been frozen for far longer than humans have been on the planet. That’s a good thing because permafrost contains over a trillion metric tons of organic carbon deposited by generations of plants, and all that carbon remains locked up when it’s frozen.
“But now, because of human activity, it’s starting to thaw,” said Collin P. Ward, an aquatic geochemist at the Woods Hole Oceanographic Institution in Woods Hole, Mass. “The big concern here is what’s going to happen to all of that organic carbon.”
The effects of thawing extend beyond climate change—buildings and roads in the Arctic are also apt to collapse when the underlying permafrost thaws.
Microbes and Sunlight
One way in which permafrost carbon gets converted to carbon dioxide is via microbes—some microscopic life-forms chow down on carbon and respire carbon dioxide.
Although this microbial process is generally taken into account in climate models, comparably little is known about the permafrost carbon that’s flushed to lakes and rivers, where it’s exposed to sunlight. “We’ve known for a while that sunlight converts organic carbon to carbon dioxide, but the governing control of this process has escaped us,” said Ward.
It’s been hypothesized that this photomineralization might be controlled by the presence of iron, which is abundant in Arctic fresh waters. “There have been lots of lab-based studies suggesting that iron is a key player, but this is the first to let nature tell us what controls this process,” said Ward.
In 2018, Ward and his colleagues collected five samples of permafrost from northern Alaska. Back in the laboratory, they thawed the permafrost, filtered out the microbes, and isolated the dissolved organic carbon and other constituents, including iron. They then exposed the samples to different wavelengths of ultraviolet and visible light.
Visible Light Wins
In nature, the highest rates of photomineralization occur in the presence of visible light, Ward and his colleagues calculated. Two factors contribute to this finding. First, Earth’s surface receives significantly more visible light than ultraviolet light. Second, iron kick-starts reactions at longer wavelengths, the team showed. (Visible light is characterized by longer wavelengths than ultraviolet light.)
Photomineralization’s wavelength dependence has important implications, said Ward. It means that permafrost carbon in deep lakes or rivers is still apt to be converted to carbon dioxide. “As you move deeper into the water column, there’s less ultraviolet light available and more visible light,” said Ward.
Older and More Effective
The researchers also determined that the older carbon found in permafrost—several thousand years old—was roughly twice as effective at producing carbon dioxide as modern carbon. Modern carbon has more sunlight-absorbing compounds, said team member Jenny Bowen, a biogeochemist at the University of Michigan, but permafrost carbon is better at reaping the reaction-promoting benefits of iron.
This unaccounted-for contribution from old carbon has the potential to fundamentally change the carbon cycle, said Ted Schuur, an ecosystem ecologist at Northern Arizona University in Flagstaff not involved in the research. “Stuff that wasn’t part of the atmosphere is suddenly ending up in the atmosphere.”
Since photomineralization of permafrost carbon isn’t presently included in climate models, estimates of future global warming are biased low, the researchers concluded. “Sunlight increases the amount of carbon dioxide coming from thawing permafrost by 14%,” said Bowen. “The planet will warm even more than expected.”
These results were published last month in Geophysical Research Letters.
In the future, Ward and his colleagues plan to study how sunlight-driven photomineralization and microbial degradation work in tandem to convert permafrost carbon into carbon dioxide. “Sunlight can facilitate microbial degradation,” said Ward. “Sunlight breaks molecules into simpler ones that microbes can readily use.”
—Katherine Kornei (@KatherineKornei), Science Writer