Arctic: From the CO₂ sink to the source

Arctic: From the CO₂ sink to the source

More and more Arctic areas are moving from carbon sinks (blue) to sources (red). © Greg Fiske / Woodwell Climate Research Center

The Arctic’s permafrost and boreal forests have served as reliable carbon sinks for thousands of years. But now advancing climate change is apparently causing this trend to increasingly reverse. As researchers have found, a third of the Arctic areas examined now emit more CO22 off as they record. Even vegetation that continues to advance further north cannot sufficiently counteract this effect.

The far north has long been considered an effective carbon sink. For example, there are large amounts of CO in the permanently frozen permafrost soils of the barren tundra2 stored in the form of undecomposed, frozen organic material. The boreal coniferous forests, which extend across Canada, Russia and Norway, among others, also absorb the greenhouse gas carbon dioxide during photosynthesis and bind it in their tissues. But climate change is currently changing the Arctic permanently. For example, forest fires are becoming more and more common in the north and global warming is thawing the permafrost. Does this also impact the Arctic’s function as a global carbon sink?

Trend reversal in the far north

To find out, researchers led by Anna-Maria Virkkala from the Woodwell Climate Research Center in the USA have now evaluated the most comprehensive data set to date. It comes from 200 measuring stations that have collected monthly data on photosynthesis and primary production as well as the breathability of soils and vegetation in different parts of the Arctic over around 30 years. From this information it was possible to determine what the net CO2-balance developed at these locations between 1990 and 2020. The database records the dynamics of this carbon balance over the course of the year as well as important recent changes in the climate and the increasing risk of forest fires in the north.

The result: After thousands of years as a carbon sink, the Arctic appears to be increasingly becoming a source, as Virkkala and her colleagues have discovered. Already a third of the areas examined emit more CO over the course of the year2 out as they record. If emissions from fires are included, this proportion rises to 40 percent, as the team reports. “While we found that many northern ecosystems still act as carbon sinks, source regions and fires are offsetting much of this net uptake and reversing long-standing trends,” emphasizes Virkkala.

Even the vegetation offers little protection

The increasing forest growth in areas that were previously too cold for this type of vegetation also appears to be doing little to counteract the negative trend. In theory, more vegetation that lasts longer throughout the year means more CO overall2 can be stored in trees, but only twelve percent of the areas that had become greener actually had such a balance. The new “greening” had the greatest effect in the summer months, when there was enough sunshine to enable photosynthesis. But in winter, CO2 predominated again in many places2emissions that were associated with thawing permafrost, for example, according to the team.

“The carbon cycle in the permafrost region is really starting to change,” emphasizes Virkkala. “Our study could be a warning sign of larger changes and provides a map of places we need to better monitor in the coming decades.”

Source: Anna-Maria Virkkala (Woodwell Climate Research Center, Falmouth, MA, USA) et al., Nature Climate Change, doi: 10.1038/s41558-024-02234-5

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