New research shows that reservoirs of ocean methane in mid-latitude regions will not be released into the atmosphere under warming conditions.
Deep below the ocean surface, the seabed contains large amounts of natural ice-like deposits made up of water and concentrated methane gas. For decades, climatologists have wondered if this reservoir of methane hydrate could “melt” and release massive amounts of methane into the ocean and atmosphere as ocean temperatures warm.
New research by scientists from the University of Rochester, the US Geological Survey and the University of California at Irvine is the first to directly show that the methane released by the breakdown of hydrates is not reaching the atmosphere.
The researchers, including John Kessler, a professor in the Department of Earth and Environmental Sciences, and DongJoo Joung, a former researcher in Kessler’s lab and now an assistant professor in the Department of Oceanography at Pusan National University in Korea, conducted the study. in mid-latitude regions – subtropical and temperate zones of the Earth.
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 signature of methane hydrate being emitted to the atmosphere,” states Joung, the first author of the study. study published in Nature geoscience.
How Methane Hydrates Form, Stabilize, and Degrade
Locked in ice-like methane hydrates, methane has no effect on the climate. But released into the atmosphere, it acts as a powerful heat-trapping gas. Today’s atmosphere contains methane emitted by human activities – such as fossil fuel extraction and use, agriculture and landfills – and methane emitted naturally by wetlands, forest fires forest, aquatic environments, coastal areas and land seeps.
Ocean sediments are massive storehouses for ancient natural reservoirs of methane in the form of methane hydrates.
“The amount of methane trapped in gas hydrates globally is staggering,” says Joung.
Scientists have speculated that releasing even part of this reservoir could significantly worsen climate change.
Says Kessler: “Imagine a bubble in your aquarium going from the bottom of the aquarium to the top and exploding and releasing whatever was in that bubble into the air above – that was how a lot of people saw it. how the breakdown of hydrates could contribute to our warming of the world.
Gas hydrates form where methane and water meet under conditions of high pressure and low temperature. In parts of the ocean in the temperate and subtropical mid-latitudes, hydrates can only remain stable at depths less than about 500 meters (about 1640 feet) below the sea surface. Generally, hydrates become more stable as they are below the surface of the sea.
This means that the upper limit of stability for methane hydrates – 500 meters – is a “sweet spot”. It’s most likely to melt under warming seawater temperatures, and it’s the shortest distance a bubble of “previously hydrated” methane would have to travel before reaching the atmosphere.
But even in this sweet spot, the researchers observed no evidence of hydrous methane being released into the atmosphere.
Methane Source Fingerprint
To conduct their study, the researchers measured unique isotopic “signatures” of ocean methane in seawater samples they collected at different depths in the mid-latitude regions of the Atlantic and Pacific oceans. This allowed them to directly identify the origin of methane in seawater.
To make even a single measurement, they need an enormous amount of water – a single sample comprises around two thousand gallons of seawater. The researchers used a giant suction pipe to collect samples and used a new technique developed by their team that involves extracting methane from each sample. The researchers compressed the methane into cylinders 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 seafloor. However, they found negligible amounts of this ancient methane in surface waters. They concluded, based on previous studies, that this methane gas first dissolves in deep water and then ocean microbes biodegrade the methane, turning it into 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 to mitigate the effects of methane released during the Deepwater Horizon oil spill.
Carbon dioxide, while also a greenhouse gas, “can be incorporated into other carbon pools in seawater,” says Kessler. Although some carbon dioxide could also be emitted into the atmosphere, this would occur on much longer time scales – thousands of years – and the warming would not be as acute.
The new study builds on previous work in Kessler’s lab, which focused on methane hydrates in the Arctic Ocean. Arctic waters are another ideal spot for studying hydrates because the cold temperature means hydrates destabilize in shallower waters, where they have a short distance to travel to reach the atmosphere.
Kessler calls these results “good news,” but news that underscores the work that still needs to be done. “This tells us that to reduce the sources of methane in the atmosphere, we can focus more of our attention on mitigating human emissions,” he says.
There are very few data on methane levels in the Great Lakes, the world’s largest collection of fresh water. Earth and environmental science professor John Kessler has invited five undergraduate students and a master’s candidate to a research venture designed to change that.
Category: Scientific technology