Now a carbon sink, Arctic could become CO2 source

The Arctic has helped absorb up to 25 per cent of the planet’s carbon dioxide, but climate change could turn it into a net emitter of the greenhouse gas, according to a new study.

If that happens, the rate of global warming — already preceding faster than scientists had predicted — could speed up even more.

Arctic land and seas have acted as a carbon sink since the end of the last Ice Age, taking up anywhere between 0 and 25 per cent of atmospheric carbon dioxide at different periods. Over time, the total amount of carbon the region has absorbed amounts to about 800 million tonnes, according to a research team led by David McGuire of the US Geological Survey and the University of Alaska at Fairbanks.

On average, the Arctic accounts for 10-15 per cent of the Earth’s carbon sink, McGuire said. But the rapid rate of climate change in the Arctic — about twice that of lower latitudes — could eliminate the sink and possibly make the Arctic a source of carbon dioxide.

Carbon generally enters the oceans and land masses of the Arctic from the atmosphere and largely accumulates in permafrost, the frozen layer of soil underneath the land’s surface. Unlike active soils, permafrost does not decompose its carbon; thus, the carbon becomes trapped in the frozen soil. Cold conditions at the surface have also slowed the rate of organic matter decomposition, allowing Arctic carbon accumulation to exceed its release.

But recent warming trends could change this balance. Warmer temperatures can accelerate the rate of surface decomposition, releasing more carbon into the atmosphere. More concerning, said McGuire, is that the permafrost has begun to thaw, exposing previously frozen soil to decomposition and erosion. These changes could reverse the historical role of the Arctic as a sink for carbon.

“In the short term, warming temperatures could expose more Arctic carbon to decomposition,” McGuire said. “And with permafrost melting, there will be more available carbon to decompose.”

On the scale of a few decades, the thawing permafrost could also result in a more waterlogged Arctic, a situation that could encourage the activity of methane-producing organisms. Currently, the Arctic is a substantial source of methane to the atmosphere: as much as 50 million metric tonnes of methane is released per year, in comparison to the 400 million metric tonnes of carbon dioxide the Arctic sequesters yearly.

A potent greenhouse gas, methane is about 23 times more effective at trapping heat than carbon dioxide on a 100-year time scale. If the release of Arctic methane accelerates, global warming could increase at much faster rates.

“We don’t understand methane very well, and its releases to the atmosphere are more episodic than the exchanges of carbon dioxide with the atmosphere,” McGuire said. “It’s important to pay attention to methane dynamics because of methane’s substantial potential to accelerate global warming.”

However, uncertainties still abound about how Arctic systems will respond to climate change. For example, the researchers write in their study, global warming could produce longer growing seasons that promote plant photosynthesis, which removes carbon dioxide from the atmosphere. Increasingly dry conditions, on the other hand, could counteract and overcome that effect. Similarly, dry conditions can lead to increased fire prevalence, releasing even more carbon.

McGuire contends that only specific regional studies can determine which areas are likely to experience changes in response to climate change.

“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,” he said.