Old carbon reservoirs unlikely to cause large greenhouse gas release

CORVALLIS, Ore. – Carbon reservoirs found in permafrost and frozen methane hydrates have the potential to emit large quantities of methane, a potent greenhouse gas, as the planet warms, but it is unlikely the gas released from those stores will reach the atmosphere, new research published this week in Science indicates.

“The findings suggest that methane from methane hydrates and permafrost is not as vulnerable to warming as some people have thought,” said Ed Brook, a paleoclimatologist at Oregon State University and a co-author of the paper. “We need to be more concerned about direct human-caused emissions.”

Using ice core samples that function like time capsules, a group of scientists analyzed changes in the methane content of the atmosphere from 8,000 to 15,000 years ago, a period in Earth’s history that is somewhat similar to the warming underway today, to better understand where they originated.

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. This forms permafrost, a type of soil that has been continuously frozen for more than a year. Permafrost is primarily found on land in Siberia, Alaska and northern Canada.

There is also an abundance of water ice in permafrost. When it 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.

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, the temperature of the ocean sediments where the methane hydrates are located will also rise. The hydrates will then destabilize and release methane gas.

“If even a fraction of that destabilizes rapidly and that methane is transferred to the atmosphere, we would have a huge greenhouse impact because methane is such a potent greenhouse gas,” said Vasilii Petrenko of the University of Rochester, which led the study.

“The concern really has to do with releasing a truly massive amount of carbon from these stocks into the atmosphere as the climate continues to warm.”

For the study, the researchers drilled and collected ice cores from Taylor Glacier in Antarctica, then used a melting chamber to extract and study the ancient air from the bubbles trapped in the ice. They focused on the composition of air dating 8,000 to 15,000 years ago, the last major deglaciation period on Earth.

“The time period is a partial analog to today, when Earth went from a cold state to a warmer state,” said Michael Dyonisius of the University of Rochester and the paper’s lead author. “But during the last deglaciation, the change was natural. Now the change is driven by human activity, and we’re going from a warm state to an even warmer state.”

The group tested whether methane from permafrost and hydrates was released by analyzing the carbon-14 isotope of methane in the samples, which should not be present in those sources. They found that methane emissions from the ancient carbon reservoirs were small, which means the likelihood of these old carbon reservoirs destabilizing and creating a large positive warming feedback in the present day is also low.

Because the amount of carbon-14 present in the methane is very small, this work required extracting air from very large ice samples – literally one ton of ice per data point. The researchers obtained their samples from a unique location on Taylor Glacier where old ice is exposed at the surface and easily sampled.

The data also support the idea 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, Brook said.

The researchers say that if methane is released from hydrates and permafrost, it does not reach the atmosphere in large quantities. They believe this is due to several natural “buffers.”

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. 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.

“It seems like whatever natural buffers are in place are ensuring there’s not much methane that gets released,” Petrenko said.

The research was supported by the National Science Foundation Office of Polar Programs. Additional co-authors from OSU are James (Andy) Menking and Michael Kalk.

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