Methane Hydrates: The Planet's Largest Single Carbon Sink?

Methane hydrates are perhaps the largest and most important carbon sink on the planet. Some scientific estimates place the amount of carbon stored in methane hydrates as greater than all the carbon stored in oil, natural gas and coal combined.[1] They are critical in maintaining the stability of earth's atmosphere and temperature.

What is a carbon sink? According to www.fern.org, as an example, "A carbon sink is anything that absorbs more carbon that it releases, whilst a carbon source is anything that releases more carbon than is absorbed. Forests, soils, oceans and the atmosphere all store carbon and this carbon moves between them in a continuous cycle. This constant movement of carbon means that forests act as sources or sinks at different times."[5]

Two primary carbon sinks, however, were not involved in that continuous cycling of carbon. Fossil fuel reserves (oil, natural gas and coal) and methane hydrate reserves (methane hydrates should properly be included in the categorization of fossil fuels), like the carbon locked in rocks, locked up carbon in stable reserves and took it out of the cycle. Until man started exploiting and burning fossil fuels those reserves were sinks only. We have, unfortunately, turned fossil fuels into one of the largest carbon sources on the planet. Now we are threatening to do the same with methane.

As recently as 1971, in fact, methane was not even on the radar as an important greenhouse gas. According to the report, Methane: A Scientific Journey from Obscurity to Climate Super-Stardom, "The first survey in 1971 on the possibility of inadvertent human modification of climate stated that "Methane has no direct effects on the climate or the biosphere [and] it is considered to be of no importance". The gas did not even appear in the index of the major climatology book of the time (Lamb's Climate Past, Present and Future)."[3]

As a result the study of methane hydrates is still very much in its infancy. Most of the research to date, in fact, has focused on the potential of using the methane in those hydrates as an energy source in light of the approaching peak and decline in oil and other fossil fuels. There has been little attention and little funding available for studying methane as a greenhouse gas and as a potential contributor to global warming, even its potential as a catalyst in a runaway greenhouse effect.

Why is all of that important? How serious a greenhouse gas is methane? Methane, when first released into the atmosphere is 62 times more potent as a greenhouse gas than carbon dioxide. However, it has a much shorter lifespan in the atmosphere. It quickly diminishes in potency to about 20 times that of carbon dioxide and will completely oxidize after about twenty years. But that's not the end of it's importance as a greenhouse gas. Methane in the upper atmosphere oxidizes into carbon dioxide and water vapour (also an important greenhouse gas) and will remain in the upper atmosphere as carbon dioxide for another hundred years. So it has a very potent early life as a greenhouse gas but also a long term life cycle as both reduced potency methane gas and then carbon dioxide.

One of the troubling aspects of methane hydrates (much more on this later) is that the methane in the hydrate is in gaseous form and under pressure. Where compressed natural gas (CNG) is artificially compressed and stored in steel cylinders or other containment vessels at pressures of 200-248 atmospheres,[6] the methane gas in methane hydrates is naturally present at a pressure of 162 atmospheres in a cage of ice.[4] Anyone who has ever seen a gas cylinder explode knows how explosive gases under pressure can be with a sudden release of that pressure.

Keith Bennett, a reader of my blog from the UK, recently sent me an e-mail in which he reminded me, "every time we have messed with nature we have found that we harm the ‘delicate balance’." This is what has bothered me with the increasing talk of exploiting methane hydrates as an energy source. We have already drastically impacted the other primary carbon sinks on this planet; cutting and burning the forests, dredging up and burning the fossil fuel reservoirs, destroying the carbon sequestration ability of our soils, saturating the oceans and diminishing their ability to absorb and sequester carbon dioxide, drastically changing the makeup of the atmosphere. We keep transferring the planet's carbon from stable sinks and reservoirs into the comparatively unstable atmosphere as carbon dioxide by burning massive volumes of fossil fuels. To date, methane hydrates were the last major carbon sink that we had not destroyed, a shortcoming we seem to be hell bent to rectify.

Ignorance may have been a legitimate excuse when we began the process of destroying the other important carbon sinks. We just did not realize the impact we were having. But we have now known for many decades and still continue to inflict damage on this planet's environment through our misuse and abuse of the carbon cycle. To now, with all that we have learned, head into the destruction of the last major carbon sink in the pursuit of more energy is to do so with no remaining excuse of ignorance to use. There is ignorance, but not such as to justify going forward. We simply do not know how important methane hydrates are as a carbon sink. We don't know what impact on the future livability of this planet we will have by exploiting methane hydrates and diminishing those reserves. We do know, as Keith Bennett suggested, that every time we have thus far "messed with" nature we have harmed the delicate balance that has evolved over millions and billions of years on this planet.

If we take the same approach with methane hydrates that we have taken with the exploitation of the other fossil fuels we most assuredly will further upset, if not destroy, that delicate balance. With fossil fuels, at every turn, we have leaned in favour of exploiting the energy resource rather than protecting the environment, both for ourselves and for future generations. Keep the wheels of industry rolling today at whatever cost to tomorrow.

