Clathrate gun hypothesis

The clathrate gun hypothesis or Methane time bomb (now effectively disproved) is the name given to the idea that as sea temperatures rise in the Arctic, this can trigger a strong positive feedback effect on climate. The hypothesis was that this warming would cause a sudden release of methane from methane clathrate compounds buried in seabeds and seabed permafrost, and then, because methane itself is a powerful greenhouse gas, temperatures rise further, and the cycle repeats. The original idea was that this runaway process, once started, could be as irreversible as the firing of a gun. It originates from a paper by Kennett et al published in 2003, which proposed that the "clathrate gun" could cause abrupt runaway warming on a time scale less than a human lifetime.

"Gun" suggests an exothermic reaction like an explosion. The clathrate decomposition is endothermic - if some of the clathrates are released they cool down the rest of the deposits. This means that the only way it can happen explosively is by a feedback with Earth's climate rapidly warming up the oceans.

The clathrates are not only kept stable by the low temperatures at the sea bed. They are also kept stable by pressure of the depth of sea above the deposits. The clathrates can slowly decompose as the result of lowering sea levels during ice ages, or by the sea floor rising along continental shelves when the ice resting on the land melts. This needs to be distinguished from clathrate dissociation due to a warming sea, which is needed for the clathrate gun hypothesis.

In December 2016, a major literature review by the 2107 USGS Hydrates project concluded that evidence is lacking for the original hypothesis. In 2017, the Royal Society review came to a similar conclusion that there is a relatively limited role for climate feedback from dissociation of the methane clathrates.

The 2018 Annual Review of Environment and Resources on Methane and Global Environmental Change concluded that "Nevertheless, it seems unlikely that catastrophic, widespread dissociation of marine clathrates will be triggered by continued climate warming at contemporary rates (0.2◦C per decade) during the twenty-first century". In 2018, the CAGE research group (Centre for Arctic Gas Hydrate, Environment and Climate) came to a much stronger conclusion when they published evidence that the methane clathrates formed over 6 million years ago and have been slowly releasing methane for 2 million years independent of warm or cold climate, rather than releasing methane only recently as had previously been thought.

The 2019 report on Ocean and Cryosphere came to a similar conclusion. It looked at all Arctic region methane emissions, rather than just the clathrates at the sea floor, and found that on the most extreme methane emissions scenario, reductions in anthropogenic sources can help mitigate natural increases in emissions of methane from all sources. . They cite a paper by Christansen et al (2019) which concluded that with a committed global effort to reduce anthropogenic emissions has far more effect on climate change than natural increases in emissions from even the extreme methane emissions scenario.

Summarizing the research for Carbon Brief, as of 2020, Dr Christina Schädel, lead coordinator of the Permafrost Carbon Network, puts it like this : "So, the latest research suggests that a methane bomb from thawing hydrates is not on the horizon."

The 2021 IPCC technical summary for AR6 Climate Change 2021: The Physical Science Basis puts it like this: :

"It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century."

At one time this hypothesis was thought to be responsible for warming events in and at the end of the Last Glacial Maximum around 26,500 years ago, but this is now also thought to be unlikely. .

At one point it was thought that a much slower runaway methane clathrate breakdown might have acted over longer timescales of tens of thousands of years during the Paleocene–Eocene Thermal Maximum 56 million years ago, and the Permian–Triassic extinction event, 252 million years ago. However, this is now thought unlikely,.

To hear what an expert says about the topic, see below.

Methane clathrates
Methane clathrate, also known commonly as methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure, which is stable under pressure, and remains stable under higher temperatures than ice, up to a few degrees above 0 °C depending on the pressure. .

The methane forms a structure I hydrate, trapped in dodecahedral cages made up of water molecules which are kept stable by a methane molecule inside each one. These are then each surrounded by tetrahedra to form part of a larger lattice with tetrakeidecahedral cavities which also contain methane molecules. Potentially large deposits of methane clathrate have been found under sediments on the ocean floors of the Earth

Methane is much more powerful as a greenhouse gas than carbon dioxide, although it has a short atmospheric lifetime of around 12 years. Shindell et al(2009) calculated that it has a global warming potential, the ratio of its warming potential to that of CO2. of between 79 and 105 over 20 years, and between 25 and 40 over 100 years, after accounting for aerosol interactions.

