Scientific Method / Science & Exploration

Methane release around Arctic islands predates recent climate change

Areas where methane bubbles up from seafloor have been around for centuries.

Precipitated carbonate at one of the sites of methane seepage.

The earth sciences have described a number of processes that are decidedly unpleasant for any living organisms unlucky enough to experience them, from volcanic eruptions to mass extinctions. Climatic “tipping points” are one of those scary processes. You can think of them along the lines of a kid who accidentally crests a steep hill on an old bike with no brakes—he won’t be stopping before the bottom of the hill (or he wipes out, whichever comes first). Crossing a tipping point in the climate system means being locked into some magnitude of climate change with no chance of halting it.

One scary tipping point involves the methane locked in molecular cages of ice called clathrates (or methane hydrates), found within the sediment on the seafloor. Heating up the water enough to melt hydrates and release methane adds an additional punch of greenhouse warming to whatever caused the heating. Such heating has been implicated in some ugly climate swings in Earth’s past.

Climate scientists don’t anticipate crossing this tipping point any time soon (the latest IPCC report judged it “very unlikely” this century), but that doesn’t mean hydrates aren’t the focus of research and concern. There is still much we don’t know about where hydrates are present and how much ocean warming it takes to begin destabilizing them.

Recent discoveries of areas in which methane is literally bubbling up from the seafloor around the Arctic have stoked worries that we could be closer to hitting this tipping point than we thought. But since we’ve only been observing them for such a short time, there’s been no way to know if those bubble plumes are new and worrisome or just the millennia-old status quo. A new paper in Science provides a little information about the history of the methane bubbling up around the Arctic islands of Svalbard, north of Scandinavia.

As organic matter in the sediments around Svalbard decays, methane is produced. Where the pressure is great enough (because of the water depth) and the temperature is cold enough, methane hydrates can form. At the shallowest edge of that zone where hydrates can exist, it doesn’t take much of a change to make them unstable. Because of the currents around Svalbard, the water there has warmed more rapidly than most of the rest of the Arctic Ocean—bottom water temperatures have risen about one degree Celsius over the past 30 years.

Plumes of rising methane bubbles have been mapped off the coast of Svalbard to where the water is about 400 meters deep—the edge of the stability zone for hydrates. In order to find out if these plumes are the result of that recent warming or are simply a feature of the area, a team of researchers led by Christian Berndt of Germany’s GEOMAR Helmholtz Centre for Ocean Research Kiel used a submersible to get a look at the seafloor where the methane is bubbling up.

There, they found crusts of calcium carbonate formed by bacteria living off the methane. (In fact, there were communities of chemosynthetic bacteria and a kind of tubeworm living at all the methane seeps they visited.) The age of geologically recent, precipitated carbonate in the ocean can be measured using radioactive isotopes of uranium and thorium, so these crusts provided a record of how long methane had been bubbling up at these spots.

The carbonate turned out to be much older than anthropogenic climate change, with measurements dating them anywhere from 500 years to over 8,000 years old. That means that, at least in the locations they sampled, methane has been bubbling for quite a long time.

The researchers also made measurements of seasonal water temperature variation and the ability of the sediment to conduct heat, which they used to create a model of the study area. The model showed that there should be a seasonal cycle in the behavior of the shallow-water hydrates just below the seafloor, with some additional hydrates forming while the water temperature is cooler and then melting when the water is warmer. That process could affect the total rate of methane coming up by clearing out pathways to the surface during the warmer part of the year.

The study allays concerns that these bubbling plumes of methane around Svalbard are a brand-new phenomenon triggered by global warming, but it’s still unknown if the rate of bubbling is changing. The researchers summarize their work by writing that “observations of large contemporary emissions reported in other studies cannot be considered proof of accelerating hydrate destabilization, although neither do they prove that catastrophic destabilization is not accelerating.” Figuring that out will simply take continued monitoring and a better understanding of the conditions on the Arctic seafloor.

Science, 2014. DOI: 10.1126/science.1246298  (About DOIs).

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