Methane release and Runaway Climate Change
Paleocene–Eocene Thermal Maximum and Clathrates
At certain temperature and pressure conditions, methane – which is being produced continually by decomposing microbes in sea bottom sediments – is stable in a complex with water, which forms ice-like cages trapping the methane in solid form. As temperature rises, the pressure required to keep this clathrate configuration stable increases, so shallow clathrates dissociate, releasing methane gas to make its way into the atmosphere. Since biogenic clathrates have a δ13C signature of −60 ‰ (inorganic clathrates are the still rather large −40 ‰), relatively small masses can produce large δ13C excursions. Further, methane is a potent greenhouse gas as it is released into the atmosphere, so it causes warming, and as the ocean transports this warmth to the bottom sediments, it destabilises more clathrates. This can take place over a short period as a few years with initial spikes and for a duration of a few thousand years. The reverse process, that of fixing methane in clathrates, occurs over a larger scale of tens of thousands of years.
In order for the clathrate hypothesis to work, the oceans must show signs of having been warmer slightly before the carbon isotope excursion, because it would take some time for the methane to become mixed into the system and δ13C-reduced carbon to be returned to the deep ocean sedimentary record. Recent (2002) work has managed to detect a short gap between the initial warming and the δ13C excursion. Chemical markers of surface temperature (TEX86) also indicate that warming occurred around 3,000 years before the carbon isotope excursion, but this does not seem to hold true for all cores. Notably, deeper (non-surface) waters do not appear to display evidence of this time gap.
Analysis of these records reveals another interesting fact: planktonic (floating) forams record the shift to lighter isotope values earlier than benthic (bottom dwelling) forams. The lighter (lower δ13C) methanogenic carbon can only be incorporated into the forams’ shells after it has been oxidised. A gradual release of the gas would allow it to be oxidised in the deep ocean, which would make benthic forams show lighter values earlier. The fact that the planktonic forams are the first to show the signal suggests that the methane was released so rapidly that its oxidation used up all the oxygen at depth in the water column, allowing some methane to reach the atmosphere unoxidised, where atmospheric oxygen would react with it.
The present-day global methane hydrate reserve is poorly constrained, but mostly considered between 2,000 ~ 10,000 Gt. However, because the global ocean bottom temperatures were ~6 degree C higher than today which induces much smaller volume of sediment hosting gas hydrate than today, global hydrate amount before PETM was thought much less than present-day estimates. So many scientists called the source of carbon for PETM as a mystery. However, a recent paper using numerical simulations suggests that enhanced organic carbon sedimentation and methanogenesis could have compensated for the smaller volume of hydrate stability. Source PETM
In this sense there is recent work done to explain further methane uptake.
Inland waters take in organic carbon and emit methane
Extreme organic carbon burial fuels intense methane bubbling in a temperate reservoir – Sobek et al. (2012)
Abstract: “Organic carbon (OC) burial and greenhouse gas emission of inland waters plays an increasingly evident role in the carbon balance of the continents, and particularly young reservoirs in the tropics emit methane (CH4) at high rates. Here we show that an old, temperate reservoir acts simultaneously as a strong OC sink and CH4 source, because the high sedimentation rate supplies reactive organic matter to deep, anoxic sediment strata, fuelling methanogenesis and gas bubble emission (ebullition) of CH4 from the sediment. Damming of the river has resulted in the build-up of highly methanogenic sediments under a shallow water column, facilitating the transformation of fixed CO2 to atmospheric CH4. Similar high OC burial and CH4 ebullition is expected in other reservoirs and natural river deltas.” Source AGU
Todays atmospheric Methane growth
Source AIRS asl.umbc.edu
From the Jenkinson effect to the compost-bomb instability and Clathrate Gun
The clathrate gun runaway effect may be used to describe more rapid methane releases. Methane in the atmosphere has a high global warming potential, but breaks down relatively quickly to form CO2, which is also a greenhouse gas. Therefore, slow methane release will have the long-term effect of adding CO2 to the atmosphere.
In order to model clathrates and other reservoirs of greenhouse gases and their precursors, global climate models would have to be ‘coupled’ to a carbon cycle model. Most climate models do not include such modelling of methane deposits.
A 2006 book chapter by Cox et al. considers the possibility of a future runaway climate feedback due to changes in the land carbon cycle:
Here we use a simple land carbon balance model to analyse the conditions required for a land sink-to-source transition, and address the question; could the land carbon cycle lead to a runaway climate feedback? [...] The simple land carbon balance model has effective parameters representing the sensitivities of climate and photosynthesis to CO2, and the sensitivities of soil respiration and photosynthesis to temperature. This model is used to show that (a) a carbon sink-to-source transition is inevitable beyond some finite critical CO2 concentration provided a few simple conditions are satisfied, (b) the value of the critical CO2 concentration is poorly known due to uncertainties in land carbon cycle parameters and especially in the climate sensitivity to CO2, and (c) that a true runaway land carbon-climate feedback (or linear instability) in the future is unlikely given that the land masses are currently acting as a carbon sink.
