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Fifteen years ago, Richard Davies of Newcastle University got hold of 3D images of the underwater sedimentary strata in the Atlantic Ocean off the coast of Mauritania. “I'm a geologist, so it's my equivalent of the medic’s CAT scan,” Davies says. “I had this data set, and I've had many, many students work on it. I’ve pored all over it for years.”
But for most of that time, something in the scans eluded the researchers. It wasn’t until the bountiful free time provided by the Covid lockdown that Davies dusted off the images and noticed something peculiar: pockmarks on the seafloor. There were 23 of them, each a kilometer wide (about two-thirds of a mile) and 50 meters (164 feet) deep. Such features are typically the work of the large-scale venting of gases, which fling sediment upwards and create divots. “They're well known,” says Davies. “But these are as big as they get.”
Davies researches one of the strangest natural substances on Earth: marine methane hydrate, also known as fire ice. Under sufficiently low temperatures and high pressure, methane gas and liquid water freeze together under the seafloor to create an ice-like formation that looks like marshmallow. Methane is flammable, so you can actually light a chunk of it on fire.
In a new paper published in the journal Nature Geoscience, Davies and his colleagues have fit the icy, gaseous pieces together to investigate the role that this weird substance could play in climate change. Not because it burns, but because it melts.
Marine methane hydrate occurs across the world’s oceans, including off the eastern seaboard of the United States. When it melts naturally, it “dissociates,” releasing methane that dissolves in the seawater or bubbles up to the surface. Methane is a wildly powerful greenhouse gas, 80 times as potent as carbon dioxide. Indeed, scientists speculate that methane hydrate has contributed to previous spans of global warming in Earth’s history. Around 52 million years ago, for instance, Davies says, “there's a suspicion that methane hydrate released might have caused one of the most dramatic climatic shifts the Earth's experienced.”
The pockmarks his team saw in their images were probably generated much more recently—in the past million years—due to climatic warming. Fire ice melted, releasing gas that traveled upslope in the sediment and erupted from the seafloor, creating the divots. One pocket of methane appears to have traveled 40 kilometers, or 25 miles.
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The finding suggests that far more fire ice is vulnerable to climate-induced melt than scientists realized, and it could be a significant source of planet-warming gas in the future. “It's a very, very, very large source of carbon,” says Davies. “What we're showing is there are routes for that carbon to be released that we hadn't appreciated.”
These particular pockmarks formed at a depth of 330 meters. But before Davies’ team dug into the data, no one was looking for melting fire ice at this location, because it's landward of where hydrate is stable in today’s climate, and therefore not a region of interest. At these relatively shallow depths, methane hydrate stops forming in the sediment, where temperatures are too high and pressure is too low.
“Everyone has been looking at a particular zone—around 450 to 750 meters below water depth—where hydrates are particularly vulnerable to melting,” says Davies. Hydrate is considered stable below 750 meters, where it is not likely to release methane into the ocean during climatic warming.
But things don’t always work out exactly as expected. Temperatures can actually increase deeper in the ocean, closer to the heat of the Earth itself. “Every 100 meters, it will get a bit warmer,” says Davies. “Although the pressure is increasing, the temperature is also increasing. They cross each other. And at that point is where hydrate goes from being stable to unstable.”
Davies thinks that when the oceans warmed in the past million years, fire ice that was very deep, perhaps several hundred meters below the seabed, at water depths around 1 to 2 kilometers, also warmed, destabilized—and then released gas that started to migrate upslope. As the methane traveled under the seafloor from deeper regions, it began to leak at around the 330 meter mark. “The ‘Eureka!’ moment was finding these giant craters. Due to interglacials—warm periods over the last million years—every time it melted, gas was then moving long distances up the shelf and venting,” says Davies. “I thought: Wow, [pockmarks are] forming due to hydrate dissociation in the deep water.”
Depth is an extremely important consideration when it comes to methane gas and climate, because it helps contain some of the damage. In the deepest parts of the ocean, fire ice might dissociate and burp up methane, but microbes will destroy the gas before it can reach the surface. Methane also readily dissolves in the seawater—which, yes, will result in its acidification, but at least it won’t reach the atmosphere. (Due to the same mechanics, higher carbon dioxide concentrations in the atmosphere acidify the ocean.)
But if fire ice is dissociating in deeper waters and the gas is traveling upslope through the seafloor to shallower waters, it might be able to bubble up through pockmarks and reach the atmosphere. “It is true that if you can get methane to do this migration trick and get as shallow as, say, 100 meters, you're starting to look at the possibility that some of that methane can reach the atmosphere,” says geophysicist Carolyn Ruppel, chief of the US Geological Survey's Gas Hydrates Project, who wasn’t involved in the research but did peer-review it. “It is likely probable that there are numerous places where hydrate is breaking down at profound water depth, and the gas is being channeled upslope.”
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“Indeed, this work does appear to suggest that more of the methane hydrate reservoir is susceptible to destabilization,” agrees John Kessler, who studies methane hydrates at the University of Rochester but wasn’t involved in the new research. “However, it should be highlighted that the destabilization of methane hydrates and the release of that methane to the overlying ocean does not necessarily mean that methane will vent to the atmosphere catastrophically.” The gas release could be significant over geologic timescales, Kessler says, but slow compared to humanity’s rapid, extensive release of greenhouse gases.
Still, the extra methane threatens to initiate a climatic feedback loop. As the Arctic warms up to four times faster than the rest of the planet, permafrost is thawing, releasing methane. That may lead to more warming and more thawing of permafrost. Under the sea, fire ice could do the same. It may melt, free more methane, and the climate would get warmer.
But to be very clear, while fire ice might seem ominous, the immediate—and much more fixable—threat is humanity’s continued insistence on emitting more climate-heating carbon. “If you want to get worried about something, let's focus on anthropogenic CO2 and methane,” says Ruppel. “Let's not think that this is the major problem.”
