The Canadian Arctic: carbon sink or source?
An article by Valérie Levée, science journalist
There's no longer any room for doubt: permafrost is thawing and thermokarst pools are emitting methane. But that doesn't mean that the Arctic and boreal forest are becoming sources of carbon, as little is known about the nature of gas flows on a landscape scale. To find out, we need to deploy large-scale gas exchange measurements and apply the eddy covariance method. This is what Oliver Sonnentag, Professor in the Department of Geography at the Université de Montréal and holder of the Canada Research Chair in Atmospheric Biogeosciences at High-Latitudes, is doing.
A landscape in transition
In northwestern Canada, the boreal forest grows on permafrost. But with global warming, thawing permafrost is subsiding, and the newly formed basins are filling with water. Spruce trees lose their support, drown and die, only to be replaced by sphagnum moss, sedges and other typical bog plants. "Forests will be replaced by wetlands, and that will change the way gas and energy are exchanged," predicts Oliver Sonnentag. This is what he wanted to measure when he took up his professorship at the Université de Montréal in 2011. "There's a landscape that's changing very rapidly, but what's happening in the atmosphere, how does this remote landscape function in the climate system?" he asks. The FLUXNET network is a global network of towers that measure gas exchange. "There are a lot of towers in Europe, in the United States, a few in southern Canada, but there is a huge lack in the far north," Oliver Sonnentag notes. He set out to fill that gap by installing towers at five observation sites along a 2,000 km south-north transect from Old Black Spruce in Saskatchewan to the Trail Valley Creek research station in the northern Northwest Territories. "In the south there is no permafrost, a few hundred kilometers to the north there is isolated or discontinuous permafrost, and in the far north there is continuous permafrost," describes Oliver Sonnentag.
The instruments are installed about twice as high as the vegetation at the interface between the biosphere and the atmosphere. For 10 years now, they have been measuring, at a rate of 20 times per second, the concentrations of CO2, methane, water vapor, heat, humidity, absorbed and reflected solar radiation... and wind speed. These measurements allow us to apply turbulence covariance. "We measure vertical wind speed and, at the same time, changes in gas concentrations. Covariance of wind speed and gas concentrations gives gas flux, gas movement per unit area and time at landscape scale," explains Oliver Sonnentag.
Complex fluxes
Methane fluxes do increase over wetlands, as numerous studies, including Oliver Sonnentag's, have already shown. But as he points out, "If you're measuring over a wetland, of course you're going to see methane flux. But you also have to consider dry mineral soils”. In the mineral soils of the Arctic, methanotrophic bacteria consume methane. This is the only natural way for the biosphere to degrade methane. But "on a landscape scale, methane is still being absorbed in the Arctic," says Oliver Sonnentag. Meanwhile, wetlands are not alone in emitting methane. In fact, he and his colleagues have shown that fugitive emissions from 170 abandoned oil and gas wells in western Canada account for 13% of total anthropogenic methane emissions.
The case of CO2 is not clear either, since sphagnum mosses are more productive than the spruce trees they replace. The CO2 fluxes measured show that they are carbon sinks, and this result is corroborated by the research of Michelle Garneau, professor at the Department of Geography at the Université du Québec à Montréal, which shows an accumulation of carbon in peat cores.
On October 13, 2022, a forest fire wiped out one of the observation sites located on the edge of the permafrost.
"It's a disaster because all the equipment burned, but scientifically it's a unique opportunity to understand how forest fires interact with permafrost thaw. Will forest fires accelerate the thaw?" asks Oliver Sonnentag.
In March 2023, the instruments were reinstalled to measure gas fluxes in a landscape recovering from a wildfire.
So, carbon sink or source?
"We can't ignore the fact that peat absorbs CO2. We still don't understand how the tundra absorbs methane, or how much fugitive methane emissions contribute. There is no simple answer to the question of whether Canada's Far North is a carbon source or sink," concludes Oliver Sonnentag. Forest fires are another unknown in the puzzle of gas exchange in the Arctic and boreal forest. Many measurements are still missing. "I am part of several global forums to determine the carbon balance of the entire Arctic, in Alaska, Canada, Iceland, Scandinavia and Russia. There are some regions, like northern Quebec, where we have no data. We have no idea what the CO2 and methane fluxes are.
To learn more :
Manuel, H., Laura E. Chasmer, L. E., Desai, A. R., Kljun, N., William L. Quinton, W. L., Sonnentag, O. (2017). Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest–wetland landscape, Global change biology, 3, 3231-3248
https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.13638
Manuel, H., William L. Quinton, W. L., Sonnentag, O. (2017) Warmer spring conditions increase annual methane emissions from a boreal peat landscape with sporadic permafrost, Environmental Research Letters, 12, 115009
https://iopscience.iop.org/article/10.1088/1748-9326/aa8c85
Virkkala, A.-M., Aalto, J., [+ 46 authors including Sonnentag, O.], Luoto, M. (2021) Statistical upscaling of ecosystem CO2 fluxes across the terrestrial tundra and boreal domain: Regional patterns and uncertainties Global change biology, 17, 4040-4059
https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15659
Fisher, J. B., Hayes, D. J., [+ 20 authors including Sonnentag, O.], Zhang, Z. (2018) Missing pieces to modeling the Arctic-Boreal puzzle, Environmental Research Letters,13, 020202
https://iopscience.iop.org/article/10.1088/1748-9326/aa9d9a/meta
Balzer, J. L., Day, N. J., [+ 20 authors], Johnstone, J. F. (2021) Increasing fire and the decline of fire adapted black spruce in the boreal forest, Proceedings of the National Academy of Sciences, 118 (45) e2024872118
https://www.pnas.org/doi/10.1073/pnas.2024872118
Turetsky, M. R., Abbott, B. W., [+ 20 authors], McGuire, A.D. (2020) Carbon release through abrupt permafrost thaw, Nature Geoscience, 13, pages138–143
https://www.nature.com/articles/s41561-019-0526-0
Miner, K. R., Turetsky, M. R., [+ 7 authors], Miller, C. E. (2022) Permafrost carbon emissions in a changing Arctic, Nature Reviews Earth & Environment, 3, 55-67
https://www.nature.com/articles/s43017-021-00230-3
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