Soil carbon and climate change: from the Jenkinson effect to the compost-bomb instability
More recently there has been a suggestion that the release of heat associated with soil decomposition, which is neglected in the vast majority of large-scale models, may be critically important under certain circumstances. Models with and without the extra self-heating from microbial respiration have been shown to yield significantly different results. The present paper presents a mathematical analysis of a tipping point or runaway feedback that can arise when the heat from microbial respiration is generated more rapidly than it can escape from the soil to the atmosphere. This ‘compost-bomb instability’ is most likely to occur in drying organic soils with high porosity covered by an insulating lichen or moss layer. However, the instability is also found to be strongly dependent on the rate of global warming. This paper derives the conditions required to trigger the compost-bomb instability, and discusses the relevance of these to the concept of dangerous rates of climate change. On the basis of simple numerical experiments, rates of long-term warming equivalent to 10°C per century could be sufficient to trigger compost-bomb instability in drying organic soils.
NSF issues world a wake-up call: “Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming.”
It is increasingly clear that if the world strays significantly above 450 ppm atmospheric concentrations of carbon dioxide for any length of time, we will find it unimaginably difficult to stop short of 800 to 1000 ppm.
A report released by the U.S. Global Change Research Program, Abrupt Climate Change, said in December 2008 (during the Bush Administration) that warming in the Arctic could cause sea levels to rise substantially beyond scientists’ previous predictions and could result in massive releases of methane. The report said that the “rapid release to the atmosphere of methane trapped in permafrost and on continental margins” was among “four types of abrupt change in the paleoclimatic record that stand out as being so rapid and large in their impact that if they were to recur, they would pose clear risks to society in terms of our ability to adapt.” Source Science stunner: Vast East Siberian Arctic Shelf methane stores destabilizing and venting
Very rapid and massive release of carbon deficient in ˆ‚13C, does put one in mind of the Methane Gun hypothesis. It postulates that methane clathrate at shallow depth begins melting and through the feed-back process accelerate atmospheric and oceanic warming, melting even larger and deeper clathrate deposits. The result: A relatively sudden massive venting of methane – the firing of the Methane Gun. Recent discovery by Davy et al (2010) of kilometer-wide (ten 8-11 kilometer and about 1,000 1-kilometer-wide features) eruption craters on the Chatham Rise seafloor off New Zealand adds further ammunition to the Methane Gun hypothesis.
It has been known for many years that methane is being emitted from Siberian swamplands hitherto covered by permafrost, trapping an estimated 1,000 billion tons of methane. Permafrost on land is now seasonally melting and with each season melting it at greater depth, ensuring that each year methane venting from this source increases.
Methane clathrate has accumulated over the East Siberian continental shelf where it is covered by sediment and seawater up to 50 meters deep. An estimated 1,400 billion tons of methane is stored in these deposits. By comparison, total human greenhouse gas emissions (including CO2) since 1750 amount to some 350 billion tons.
Significant methane release can occur when on-shore permafrost is thawed by a warmer atmosphere (unlikely to occur in significance on less than a century timescale) and undersea clathrate at relatively shallow depths is melted by warming water. This is now occurring. In both cases, methane gas bubbles to the surface with little or no oxidation, entering the atmosphere as CH4 – a powerful greenhouse gas which increases local, then Arctic atmospheric and ocean temperature, resulting in progressively deeper and larger deposits of clathrate melting.
Methane released from deeper deposits such as those found off Svalbard has to pass through a much higher water column (>300 meters) before reaching the surface. As it does so, it oxidises to CO2, dissolving in seawater or reaching the atmosphere as CO2 which causes far slower warming, but can nevertheless contribute to ocean acidification. Source The methane hydrate feedback revisited
The sudden release of large amounts of natural gas from methane clathrate deposits in runaway climate change could be a cause of past, future, and present climate changes. The release of this trapped methane is a potential major outcome of a rise in temperature; it is thought that this is a main factor in the global warming of 6°C that happened during the end-Permian extinction as methane is much more powerful as a greenhouse gas than carbon dioxide (despite its atmospheric lifetime of around 12 years, it has a global warming potential of 72 over 20 years and 25 over 100 years). The theory also predicts this will greatly affect available oxygen content of the atmosphere.
Focusing on the Permian-Triassic boundary, Gregory Ryskin  explores the possibility that mass extinction can be caused by an extremely fast, explosive release of dissolved methane (and other dissolved gases such as carbon dioxide and hydrogen sulfide) that accumulated in the oceanic water masses prone to stagnation and anoxia (e.g., in silled basins).
The consequences of a methane-driven oceanic eruption for marine and terrestrial life are likely to be catastrophic. Figuratively speaking, the erupting region “boils over,” ejecting a large amount of methane and other gases (e.g., CO2, H2S) into the atmosphere, and flooding large areas of land. Whereas pure methane is lighter than air, methane loaded with water droplets is much heavier, and thus spreads over the land, mixing with air in the process (and losing water as rain). The air-methane mixture is explosive at methane concentrations between 5% and 15%; as such mixtures form in different locations near the ground and are ignited by lightning, explosions and conflagrations destroy most of the terrestrial life, and also produce great amounts of smoke and of carbon dioxide. Firestorms carry smoke and dust into the upper atmosphere, where they may remain for several years; the resulting darkness and global cooling may provide an additional kill mechanism. Conversely, carbon dioxide and the remaining methane create the greenhouse effect, which may lead to global warming. The outcome of the competition between the cooling and the warming tendencies is difficult to predict.
The evolution of dust and smoke, if it caused global cooling, would likely only last a short time before the particulates washed out of the atmosphere. Then the raised levels of methane and the derivative carbon dioxide would take over. The likely result would be an alternating series of extra cold and extra warm years, arguably more devastating to crop production than a trend in one direction or the other.
Source Clathrate Gun Hypothesis