Huge amounts of greenhouse gases lurk in the oceans, and could make warming far worse

Scientists are finding hidden climate time bombs—vast reservoirs of carbon dioxide and methane—scattered under the seafloor across the planet.

And the fuses are burning.

Caps of frozen CO₂ or methane, called hydrates, contain the potent greenhouse gases, keeping them from escaping into the ocean and atmosphere. But the ocean is warming as carbon emissions continue to rise, and scientists say the temperature of the seawater surrounding some hydrate caps is within a few degrees of dissolving them.

That could be very, very bad. Carbon dioxide is the most common greenhouse gas, responsible for about three-quarters of emissions. It can remain in the atmosphere for thousands of years. Methane, the main component of natural gas, doesn't stay in the atmosphere as long as CO₂—about 12 years—but it is at least 84 times more potent over two decades.

The oceans absorb a third of humanity’s carbon dioxide emissions and 90 percent of the excess heat generated by increased greenhouse gas emissions; it’s the largest carbon sink on the planet. If warming seas melt hydrate caps, there’s a danger that the oceans will become big carbon emitters instead, with grave consequences for climate change and sea level rise.

“If that hydrate becomes unstable, in fact melts, that enormous volume of CO₂ will be released to the ocean and eventually the atmosphere,” says Lowell Stott, a paleoceanographer at the University of Southern California.

The discovery of these deep ocean CO₂ reservoirs, as well as methane seeps closer to shore, comes as leading scientists warned this month that the world is now surpassing a number of climate tipping points, with ocean temperatures at record highs.

The few CO₂ reservoirs that have been found so far are located adjacent to hydrothermal vent fields in the deep ocean. But the global extent of such reservoirs remains unknown.

“It's a harbinger, if you will, of an area of research that is really important for us to investigate, to find out how many of these kinds of reservoirs are out there, how big they are, and how susceptible they are to releasing CO₂ to the ocean,” Stott says. “We have totally underestimated the world’s total carbon budget, which has profound implications.”

Jeffrey Seewald, a senior scientist at Woods Hole Oceanographic Institution who studies the geochemistry of hydrothermal systems, questioned the magnitude of hydrate-capped reservoirs.

“I don't know how globally significant they are as most hydrothermal systems that we know of are not associated with large accumulations of carbon, though there’s still a lot to be explored,” he says. “So I would be a little careful about suggesting that there are significant accumulations of CO₂ that are just waiting to be released.”

Hydrothermal vent scientist Verena Tunnicliffe of the University of Victoria in Canada notes that data has been collected at just 45 percent of known hydrothermal sites and most are not well surveyed.

A threat closer to home

Other scientists are far more concerned about potential climate time bombs much closer to home—methane hydrates that form on the shallower seafloor at the margins of continents.

For one thing, there apparently are a lot of them. Between 2016 and 2018, for instance, researchers at Oregon State University and the National Oceanic and Atmospheric Administration (NOAA) deployed a new sonar technique to discover 1,000 methane seeps off the Pacific Northwest coast of the United States.

In contrast, just 100 had been identified between 2015 and the late 1980s, when scientists first stumbled across methane deposits. There are likely many more to be located, given that as of 2018, researchers only had mapped 38 percent of the seafloor between Washington State and Northern California.

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“Because a lot of methane is stored on the continental margins in relatively shallow water, the effects of ocean warming will get to it sooner and potentially destabilize the methane hydrates that are present in the sediment,” says Dave Butterfield, a senior research scientist and hydrothermal vent expert at NOAA’s Pacific Marine Environmental Laboratory in Seattle.

He noted that these methane seeps likely constitute a far larger global reservoir of greenhouse gases than pools of carbon dioxide under the deep ocean floor.

“This idea is that if you destabilize the methane hydrates, that methane would be injected into the atmosphere and cause more extreme global warming,” says Butterfield, who in 2003 was part of an expedition that discovered a hydrate-capped reservoir of liquid CO₂ at a hydrothermal system on the Mariana Arc in the Pacific.

Stott and colleagues earlier this year published a paper presenting evidence that the release of carbon dioxide from hydrothermal seafloor reservoirs in the eastern equatorial Pacific some 20,000 years ago helped trigger the end of the last glacial era. And in a new paper, Stott finds geological indications that during the end of Pleistocene glaciations, carbon dioxide was released from seafloor reservoirs near New Zealand.

The spike of atmospheric temperatures during previous periods when ice ages were ending mirrors today’s rapid rise as a result of greenhouse gas emissions. While the oceans have long been suspected as significant contributors to ancient global warming, the prevailing consensus was that the CO₂ was released from a layer of water resting deep in the ocean. But research from Stott and other oceanographers over the past decade points to a geological culprit.

“Even if only a small percentage of the unsampled hydrothermal systems contain separate gas or liquid CO₂ phases it could change the global marine carbon budget substantially,” Stott and his co-authors write of present-day carbon reservoirs.

Like a needle in a haystack

Take the hydrate-capped liquid CO₂ reservoir found by Butterfield and his colleagues on a volcano in the Pacific. They calculated that the rate that liquid CO₂ bubbles were escaping the seafloor equaled 0.1 percent of the carbon dioxide emitted on the entire Mid-Ocean Ridge. That may seem like a small amount, but consider that the CO₂ is escaping from a single, small site along a 40,390 mile-long system of submerged volcanoes that rings the planet.

“That's an astonishing number,” says Stott.

Scientists believe such reservoirs can be formed when volcanic magma deep beneath the ocean floor interacts with seawater to produce superheated fluids rich in carbon or methane that rise toward the surface. When that plume collides with cooler water, an ice-like hydrate forms that traps the carbon or methane in subsurface sediments.

The risk the reservoirs pose depends on their location and depth. For example, rising ocean temperatures could in coming years melt a hydrate capping a lake of liquid CO₂ in the Okinawa Trough west of Japan, according to Stott. But the absence of upwelling currents there means a mass release of carbon dioxide at a depth of 4,600 feet would likely acidify the surrounding waters but not enter the atmosphere for an extremely long time.

Stott notes that finding CO₂ and methane reservoirs in the deep ocean is a “needle and haystack situation.”

But in a paper published in August, scientists from Japan and Indonesia revealed that they had detected five large and previously unknown CO₂ or methane gas reservoirs under the seafloor in the Okinawa Trough by analyzing seismic pressure waves generated by an acoustical device. Since those waves travel more slowly through gas than solids under the seafloor, the researchers were able to locate the reservoirs. The data indicates that hydrates are trapping the gas.

“Our survey area is not broad, so there could be more reservoirs outside of our survey area,” Takeshi Tsuji, a professor of exploration geophysics at Kyushu University in Japan and a co-author of the paper, says in an email.

“Methane or CO₂ in this environment is not stable, because of intensive hydrothermal activities in the axis of the Okinawa Trough. Therefore, the CO₂ or methane could be leaked to seafloor (and atmosphere).”