Marine methane hydrate reserves are relatively stable but remain so within a fairly narrow range of temperature and pressure known as the Hydrate stability zone. In my article (also in the blog), The real problem with Methane Hydrates is Sliding under the Radar, I dealt with this issue at length. Here is an excerpt but I would seriously encourage you to read the whole article. "The physical nature of methane hydrates and the quite distinct physical properties of water - specifically H2O - and of methane (CH4) independently function both as a barrier to exploitation and as a serious environmental risk in conjunction with global warming. ..... H2O which is only water above 0C [at 1 atmosphere] and becomes vapour at higher temperatures - reaches its maximum density of 999.9720 kilograms per cubic meter at a temperature of 3.98C. At the freezing temperature of 0C its density has reduced to 998.8395 kilograms per cubic meter, 988.1170 at -10C. The critical part of that range, with regard to methane hydrates, is that from 0C to 3.98C. ..... The lower density of H2O as ice (998.8395) at 0C (even lower if the ice is super cooled) is what allows ice to float on the surface of water. Average global ocean temperatures today (this has varied over geological time, especially during different eras of ice age and global warming) is 2C. At 2C H2O has a density 999.9400, between that of ice at 0C of 998.8395 and the maximum density at 3.98C of 999.9720. It still supports, therefore, the lighter ice even in the Arctic. ..... Because of the lower density (greater buoyancy) of ice relative to sea water, submarine methane hydrates are always under pressure, physically wanting to rise to the surface. The [hydrate] deposits only become "relatively" stable when anchored by sufficient sediment on the ocean bottom. When and if that "anchorage" breaks down or is swept away, for example, by a sub-surface landslide, the hydrates can suddenly be released into the water and rise toward the surface. ..... The density of the gaseous methane in hydrates is 162 times greater than methane gas in the atmosphere. At the temperature and pressure of the sea water around and above the hydrate deposits, the methane gas contained in the hydrates should have much lower density (occupy much more space) than it does. This physical anomaly means that the pressure on the methane gas to expand is constantly at odds with and pushing against the ice cage enclosing it. This is a key component of the essential instability of methane hydrates. ..... Gas density generally decreases far more rapidly for gases than liquids or solids as temperature rises or pressure decreases. That means two factors can affect the stability of methane hydrates currently in the hydrate stability zone. Changes in sea level can affect the water pressure in the zone: a drop in sea level can decrease the pressure. Changes in temperature of the water can have the same effect. Increase of the temperature above the current average 2C can also dramatically affect that stability."

In view of the threat of global warming, the potential impact from rising sea temperatures warrants particular attention. As the temperature of the hydrate deposit rises two opposing things begin to happen. The ice cage around the methane shrinks, further increasing the pressure on the methane gas inside, similar to squeezing a cylinder containing a gas. This increases the tendency of the gas to seek escape from the containment. At the same time the ice cage containing the methane is softening and weakening, making it more susceptible to rupture. This increases the probability that the submarine methane hydrate deposits will destabilize and that they will do so explosively.

There is now considerable accepted scientific evidence that this has happened several times in the geological past,[10] most notably 55 million years ago, as per NASA.[11] Of more immediate concern, however, is the growing evidence that there is a measurable and significant increase in methane venting from hydrate deposits on the Arctic sea floor.[7][8] The temperatures in the Arctic have been increasing much more rapidly over the past century than elsewhere on earth. In fact, atmospheric methane concentrations have more than doubled over the past 200 years "due to decomposing organic materials in wetlands and swamps and human aided emissions from gas pipelines, coal mining, increases in irrigation and livestock flatulence."[11] The Arctic is a kind of canary in the coal mine when it comes to showing the early signs of global warming. The concern over Arctic sub sea methane venting is doubled when considering the potential positive feedback on releasing the massive amount of methane hydrates trapped in Arctic permafrost, both in northern North America and Europe/Asia. Large areas of Arctic permafrost coastline are, quite literally, oozing into the ocean and releasing their sequestered methane.
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1) Methane hydrate - A major reservoir of carbon in the shallow geosphere?
2) Siberian Peatlands a Net Carbon Sink and Global Methane Source Since the Early Holocene
3) Methane: A Scientific Journey from Obscurity to Climate Super-Stardom
4) The real problem with Methane Hydrates is Sliding under the Radar
5) WHAT ARE CARBON SINKS?
6) Compressed natural gas
7) JGR/MIT Study - Subsea Methane Clathrates May Already Be Venting Far More Quickly Than Projected
8) Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf
http://www.sciencemag.org/cgi/content/abstract/327/5970/1246
9) Computer simulation strengthens link between climate change and release of subsea methane
10) Explosive methane venting at hydrate/gas transition in the bedrock
11) METHANE EXPLOSION WARMED THE PREHISTORIC EARTH, POSSIBLE AGAIN