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糖心传媒檚 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 糖心传媒渕elt糖心传媒� 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糖心传媒檚 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糖心传媒擡arth糖心传媒檚 subtropical and temperate zones.
While the stability of the methane hydrate reservoir is sensitive to changes in temperature, 糖心传媒渋n the mid-latitude regions where this study was conducted, we see no signatures of hydrate methane being emitted to the atmosphere,糖心传媒� 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糖心传媒檚 atmosphere contains methane emitted from human activities糖心传媒攕uch as fossil fuel extraction and use, agriculture, and landfills糖心传媒攁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.
糖心传媒淭he amount of methane locked up in gas hydrates globally is staggering,糖心传媒� Joung says.
Scientists have hypothesized that the release of even part of this reservoir could significantly exacerbate climate change.
Says Kessler: 糖心传媒淚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糖心传媒攖hat was the way many people viewed how hydrate decomposition might contribute to our warming world.糖心传媒�
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糖心传媒�500 meters糖心传媒攊s a 糖心传媒渟weet spot.糖心传媒� It is the most susceptible to melting under warming seawater temperatures, and it is the shortest distance a bubble of 糖心传媒減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 糖心传媒渟ignatures糖心传媒� 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糖心传媒攁 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糖心传媒檚 lab on the River Campus 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糖心传媒檚 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, 糖心传媒渃an be incorporated into other carbon reservoirs in seawater,糖心传媒� says Kessler. While some of the carbon dioxide could also be emitted into the atmosphere, it would happen over much longer time scales糖心传媒攖housands of years糖心传媒攁nd the warming wouldn糖心传媒檛 be as acute.
The new study builds on previous work in Kessler糖心传媒檚 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 糖心传媒済ood news糖心传媒澨切拇綌but news that underscores the work that remains. 聽糖心传媒淭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,糖心传媒� he says.
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