We’ve talked quite a bit about the Arctic Methane Monster — the potential that a rapidly warming Arctic will force the release of disproportionately large volumes of methane from organic material locked in permafrost and in frozen sea bed hydrates composing volumes of this powerful greenhouse gas large enough to significantly increase the pace of human-forced global warming. But if we consider the globe as a whole, the Arctic isn’t the only place where large methane stores lurk — laying in wait for the heat we’ve already added to the world’s oceans and atmosphere to trigger their release. And a new study out of the University of Washington provides yet another indication that the continental shelf off Oregon and Washington may be one of many emerging methane release hot spots.
For all around the world, and beneath the broad, blue expanse of the world’s seas, rest billions and billions of tons of frozen methane hydrate.
A kind of methane and ice combination, frozen hydrate is one of the world’s most effective natural methods of trapping and sequestering carbon. Over long ages, organic material at the bottom of the oceans decompose into hydrocarbons, often breaking down into methane gas. At high pressure and low temperature, this methane gas can be locked away in a frozen water-ice hydrate lattice, which is then often buried beneath the sea bed where it can safely remain for thousands or even millions of years.
(Plume of methane bubbles rising from the sea floor off the Oregon Coast. This image shows methane bubbles originating from the sea bed about 515 meters below the surface before dissolving into the water column at about 180 meters depth. Image source: American Geophysical Union.)
Most of these deposits lay well beneath the sea bed or at extreme ocean depths of one mile or greater. And so far, human forced warming hasn’t been great enough to risk the destabilization of most of these deep ocean carbon stores. But some hydrate deposits rest in the shallower waters of continental slope systems and at depths where current warming may now be causing them to destabilize.
Scientists Think Methane Hydrates May be Destabilizing off Oregon
Enter a new study by University of Washington scientists which found “an unusually high number of bubble plumes at the depth where methane hydrate would decompose if seawater has warmed.” The scientists concluded that these bubble plumes were likely evidence of methane hydrate destabilization due to a human forced warming of the water column in the range of about 500 meters of depth.
The warm waters, ironically, come from a region off Siberia where the deep waters have, over recent decades, been heated to unprecedented temperatures. These waters have, in turn, through ocean current exchange, circulated to the off-shore region of Washington and Oregon where they appear to have gone to work destabilizing methane hydrate in the continental slope zone. A paper published during 2014 hypothesized that these warm waters would have an impact on hydrates. And the new paper is the first potential confirmation of these earlier predictions.
In total about 168 methane plumes are now observed to be bubbling out of the sea bed off the Washington and Oregon coasts. Of these, 14 are located in the 500 meter depth range where ocean warming has pushed temperatures to levels at which hydrate could begin to destabilize. University of Washington researchers noted that the number of plumes at this depth range was disproportionately high, which also served as an indirect indicator that human heating may be causing this methane to release.
(Locations of methane plumes in the continental slope zone off Washington and Oregon. The location of a disproportionate number of these plumes in a zone now featuring a warming water column is an indication that the human-forced heating of ocean currents is starting to drive some methane hydrate structures to destabilize. Image source: AGU.)
Lead author H. Paul Johnson, a University of Washington professor of oceanography noted in AGU:
“So it is not likely to be just emitted from the sediments; this appears to be coming from the decomposition of methane that has been frozen for thousands of years… What we’re seeing is possible confirmation of what we predicted from the water temperatures: Methane hydrate appears to be decomposing and releasing a lot of gas. If you look systematically, the location on the margin where you’re getting the largest number of methane plumes per square meter, it is right at that critical depth of 500 meters.””
Implications For Ocean Health, Carbon Cycle
Most methane released at this depth never reaches the atmosphere. Instead, it either oxidizes to CO2 in the water column or is converted by ocean bacteria. That said, expanding zones of methane release can rob the surrounding ocean of vital oxygen even as it can saturate the water column with carbon — increasing ocean acidification and reducing the local ocean’s ability to draw carbon out of the atmosphere. Such a response can indirectly increase the volume of heat trapping gasses in the atmosphere by reducing the overall rate of ocean carbon uptake. In more extreme cases, methane bubbles reach the surface where they then vent directly into the atmosphere, proportionately adding to the human-produced greenhouse gasses that have already put the world into a regime of rapid warming.
It has been hypothesized that large methane releases from ocean hydrate stores contributed to past hothouse warming events and related mass extinctions like the Permian and the PETM (See A Deadly Climb From Glaciation to Hothouse). But the more immediate consequences of smaller scale releases are related to declining ocean health.
According to AGU and Dr. Johnson, the study author:
Marine microbes convert the methane into carbon dioxide, producing lower-oxygen, more-acidic conditions in the deeper offshore water, which eventually wells up along the coast and surges into coastal waterways. “Current environmental changes in Washington and Oregon are already impacting local biology and fisheries, and these changes would be amplified by the further release of methane,” Johnson said.
Instances of mass sea life die-off have already occurred at a very high frequency off the Washington and Oregon Coasts. And many of these instances have been associated with a combination of low oxygen content in the near and off shore waters, increasing ocean acidification, increasing dangerous algae blooms, and an overall warming ocean system. It’s important to note that ocean acidification, though often cited in the media, is just one of many threats to ocean life and health. In many cases, low oxygen dead zones and large microbial blooms can be even more deadly. And in the most extreme low oxygen regions, the water column can start to fill up with deadly hydrogen sulfide gas — a toxic substance that, at high enough concentrations, kills off pretty much all oxygen-based life (See Hydrogen Sulfide in the World’s Warming Oceans).
During recent years, mass sea life deaths have been linked to a ‘hot blob’ forming in nearby waters (See Mass Whale Death in Northeast Pacific — Hot Blob’s Record Algae Bloom to Blame?). However, indicators of low oxygen in the waters near Washington and Oregon have been growing in frequency since the early 2000s. Though the paper does not state this explicitly — increasing rates of methane release in the off-shore waters due to hydrate destabilization may already be contributing to declining ocean health in the region.
Slope Collapse, Conditions in Context
A final risk associated with methane hydrate destabilization in the continental slope zone is an increased prevalence of potential slope collapse. As methane hydrate releases, it can deform the sea bed structures within slope systems. Such systems become less stable, increasing the potential for large underwater landslides. Not only could these large landslides displace significant volumes of water or even set off tsunamis, slope collapse events also risk uncovering and exposing more hydrate systems to the warming ocean in a kind of amplifying feedback.
In context, the total volume of methane being released into the off-shore environment is currently estimated to be about 0.1 million metric tons each year. That’s about the same rate of hydrocarbon release seen from the Deepwater Horizon blowout. A locally large release but still rather small in size compared to the whopping 10+ billion tons of carbon being dumped into the atmosphere each year through human fossil fuel burning. However, this release is widespread, uncontrolled, un-cappable and, if scientists are correct in their indications of a human warming influence, likely to continue to increase as the oceans warm further.
Hat tip to Humortra