How much CO2 will melting permafrost release?

Highway to the West Fjords region of Iceland
The warming of the world’s tundra and permafrost areas will have a huge effect on global climate. @bberwyn photo.

New study shows soil moisture is a big factor in global warming equation

Staff Report

Methane won’t be the only problem as Arctic permafrost thaws in the coming decades. A new study shows that, as frozen permafrost areas warm and dry out, they will also release more CO2. The study was led by Northern Arizona University assistant research professor Christina Schädel and published in the journal Nature Climate Change.

The findings show that a 10 degree Celsius increase in soil temperature released twice as much carbon into the atmosphere, and drier, aerobic soil conditions released more than three times more carbon than wetter, anaerobic soil conditions.

“Our results show that increasing temperatures have a large effect on carbon release from permafrost but that changes in soil moisture conditions have an even greater effect,” said Schädel. “We conclude that the permafrost carbon feedback will be stronger when a larger percentage of the permafrost zone undergoes thaw in a dry and oxygen-rich environment.”

The study was part of an ongoing effort to quantify greenhouse gas releases from thawing permafrost, a critical part of the global warming equation because so much carbon is currently locked up in frozen organic soils in the Arctic. Much of the data came from the international Permafrost Carbon Network that Schädel co-leads with Northern Arizona University professor of ecosystem ecology, Ted Schuur.

As permafrost thaws, microbes wake up and begin digesting the newly available remains of ancient plants and animals stored as carbon in the soil. This digestion produces either carbon dioxide or methane, depending on soil conditions.

Scientists want to understand the ratio of carbon dioxide to methane gas released by this process because it affects the strength of the permafrost carbon feedback loop: greenhouse gases released due to thawing permafrost cause temperatures to rise, leading to even more thawing and carbon release. Furthermore, the Arctic permafrost is like a vast underground storage tank of carbon, holding almost twice as much as the atmosphere. At that scale, small changes in how the carbon is released will have big effects.

The new study looked at how soil temperature and the availability of oxygen affect that process. Soils in the lab were incubated at a range of warmer temperatures projected for the future. The availability of oxygen is important because it determines how microbes digest carbon. Oxygen-rich, or aerobic, conditions are found in dry soils and produce carbon dioxide. Oxygen-poor, or anaerobic, conditions are found in wet soils and produce both carbon dioxide and methane. Lab incubations mimicked these two conditions.

Scientists don’t know for sure if wet or dry soils will dominate in the future, but the new findings will help climate models make more accurate projections and  highlights the need to monitor changes in wetness associated with permafrost thaw, changes that ultimately sculpt the topography of waterlogged depressions and dry uplands across the Arctic landscape.

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