How the clathrates dissociate
As the oceans warm then methane can be released as the methane dissociates. The deposits extend to a depth of many meters and how much of an effect this is depends on how far down into the deposits the dissociation proceeds. The reaction absorbs heat rather than generating it, so as the reaction proceeds it cools surrounding sediments rather than warming them.

The 2017 and 2018 studies   have suggested only the topmost layers would be affected, while the original hypothesis was based on the supposition that deep layers would dissociate. The amount of the effect also depends on what happens to the methane as it rises in the water column above it after it is released from the clathrates. If the deposit is more than a hundred meters below the surface then most of the methane in the bubbles dissolves into the sea before it reaches the surface, since the sea is undersaturated in methane.

At a density of around 0.9 g/cm3, methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment. So the clathrate deposits all consist of clathrates firmly bound within the ocean sediments.

USGS and Royal Society metastudies (2016 and 2017)
A USGS metastudy first published December 2016 by the USGS Gas Hydrates Project concluded

""“Our review is the culmination of nearly a decade of original research by the USGS, my coauthor Professor John Kessler at the University of Rochester, and many other groups in the community,” said USGS geophysicist Carolyn Ruppel, who is the paper’s lead author and oversees the USGS Gas Hydrates Project. “After so many years spent determining where gas hydrates are breaking down and measuring methane flux at the sea-air interface, we suggest that conclusive evidence for release of hydrate-related methane to the atmosphere is lacking.”"

From the Royal Society report:Climate updates What have we learnt since the IPCC 5th Assessment Report? Royal Society update on questions from IPCC review of methane clathrate research from 2017 ""Clathrates: Some economic assessments continue to emphasise the potential damage from very strong and rapid methane hydrate release (Hope and Schaefer, 2016), although AR5 did not consider this likely. Recent measurements of methane fluxes from the Siberian Shelf Seas (Thornton et al., 2016) are much lower than those inferred previously (Shakhova et al., 2014). A range of other studies have suggested a much smaller influence of clathrate release on the Arctic atmosphere than had been suggested (Berchet et al., 2016; Myhre et al., 2016). New model ling work confirms (Kretschmer et al., 2015) that the Arctic is the region where methane release from clathrates is likely to be most important in the next century, but still estimates methane release to the water column to be negligible compared to anthropogenic releases to the atmosphere. A recent review (Ruppel and Kessler, 2017) emphasises that there remains little evidence that clathrate methane is reaching the atmosphere at present. Although methane that is oxidised in the water column will not reach the atmosphere, it will have the effect of further lowering the pH of the ocean (Boudreau et al., 2015). A recent modelling study joined earlier papers in assigning a relatively limited role to dissociation of methane hydrates as a climate feedback (Mestdagh et al., 2017). Methane concentrations are rising globally, raising interesting questions (see section on methane) about what the cause is (Nisbet et al., 2016; Rigby et al., 2017; Schaefer et al., 2016; Turner et al., 2017). finally new measurements of the 14C content of methane across the warming out of the last glacial period (Petrenko et al., 2017) show that the release of old carbon reservoirs (including methane hydrates) played only a small role in the methane concentration increase that occurred then"" ""Clathrates: Some economic assessments continue to emphasize the potential damage from very strong and rapid methane hydrate release, although AR5 did not consider this likely. Recent measurements of methane fluxes from the Siberian Shelf Seas are much lower than those inferred previously. A range of other studies have suggested a much smaller influence of clathrate release on the Arctic atmosphere than had been suggested. …. A recent modeling study joined earlier papers in assigning a relatively limited role to dissociation of methane hydrates as a climate feedback. Methane concentrations are rising globally, raising interesting questions (see section on methane) about what the cause is, finally new measurements of the 14C content of methane across the warming out of the last glacial period show that the release of old carbon reservoirs (including methane hydrates) played only a small role in the methane concentration increase that occurred then.""

Timeline with original hypothesis, and later developments
This is a timeline of clathrates research with some of the milestones. For the latest research see and.

