New research shows reservoirs of ocean methane in mid-latitude regions will not be released to the atmosphere under warming conditions.
Deep below the ocean鈥檚 surface, the seafloor contains large quantities of naturally occurring, ice-like deposits made up of water and concentrated methane gas. For decades, climate scientists have wondered if this methane hydrate reservoir might 鈥渕elt鈥� and release massive amounts of methane to the ocean and the atmosphere as ocean temperatures warm.
New research from scientists at the , the US Geological Survey, and the University of California Irvine is the first to directly show that methane released from decomposing hydrates is not reaching the atmosphere.
The researchers, including , a professor in the , and DongJoo Joung, a former research scientist in Kessler鈥檚 lab and now an assistant professor in the Department of Oceanography at Pusan National University in Korea, carried out the study in mid-latitude regions鈥擡arth鈥檚 subtropical and temperate zones.
While the stability of the methane hydrate reservoir is sensitive to changes in temperature, 鈥渋n the mid-latitude regions where this study was conducted, we see no signatures of hydrate methane being emitted to the atmosphere,鈥� says Joung, the first author of , published in Nature Geoscience.
How methane hydrates form, stabilize, and degrade
Locked away in ice-like methane hydrates, methane has no effect on climate. But released into the atmosphere, it acts as a powerful, heat-trapping gas. Today鈥檚 atmosphere contains methane emitted from human activities鈥攕uch as fossil fuel extraction and use, agriculture, and landfills鈥攁nd methane emitted naturally from wetlands, wildfires, aquatic environments, and coastal zones and onshore seeps.
Ocean sediments are massive storehouses for ancient reservoirs of natural methane in the form of methane hydrates.
鈥淭he amount of methane locked up in gas hydrates globally is staggering,鈥� Joung says.
Scientists have hypothesized that the release of even part of this reservoir could significantly exacerbate climate change.
Says Kessler: 鈥淚magine a bubble in your fish tank going from the bottom of the tank to the top and exploding and releasing whatever was in that bubble to the air above it鈥攖hat was the way many people viewed how hydrate decomposition might contribute to our warming world.鈥�
Gas hydrates form where both methane and water meet at high-pressure and low-temperature conditions. In the parts of the ocean located in the temperate and subtropical mid-latitudes, hydrates can remain stable only at depths below about 500 meters (approximately 1640 feet) beneath the sea surface. Generally, hydrates become more stable the deeper they are beneath the sea surface.
That means the upper stability boundary for methane hydrates鈥�500 meters鈥攊s a 鈥渟weet spot.鈥� It is the most susceptible to melting under warming seawater temperatures, and it is the shortest distance a bubble of 鈥減reviously-hydrated” methane would have to travel before reaching the atmosphere.
But even in this sweet spot, the researchers did not observe evidence of hydrate methane being emitted to the atmosphere.
Fingerprinting the methane source
To conduct their study, the researchers measured unique isotopic 鈥渟ignatures鈥� of oceanic methane in samples of seawater they collected from various depths in the mid-latitude regions of both the Atlantic and Pacific oceans. This allowed them to directly identify the origin of methane in seawater.
To make even one measurement, they need an enormous amount of water鈥攁 single sample includes about two thousand gallons of seawater. The researchers used a giant suction hose to collect the samples and employed a novel technique their team developed that involves extracting methane from each sample. The researchers compressed the methane into cylinders that they then brought back to Kessler鈥檚 lab on the River 人妻少妇专区 to prepare for analysis.
As the researchers documented, ancient methane is being released from the seafloor. However, they found negligible amounts of this ancient methane in the surface waters. They concluded, based on earlier studies, that this methane gas first dissolves in the deeper waters and then oceanic microbes biodegrade the methane, turning it into carbon dioxide before it leaves the water.
Previous work by Kessler鈥檚 group and others found that these processes are active in the mid-latitude regions and that similar processes helped to mitigate the effects of methane released during the Deepwater Horizon oil spill.
Carbon dioxide, while also a greenhouse gas, 鈥渃an be incorporated into other carbon reservoirs in seawater,鈥� says Kessler. While some of the carbon dioxide could also be emitted into the atmosphere, it would happen over much longer time scales鈥攖housands of years鈥攁nd the warming wouldn鈥檛 be as acute.
The new study builds on previous work in Kessler鈥檚 lab, focused on methane hydrates in the Arctic Ocean. Arctic waters are another sweet spot for the study of hydrates because the cold temperature means that hydrates destabilize in shallower waters, where they have a short distance to travel to reach the atmosphere.
Kessler calls these results 鈥済ood news鈥濃�攂ut news that underscores the work that remains. 聽鈥淭his tells us that in order to reduce sources of methane to the atmosphere, we can focus more of our attention on mitigating human emissions,鈥� he says.
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