An international team probed Arctic permafrost samples to better understand the carbon feedback loop that could be set off by future thawing.

A massive store of carbon—relics of long-dead plants and other living things—is preserved within the ancient permafrost of the Arctic. Ever-increasing global temperatures, however, threaten to set off a costly chain reaction: As the Arctic permafrost, frozen solid for millennia, begins to thaw, it will release carbon from these stores.

Scientists project that up to 300 billion metric tons of Arctic carbon may be released into the atmosphere by 2100. For reference, the current atmosphere has accumulated about 250 billion metric tons of carbon from around the globe since the start of the industrial revolution, which has already been linked to changes such as increasing temperatures and extreme weather events.

To study how this vulnerable permafrost carbon store would most likely be converted into greenhouse gases (such as carbon dioxide or methane) and emitted into the atmosphere, Stubbins et al. collected water samples from a permafrost stream and other rivers in the Kolyma River Basin, the sixth-largest river in the Arctic and the largest watershed on Earth that is completely underlain by continuous permafrost.

Previous work by the researchers, hailing from the United States, Russia, and Europe, revealed that the ancient carbon stored in permafrost soils is rapidly converted to carbon dioxide by stream bacteria. The carbon dioxide is then emitted into the Earth’s atmosphere. This process creates a feedback loop, continuing to further amplify and accelerate climate change.

As ice coverage declines because of Arctic warming, Arctic waters receive more sunlight. This sunlight leads to photoreactions that can also convert river carbon to carbon dioxide. Permafrost carbon has unique chemical properties. So it was unclear whether sunlight converts permafrost carbon into carbon dioxide. To find out, the team placed water samples containing ancient permafrost carbon under a scientific sunbed. Samples were radiocarbon dated to determine if ancient permafrost carbon was converted to carbon dioxide by sunlight. Permafrost carbon chemistry was also studied.

The results showed that sunlight-driven photoreactions did change the chemistry of permafrost carbon. This chemical change may influence what happens to permafrost carbon in rivers. However, sunlight did not convert ancient permafrost carbon to carbon dioxide. Instead, the researchers concluded that biodegradation (the decomposition of organic materials via microscopic organisms, such as bacteria) is most likely the key process for converting permafrost carbon within rivers into carbon dioxide.

These findings not only improve scientific understanding of carbon cycling through Arctic ecosystems but also help shed light on the far-reaching effects of climate change. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2016JG003688, 2017)

—Sarah Witman, Freelance Writer

Correction, 5 March 2017: Due to editing errors, an earlier version of this research spotlight gave the incorrect number for the scientists’ projections of carbon release, and for the current atmosphere’s accumulation of carbon since the industrial revolution. These errors have now been fixed.