2007 - Most deposits are too deep, focus is on shallow deposits
Most deposits of methane clathrate are in sediments too deep to respond rapidly, and modeling by Archer (2007) suggests the methane forcing should remain a minor component of the overall greenhouse effect. Clathrate deposits destabilize from the deepest part of their stability zone, which is typically hundreds of meters below the seabed. A sustained increase in sea temperature will warm its way through the sediment eventually, and cause the shallowest, most marginal clathrate to start to break down; but it will typically take on the order of a thousand years or more for the temperature signal to get through.

Subsea permafrost occurs beneath the seabed and exists in the continental shelves of the polar regions. This source of methane is different from methane clathrates, but contributes to the overall outcome and feedbacks.

From sonar measurements in recent years researchers quantified the density of bubbles emanating from subsea permafrost into the ocean (a process called ebullition), and found that 100–630 mg methane per square meter is emitted daily along the East Siberian Shelf, into the water column. They also found that during storms, when wind accelerates air-sea gas exchange, methane levels in the water column drop dramatically. Observations suggest that methane release from seabed permafrost will progress slowly, rather than abruptly. However, Arctic cyclones, fueled by global warming, and further accumulation of greenhouse gases in the atmosphere could contribute to more rapid methane release from this source.

2008 - Original hypothesis, idea of a fast release of 50 gigatons of methane
Research carried out in 2008 in the Siberian Arctic showed millions of tons of methane being released, apparently through perforations in the seabed permafrost, with concentrations in some regions reaching up to 100 times normal levels. The excess methane has been detected in localized hotspots in the outfall of the Lena River and the border between the Laptev Sea and the East Siberian Sea. At the time, some of the melting was thought to be the result of geological heating, but more thawing was believed to be due to the greatly increased volumes of meltwater being discharged from the Siberian rivers flowing north. The current methane release had previously been estimated at 0.5 megatonnes per year. Shakhova et al. (2008) estimate that not less than 1,400 gigatons of carbon is presently locked up as methane and methane hydrates under the Arctic submarine permafrost, and 5–10% of that area is subject to puncturing by open taliks. They conclude that "release of up to 50 gigatonnes of predicted amount of hydrate storage [is] highly possible for abrupt release at any time". That would increase the methane content of the planet's atmosphere by a factor of twelve, equivalent in greenhouse effect to a doubling in the current level of.

This is what lead to the original Clathrate gun hypothesis, and in 2008 the United States Department of Energy National Laboratory system and the United States Geological Survey's Climate Change Science Program both identified potential clathrate destabilization in the Arctic as one of four most serious scenarios for abrupt climate change, which have been singled out for priority research. The USCCSP released a report in late December 2008 estimating the gravity of this risk.

2010 - Taliks or pongos could lead to gas migration pathways
There is a possibility for the formation of gas migration pathways within fault zones in the East Siberian Arctic Shelf, through the process of talik formation, or pingo-like features.

2012 - Possible abrupt release of clathrates stabilized by low temperatures or after landslips
A 2012 assessment of the literature identifies methane hydrates on the Shelf of East Arctic Seas as a potential trigger.

The Arctic ocean clathrates can exist in shallower water than elsewhere, stabilized by lower temperatures rather than higher pressures; these may potentially be marginally stable much closer to the surface of the sea-bed, stabilized by a frozen 'lid' of permafrost preventing methane escape.

The so-called self-preservation phenomenon has been studied by Russian geologists starting in the late 1980s. This metastable clathrate state can be a basis for release events of methane excursions, such as during the interval of the Last Glacial Maximum. A study from 2010 concluded with the possibility for a trigger of abrupt climate warming based on metastable methane clathrates in the East Siberian Arctic Shelf (ESAS) region. A trapped gas deposit on the continental slope off Canada in the Beaufort Sea, located in an area of small conical hills on the ocean floor is just 290 meters below sea level and considered the shallowest known deposit of methane hydrate.

Seismic observation (in 2012) of destabilizing methane hydrate along the continental slope of the eastern United States, following the intrusion of warmer ocean currents, suggests that underwater landslides could release methane. The estimated amount of methane hydrate in this slope is 2.5 gigatonnes (about 0.2% of the amount required to cause the PETM), and it is unclear if the methane could reach the atmosphere. However, the authors of the study caution: "It is unlikely that the western North Atlantic margin is the only area experiencing changing ocean currents; our estimate of 2.5 gigatonnes of destabilizing methane hydrate may therefore represent only a fraction of the methane hydrate currently destabilizing globally."

