Shallow water, heat, and methane
by Matt Owens February 1, 2013
We know that warming the methane hydrates will release them, the question is how fast and how much.
The image to the left (click for larger view) shows ocean water temperatures in the uppermost layer of the ocean by 2100, if global warming continues at its current pace. There are three large areas that are modeled to have annual temperatures rise above 7ºC. These are also areas of very shallow water (see image below, right) where the methane hydrates are most vulnerable to melting and atmospheric release.
Melting methane hydrates have not received much attention, either in the public eye or in the scientific community. This stems from a pervasive unconscious bias in outlook held by people who have experienced most of their lives under a phase of improving (or at least stable) living conditions. The same phenomena is apparent in dogs: ones that have only known beatings and deprivation cower and grimace at the slightest movement, while well-cared for dogs are usually eager to run up to complete strangers. This bias actually verges on a complete ignorance of what "very bad" actually means; and this is why reasonable scientists can conclude that a catastrophic release of methane into the atmosphere appears very unlikely, while they also acknowledge that many of the processes involved are poorly understood."
Poorly understood indeed. For instance: how ocean currents will change, how human emissions will change, how the melting/disintegrating ice caps will interact, how much methane is actually there (choices: an enormous volume, or 1oo times more than an enormous volume?), where the methane is, how vulnerable the methane is to release, and so on...
The outcome of warming oceans is like a game of Plinko. But the grand prize is global devastation. And we don't actually know how the board is set up, how it works, or much of anything about it. Because we haven't bothered to survey the ocean floor. How this equates to a "very unlikely risk" is beyond me.
Image above, right: a bathymetry map of the Arctic, click for a larger view and compare to the image of projected warming. Image from NOAA.
The shallow ocean region off the northeast tip of North America has already started warming significantly, and in the past few years, it's been much above normal, well on the its way to achieving the projected temperatures of the model. The image at right is from NOAA, showing the mixed surface layer, aka sea surface temperature (SST) anomaly (ºC) in 2010.
Images below: surface layer temperature warming by 2100 around the globe. These images were created using the GISS II global climate model, using RCP 8.5 scenario assumptions. The second image below has shading to help match the temperature key to the map image. Besides the three areas around the Arctic, the other shallow water spots projected by this model to warm rapidly are all comparatively much smaller in total area.
Background:
Methane is produced by microbes when they have no access to the oxygen (the same type we breathe: O2), either from the air or as a gas in water. This microbial production of methane can be fairly copious too. At the shore of just about any creek, pond, or lake if you use a stick to probe into the underwater sediment, methane (among other things, possibly including stinking sulfur gas) bubbles rise up. The process, biological methanogenesis, happens in most aquatic sediments, including in the ocean, and it starts at just a few centimeters below the surface. It's all this methane (CH4) that gets turned into methane hydrate in ocean sediments.
Under the pressure of ocean water, and especially at cold temperatures, methane and water combine to form a type of ice-like material, methane hydrate. Because this material only forms when both pressure and temperature are right, there exists a zone of formation and stability, also known as the gas hydrate stability zone, or GHSZ.
As more sediments accumulate on the ocean floor, the sediment layer grows thicker. Deep below the surface of the ocean floor, temperatures are much warmer, heated by the natural geothermal heat of the Earth. Once this geothermal heat becomes too hot, the GHSZ comes to a lower boundary. So as the sediments build, millimeter by millimeter at the surface, geothermal heat creeps up and melts the very deepest portion of the GHSZ.
This creeping movement upwards leads to a natural migration of the GHSZ, and the methane hydrate that decays at the very bottom edge simply migrates upward, only to be trapped again. Thus, the methane volume in the GHSZ builds over time, accumulating methane hydrate at ever increasing concentrations.
Scientists only know a small amount about methane hydrates in the oceans, and the ultimate fate of all this methane is unclear. Some volume of hydrate may get more permanently buried and transformed into natural gas where sedimentation rates are high enough and the depth of the GHSZ is large enough to slow the methane from rising before sedimentary rock can form.
One clear answer to where this hydrate goes is: back into the air. When the ocean water above the sediment surface warms, the GHSZ shrinks, but this time from the top down. The now gaseous methane naturally rises as bubbles or it diffuses into the water. In either case, the movement of the methane molecule is upwards, into the water column.
When temperatures rise slowly and the GHSZ shrinks slowly, microbes in the sediments and in the overlying water column have a chance to consume the methane, turning it into carbon dioxide (CO2). That CO2 then will stay dissolved in the oceans. The oceans and the atmosphere are in equilibrium with respect to CO2, so increasing the amount in the deep ocean will eventually lead to an increase in the atmosphere. Increasing the amount of CO2 in the shallow oceans however, will lead to a rise in atmospheric CO2 levels much faster. Methane on the other hand, if it isn't consumed by microbes, reaches the surface of the ocean very quickly. So shallow water is at a higher risk of seeing substantial methane releases to the atmosphere.
Shallow water is also where temperature can change the fastest. That's because the surface layer of the ocean, often 100 to 200 meters deep, is close to the air temperature above it, while deeper layer of ocean stay insulated at much cooler temperatures. The layers of the ocean don't often mix very thoroughly, except in a few areas of the world. Water from the top and bottom exchange places in just a few places too, and it takes a few thousand years for a complete circuit of the ocean. At least, that's how it works when climate is stable.
This all means that a rapid rise in ocean water temperature (on geologic scales of time) translates to methane escaping into the atmosphere from the oceans, and the shallowest, coldest regions of ocean are possibly the most vulnerable to rapid methane releases.
