Ancient Carbon Unlikely to Cause Massive Greenhouse Gas Release

As soil and ocean temperatures rise, the reservoirs have the potential to break down, releasing enormous quantities of the potent greenhouse gas methane. (Image: via   pixabay  /  CC0 1.0)
As soil and ocean temperatures rise, the reservoirs have the potential to break down, releasing enormous quantities of the potent greenhouse gas methane. (Image: via pixabay / CC0 1.0)

Permafrost in the soil and methane hydrates deep in the ocean are large reservoirs of ancient carbon. As soil and ocean temperatures rise, the reservoirs have the potential to break down, releasing enormous quantities of the potent greenhouse gas methane. But would this methane actually make it to the atmosphere?

Researchers at the University of Rochester — including Michael Dyonisius, a graduate student in the lab of Vasilii Petrenko, professor of earth and environmental sciences — and their collaborators studied methane emissions from a period in the Earth’s history partly analogous to the warming of the Earth today. Their research, published in Science, indicates that even if methane is released from these large natural stores in response to warming, very little will actually reach the atmosphere. Dyonisius said:

What are methane hydrates and permafrost?

When plants die, they decompose into carbon-based organic matter in the soil. In extremely cold conditions, the carbon in the organic matter freezes and becomes trapped instead of being emitted into the atmosphere. This forms permafrost, soil that has been continuously frozen — even during the summer — for more than one year. Permafrost is mostly found on land, mainly in Siberia, Alaska, and Northern Canada.

Researchers including Michael Dyonisius, left, drill ice cores in Antarctica. The researchers used the ice cores to determine how much of the potent greenhouse gas methane from ancient carbon deposits might be released to the atmosphere in warming conditions. (University of Rochester image / Vasilii Petrenko)

Researchers, including Michael Dyonisius, left, drill ice cores in Antarctica. The researchers used the ice cores to determine how much of the potent greenhouse gas methane from ancient carbon deposits might be released to the atmosphere in warming conditions. (Image: University of Rochester / Vasilii Petrenko)

Along with organic carbon, there is also an abundance of water ice in permafrost. When the permafrost thaws in rising temperatures, the ice melts and the underlying soil becomes waterlogged, helping to create low-oxygen conditions — the perfect environment for microbes in the soil to consume the carbon and produce methane.

Methane hydrates, on the other hand, are mostly found in ocean sediments along the continental margins. In methane hydrates, cages of water molecules trap methane molecules inside. Methane hydrates can only form under high pressures and low temperatures, so they are mainly found deep in the ocean. If ocean temperatures rise, so will the temperature of the ocean sediments where the methane hydrates are located. The hydrates will then destabilize, fall apart, and release the methane gas. Petrenko said:

Gathering climate change data from ice cores

In order to determine how much methane from ancient carbon deposits might be released to the atmosphere in warming conditions, Dyonisius and his colleagues turned to patterns in Earth’s past. They drilled and collected ice cores from Taylor Glacier in Antarctica. The ice core samples act like time capsules: They contain tiny air bubbles with small quantities of ancient air trapped inside. The researchers use a melting chamber to extract the ancient air from the bubbles and then study its chemical composition.

Dyonisius’s research focused on measuring the composition of air from the time of Earth’s last deglaciation, 8,000-15,000 years ago, saying:

A thin section of an ice core collected at Taylor Glacier in Antarctica. The ice core samples contain tiny air bubbles with small quantities of ancient air trapped inside. The researchers use a melting chamber to extract the ancient air from the bubbles and then study its chemical composition. The Rochester research focused on measuring the composition of air from the time of Earth’s last deglaciation, 8,000-15,000 years ago. This time period is a partial analog to today. (University of Rochester photo/ Vasilii Petrenko)

A thin section of an ice core collected at Taylor Glacier in Antarctica. The ice core samples contain tiny air bubbles with small quantities of ancient air trapped inside. The researchers use a melting chamber to extract the ancient air from the bubbles and then study its chemical composition. The Rochester research focused on measuring the composition of air from the time of Earth’s last deglaciation, 8,000-15,000 years ago. This time period is a partial analog to today. (Image: University of Rochester / Vasilii Petrenko)

Analyzing the carbon-14 isotope of methane in the samples, the group found that methane emissions from the ancient carbon reservoirs were small. Thus, Dyonisius concludes:

Dyonisius and his collaborators also concluded that the methane released does not reach the atmosphere in large quantities. The researchers believe this is due to several natural “buffers.”

Buffers protect against release to the atmosphere

In the case of methane hydrates, if the methane is released in the deep ocean, most of it is dissolved and oxidized by ocean microbes before it ever reaches the atmosphere. Rochester earth and environmental science professor John Kessler studies these processes. If the methane in permafrost forms deep enough in the soil, it may be oxidized by bacteria that eat the methane, or the carbon in the permafrost may never turn into methane and may instead be released as carbon dioxide. Patrenko said:

The data also shows that methane emissions from wetlands increased in response to climate change during the last deglaciation, and it is likely wetland emissions will increase as the world continues to warm today. Even so, Petrenko says:

Provided by: University of Rochester [Note: Materials may be edited for content and length.]

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