2015 - Model based on the hypothesis suggests an extra 6 °C rise within 80 years
A study of the effects for the original hypothesis, based on a coupled climate–carbon cycle model (GCM) assessed a 1000-fold (from <1 to 1000 ppmv) methane increase—within a single pulse, from methane hydrates (based on carbon amount estimates for the PETM, with ~2000 GtC), and concluded it would increase atmospheric temperatures by more than 6 °C within 80 years. Further, carbon stored in the land biosphere would decrease by less than 25%, suggesting a critical situation for ecosystems and farming, especially in the tropics.

2016 Methane in upper continental slope clathrates doesn't get to surface
Some of the shallow methane clathrates are indeed decomposing and there are higher concentrations of methane near the sea floor that do indeed come from the clathates. But it is taken up by the sea water and from the measurements made by many scientists, almost none reaches the surface of the sea. Methane in the upper layers of the sea do not come from the sea floor and there aren't any significant atmospheric additions.Ruppel, C.D. and Kessler, J.D., 2017. The interaction of climate change and methane hydrates. Reviews of Geophysics, 55(1), pp.126-168. ""While upper continental slope gas hydrates are generally viewed as being in a net dissociation regime in light of contemporary climate warming, the details are certainly more complicated, with gas hydrate dissociating and re‐forming at the shallowly buried BGHS in response to oscillating temperatures and pressures on the slopes. Key questions include the degree to which gas hydrates remain out of equilibrium with local P‐T conditions, the rate at which upper slope gas hydrates respond to climate forcing, and whether upper slope hydrate dissociation processes that do not produce seafloor seepage can be detected by geophysical surveys.

Despite the expectation that upper continental slopes host the most climate‐susceptible gas hydrate populations, widespread upper slope seepage has so far only been recognized on the West Spitsbergen margin], and the northwestern U.S. Pacific margin

... [discussion of satellite data] Furthermore, the strength of water column sinks makes it unlikely that upper continental slope methane is reaching the ocean‐atmosphere interface in any case, leading to questions about the origin of the signal being detected in the satellite data.

... Graves et al. [2015] recorded near‐bottom methane concentrations as high as 825 nM over the upper slope seeps but show that most of this methane remains near the bottom and that the methane at shallower depths in the water column does not originate with in situ seafloor emissions.

... Steinle et al. [2015] documented the strength of the MOx sink in these cold waters, demonstrating that changing ocean currents have a profound effect on the efficiency of the sink. Myhre et al. [2016] and Graves et al. [2015] used direct measurements and indirect arguments to demonstrate that sea‐air flux over the upper continental slopes is not elevated, while Fisher et al. [2011] concluded that the atmospheric methane in this area lacks a signal related to gas hydrate dissociation. Nonetheless, at least for the CH4 detected near the seafloor, there is strong evidence that it is being sourced in gas hydrate dissociation

... The types of exhaustive studies of seafloor CH4 flux rates that are available for the West Spitsbergen margin have not yet been completed for the U.S. Atlantic or northwestern U.S. Pacific margin seeps. Based on limited bubble observations, Skarke et al. [2014] gave a conservative estimate of 15–90 Mg yr−1 CH4 for seafloor flux for the ~570 seeps they describe along 950 km of the Atlantic margin, while Weinstein et al. [2016] used indirect methods to infer 70–280 Mg yr−1 CH4 in Hudson Canyon alone. How much, if any, of these emissions originate in dissociating gas hydrate remains unknown. Preliminary continuous sea‐air flux measurements indicate that the Atlantic margin seeps are unlikely to be contributing CH4 to the atmosphere"

This is also the date of publication of the USGS metastudy

2017 - Fertilizing effect of methane at continental margins may lead to net CO2 sink
One paper published in 2017 found from measurements on the Svalbard margin that CO2 sequestration due to the fertilizing effect of the methane on surface microbes lead to a net negative effect on radiative forcing, 231 times greater than the effect of the methane emissions

