Deep below the sea surface, the seafloor contains large amounts of naturally occurring, ice-like deposits of water and concentrated methane gas. For decades, climate scientists have wondered if this reservoir of methane hydrate could “melt” as sea temperatures rise, releasing vast amounts of methane into the ocean and atmosphere.
New research from scientists at the University of Rochester, the US Geological Survey and the University of California Irvine is the first to show directly that methane is released from decomposing hydrates Not reach the atmosphere.
The researchers, including John Kessler, a professor in the Department of Earth and Environmental Sciences, and DongJoo Joung, a former research scientist in Kessler’s lab and now an assistant professor in the Department of Oceanography at Pusan National University in Korea, conducted the study in the mid-latitude regions – the subtropical and temperate zones of the world.
While the stability of the methane hydrate reservoir is sensitive to temperature changes, “in the mid-latitude regions where this study was conducted, we see no evidence of methane hydrate being emitted to the atmosphere,” says Joung, the study’s first author nature geosciences.
How methane hydrates form, stabilize and break down
Trapped in ice-like methane hydrates, methane is climate-neutral. But released into the atmosphere, it acts as a potent, heat-retaining gas. Today’s atmosphere contains methane released by human activities – such as extraction and use of fossil fuels, agriculture, and landfills – as well as methane released naturally from wetlands, wildfires, aquatic environments and coastal areas, and onshore seeps.
Ocean sediments are vast deposits of ancient reservoirs of natural methane in the form of methane hydrates.
“The amount of methane trapped in gas hydrates around the world is staggering,” says Young.
Scientists have hypothesized that releasing even a portion of this reservoir could significantly exacerbate climate change.
Kessler says, “Imagine a bubble in your aquarium going up from the bottom of the tank and exploding, releasing whatever was in that bubble into the air above — as many people saw, like the decomposition of hydrates our warming world could contribute to this.”
Gas hydrates form where methane and water meet under high pressure and low temperature conditions. In the parts of the ocean in the temperate and subtropical mid-latitudes, hydrates can remain stable only at depths less than about 500 meters (about 1640 feet) below the sea surface. In general, hydrates become more stable the deeper they lie below the sea surface.
This means that the upper stability limit for methane hydrates – 500 meters – is a “sweet spot”. It’s most prone to melting with rising seawater temperatures, and it’s the shortest distance a bubble of “previously hydrated” methane would have to travel before reaching the atmosphere.
But even at this optimal point, the researchers saw no sign of methane hydrate being released into the atmosphere.
Methane source fingerprint
To carry out their study, the researchers measured unique isotopic ‘signatures’ of oceanic methane in seawater samples collected from different depths in the mid-latitude regions of both the Atlantic and Pacific. This enabled them to directly identify the origin of methane in seawater.
To make even one measurement, they need an enormous amount of water — a single sample contains about two thousand gallons of seawater. The researchers used a giant suction hose to collect the samples and applied a novel technique their team developed that extracts methane from each sample. The researchers compressed the methane into bottles, which they then took back to Kessler’s lab on the River Campus to prepare for analysis.
As the researchers documented, ancient methane is being released from the sea floor. However, they found negligible amounts of this old methane in the surface waters. Based on previous studies, they concluded that this methane gas first dissolves in the deeper waters and then ocean microbes biodegrade the methane and convert it to carbon dioxide before it leaves the water.
Previous work by Kessler’s group and others found that these processes are active in mid-latitude regions and that similar processes helped mitigate the effects of methane released during the Deepwater Horizon oil spill.
While carbon dioxide is also a greenhouse gas, “it can also be incorporated into other carbon stores in seawater,” says Kessler. While some of the carbon dioxide could also be emitted into the atmosphere, this would happen over much longer periods of time – thousands of years – and the warming would not be as severe.
The new study builds on previous work in Kessler’s lab that focused on methane hydrates in the Arctic Ocean. Arctic waters are another ideal location for studying hydrates, as cold temperatures cause hydrates to destabilize in shallower waters, where they have to travel a short distance to reach the atmosphere.
Kessler calls these results “good news” — but news that underscores the work that remains. “This tells us that to reduce sources of methane in the atmosphere, we can focus more on reducing human emissions,” he says.
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