Off Siberia’s Arctic Coast, The Seafloor Belches Methane
by: Scott K. Johnson | Published: November 30, 2013
If you can’t find the hole in a leaky bike tire, one thing you can do is stick it under water. The line of rising bubbles will lead you right to the damaged patch of rubber. You can use a similar trick if you’re trying to work out how methane is being released from thawing permafrost—you just have to look in the shallow Arctic waters off the Siberian coast.
The continental shelf here is broad, and much of it was exposed during the last ice age when sea levels were about 130 meters (nearly 430 feet) lower. As a result, the area was permafrost before it was inundated over 5,000 years ago. The Arctic Ocean is cold, to be sure, but it’s not frozen solid. That means it’s warmer than the frigid air temperatures on land, and the inundated permafrost has slowly been thawing—very slowly.
Within and below that thick layer of permafrost, there’s organic matter and methane. Some of that methane—particularly the deeper stuff—is in the form of methane hydrates, which are made of molecular cages of ice that hold onto methane. As the permafrost melts, the frozen organic matter can start to rot, generating carbon dioxide and methane. If the thaw were to reach down to the depths where methane hydrates are present, they could release their molecular prisoners too.
Thawing permafrost gets a lot of attention as a positive feedback that could amplify global warming by releasing carbon dioxide and methane, both of which are greenhouse gases. Because of this, a lot of effort goes into studying Arctic permafrost. An international group of researchers led by Natalia Shakhova at the University of Alaska Fairbanks has been plying the remote waters of the Siberian Shelf for about a decade to find out how much methane was coming up from the thawing permafrost. They didn’t expect to find it bubbling.
The researchers have discovered a number of these bubbling plumes, but it’s difficult to figure out just how important they are to the total amount of methane escaping from the Siberian Shelf. To make progress toward that end, their latest work involved surveys around the Lena River Delta to measure methane in and above the water and learn more about the bubble plumes in the area by measuring them using sonar.
Apart from being unusual, the bubbles are actually an important phenomenon. Some of the methane doesn't form large bubbles. This moves slowly through the sediment and water and is oxidized by microbes, becoming CO2, which is less potent as a greenhouse gas, molecule-for-molecule, than methane. Large, buoyant bubbles take the express route, heading straight for the atmosphere.
The methane that ends up dissolved in the bottom waters can also build up, trapped beneath the warmer surface water. By churning up the water, the storms help ventilate the deeper water, effectively emptying this methane bucket and allowing it to start refilling. However, when the team rushed out after several storms to take water samples, they found that most of that methane was already gone.
The sonar work found areas where bubbles were seeping out of the sediment all around the delta. Using their methane measurements and the density of the bubbles picked up with the sonar, the researchers revised their estimate of the total amount of methane being released from the Siberian Shelf. The number roughly doubled to 17 billion kilograms per year.
So does this mean we’ve already popped the cork on a deadly bottle of champagne? Probably not. First, it’s important to put this in perspective—17 billion kilograms represents about three percent of the methane released from natural and anthropogenic sources around the globe each year. And we know that the increase in atmospheric methane is not accelerating. In fact, its rise has slowed over the past couple decades. That could obviously change, but it does indicate that we’re not currently in the midst of some catastrophic release of methane.
Even in this area of intensive study, the picture isn't clear. We have no idea yet if the methane release from the Siberian Shelf is increasing or perfectly in line with what it’s been doing for the past few thousand years. It’s true that the warming of the water, an increase in storminess, and dwindling sea ice cover could potentially boost that release, but we don’t know if those things have already had a significant effect or not. It will take time and hard work to separate any trends from variability.
Finally, this is not the first time this region has experienced warmer temperatures. During some of the warm periods between past ice ages, it has been as warm as, or warmer than, it is today. No sudden spike in atmospheric methane shows up in climate records from those times, however. That tells us that, fortunately, it takes a pretty strong kick to awaken a methane giant.