"Continuous sea−air gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO2 uptake rates (−33,300±7,900 μmol m−2·d−1) twice that ofsurrounding waters and ∼1,900 times greater than the diffusive sea−air methane efflux (17.3±4.8μmol m−2·d−1). The negative radiative forcing expected from this CO2 uptake is up to 231 times greater than the positive radiative forcing from the methane emissions"

2017 - Methane clathrates only decompose to a depth of 1.65 meters


However, later research cast doubt on this picture. Hong et al (2017) studied the seepage from large mounds of hydrates in the shallow arctic seas at Storfjordrenna, in the Barents Sea close to Svalbard. They showed that though the temperature of the sea bed has fluctuated seasonally over the last century, between 1.8 and 4.8 °C, it has only affected release of methane to a depth of about 1.6 meters. The areas that do destabilize do so only very slowly (centuries) because they are only warmed sufficiently for less than half the year, from April to August - and this doesn’t seem to be enough for fast destabilizing (see figure 6 on Nature ).

Hydrates can be stable through the top 60 meters of the sediments and the current rapid releases came from deeper below the sea floor. They concluded that the increase in flux started hundreds to thousands of years ago well before the onset of warming that others speculated as its cause, and that these seepages are not increasing due to momentary warming. Summarizing his research, Hong stated: ""The results of our study indicate that the immense seeping found in this area is a result of natural state of the system. Understanding how methane interacts with other important geological, chemical and biological processes in the Earth system is essential and should be the emphasis of our scientific community,""

Further research by Klaus Wallmann et al (2018) found that the hydrate release is due to the rebound of the sea bed after the ice melted. The methane dissociation began around 8,000 years ago when the land began to rise faster than the sea level, and the water as a result started to get shallower with less hydrostatic pressure. This dissociation therefore was a result of the uplift of the sea bed rather than anthropogenic warming. The amount of methane released by the hydrate dissociation was small. They found that the methane seeps originate not from the hydrates but from deep geological gas reservoirs (seepage from these formed the hydrates originally). They concluded that the hydrates acted as a dynamic seal regulating the methane emissions from the deep geological gas reservoirs and when they were dissociated 8,000 years ago, weakening the seal, this led to the higher methane release still observed today.

This is also the date of publication of the Royal Society metastudy

2018 - CAGE group findings, the methane has been escaping at the same rate for millions of years
Research by the CAGE group in 2018 showed that the methane there has been escaping at the same rate for millions of years! "Recent observations of extensive methane release from the seafloor into the ocean and atmosphere cause concern as to whether increasing air temperatures across the Arctic are causing rapid melting of natural methane hydrates. Other studies, however, indicate that methane flares released in the Arctic today were created by processes that began way back in time – during the last Ice Age."

"Newest research from the Center for Arctic Gas Hydrate, Climate and Environment (CAGE) shows that methane has been leaking in the Arctic for millions of years, independent of warm or cold climate. Methane has been forming in organic carbon rich sediments below the leakage spots off the coast of western Svalbard for a period of about 6 million years (since the late Miocene). According to our models, methane flares occurred at the seafloor for the first time at around 2 million years ago; at the exact time when ice sheets started to expand in the Arctic."

"The acceleration of leakage occurred when the ice sheets were big enough to erode and deliver huge amounts of sediments towards the continental slope. Methane leakage was promoted due to formation of natural gas in organic-rich sediments under heavy loads of glacial sediments. Faults and fractures opened within the Earth’s crust as a consequence of growth and decay of the massive ice masses. This brought up the gases from deeper sediments higher up towards the seafloor. These gases then fueled the gas hydrate system off the Svalbard coast for the past 2 million years. It is, to this day, controlling the leakage of methane from the seabed."

So, the methane deposits formed in the late Miocene starting 6 million years ago, and the methane leaks have been going on for two million years through multiple ice ages.

Also published in 2018, the Review of Environment and Resources on Methane and Global Environmental Change concluded that ""Although the clathrate gun hypothesis remains controversial (21), a good understanding of how environmental change affects natural CH4 sources is vital in terms of robustly projecting future fluxes under a changing climate.""

Then later: ""Nevertheless, it seems unlikely that catastrophic, widespread dissociation of marine clathrates will be triggered by continued climate warming at contemporary rates (0.2◦C per decade) during the twenty-first century"" .

They did however urge caution about extraction of methane clathrates as a fuel, as this could lead to leaks of methane.

