Huge amounts of greenhouse gases lurk in the oceans, and could make warming far worse
18 December 2019 | Adaptation
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.
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).”
Source: National Geographic