Arctic’s big carbon sink could shift to a source

Scientists have known for some time now that the land and seas in the Arctic act as a sink for atmospheric carbon. In a new review paper in the journal Ecological Monographs, ecologists now have a sense of just how much carbon the Arctic has historically handled – up to a whopping 25 percent of the world’s carbon flux.

David McGuire of the University of Alaska Fairbanks and the USGS is the lead author on the paper, which reviews 265 published papers related to the Arctic and global carbon cycling. Since the end of the last Ice Age, the Arctic has oscillated between being carbon-neutral, or neither sequestering nor emitting carbon, to trapping up to 800 million metric tons of carbon from the atmosphere and the surrounding ecosystems.

This figure shows (a) the extent of permafrost in 2000, (b) the estimate extent of permafrost in 2100 and (c) b overlaid on a.
This figure shows (a) the extent of permafrost in 2000, (b) the estimate extent of permafrost in 2100 and (c) b overlaid on a.

The authors note that recent warming trends have led to a steady decrease in the extent of permafrost, or the frozen underground earth beneath the active soil. Unlike active soils, permafrost does not decompose its carbon; thus, the carbon becomes trapped in the frozen soil. Cold conditions at the surface also slow the rate of organic matter decomposition, allowing Arctic carbon accumulation to exceed its release.

The decrease in permafrost thus could tip the current balance toward the Arctic releasing instead of storing carbon. Warmer temperatures will create more active soils, accelerating decomposition and exposing layers formerly frozen in permafrost to decomposition and erosion.

And all this doesn’t even touch on the methane problem. As the Arctic thaws, the lands become more waterlogged, encouraging anaerobic metabolism, which releases methane to the air. This is particularly concerning because methane can be up to 23 times more effective as a greenhouse gas than carbon dioxide.

At this point, the carbon fate of the Arctic is far from certain, say the authors. Global warming may produce longer growing seasons and promote plant photosynthesis, removing carbon dioxide from the atmosphere. But on the other hand, increasingly dry conditions in the long term might counteract and even overcome this effect through decreased photosynthesis and increased fire prevalence.  Says McGuire:

If the response of the arctic carbon cycle to climate change results in substantial net releases of greenhouse gases, this could compromise mitigation efforts that we have in mind for controlling the carbon cycle.

Note also that this is the first review paper published by Ecological Monographs, a trend which will continue in future issues.

Read more in the ESA press release.

Author: Christine Buckley

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