"As discussed previously(Section 4.1), the stability of CH4 clathrate deposits may already be at risk from climate change.Accidental or deliberate disturbance, due to fossil fuel extraction, has the potential for extremelyhigh fugitive CH4 losses to the atmosphere"

2019 - IPCC report on ocean and cryosphere and Christensen et al.
The 2019 Special Report on the Ocean and Cryosphere in a Changing Climate say that most models do not explicitly take account of methane release because fluxes are small even taking account of the higher global warming potential. There are some more detailed models however, and with these there is still a wide range of views on the matter.

The range is so great that some of the more detailed models suggest that if you include the effect of the land based permafrost changes, the Arctic could remain a net sink through to 2100 even on high emissions (see their figure 3.11). This is because of stimulation of plant growth from the effects of nutrients, warming, extended growing season and CO2 fertilization, which could offset some or all the losses, by sequestring new carbon into the plant biomass and increasing the carbon inputs into the surface soil. (3.4.1.1)

With the most extreme methane emissions scenario, the effects of increases in natural sources are still less than increases due to anthropogenic methane emissions, and reductions in anthropogenic sources can help mitigate natural increases in emissions of methane from all sources.

They cite a 2019 paper by Christensen et al

This includes an "Extreme" scenario, in which methane sources from Tundra and lakes increase markedly, and there are extreme increase in ocean emissions. If we keep to existing legislation on methane then even with no increase in natural emissions, methane levels almost double by 2100 from 1423 to 2842 ppb. With the maximum reduction of anthropogenic emissions levels decrease on all except the extreme natural emissions scenario and even the extreme case only increases it by 42% over the case of no change in natural emissions, increasing it to just a little over present day levels.

So with a committed global effort to offset methane emissions we can compensate for just about all the effect of any increase in natural emissions, even with the extreme scenario, and reducing anthropogenic emissions has far more effect on climate change than even the extreme methane emissions scenario. This paper also cites and earlier study that found that the maximum feasible reductions in anthropogenic methane can reduce global warming by around 0.1 to 0.2°C for the 2036–2050 period relative to continually increasing emissions. That’s a significant impact on the goal to keep the rise well below 2.0 °C, preferably 1.5 °C,

Christansen et al conclude:

"Methane, however, is too often portrayed as solely being able to cause runaway climate change. Despite large uncertainties associated with future projections of Arctic natural methane emissions, our current best estimates of potential increases in natural emissions remain lower than anthropogenic emissions. In other words, claims of an apocalypse associated solely with Arctic natural methane emission feedbacks are misleading, since they guide attention away from the fact that the direction of atmospheric methane concentrations, and their effect on climate, largely remain the responsibility of anthropogenic GHG emissions."

2020 - evidence from ice cores of low methane emissions
Researchers from the University of Rochester analysed ice cores from Antarctica and found that the amount of methane in the atmosphere was low during the last deglaciation between 8000 and 18000 years ago despite a rapid rise in temperature, similar to what is predicted for our immediate future. M. N. Dyonisius, V. V. Petrenko, A. M. Smith, Q. Hua, B. Yang, J. Schmitt, J. Beck, B. Seth, M. Bock, B. Hmiel, I. Vimont, J. A. Menking, S. A. Shackleton, D. Baggenstos, T. K. Bauska, R. H. Rhodes, P. Sperlich, R. Beaudette, C. Harth, M. Kalk, E. J. Brook, H. Fischer, J. P. Severinghaus, R. F. Weiss. Old carbon reservoirs were not important in the deglacial methane budget. Science, 2020; 367 (6480): 907 DOI: 10.1126/science.aax0504, press release To combat climate change, human activities more important than natural feedbacks
 * Permafrost and methane hydrates are large, climate-sensitive old carbon reservoirs that have the potential to emit large quantities of methane, a potent greenhouse gas, as the Earth continues to warm. We present ice core isotopic measurements of methane (Δ14C, δ13C, and δD) from the last deglaciation, which is a partial analog for modern warming. Our results show that methane emissions from old carbon reservoirs in response to deglacial warming were small (<19 teragrams of methane per year, 95% confidence interval) and argue against similar methane emissions in response to future warming. Our results also indicate that methane emissions from biomass burning in the pre-Industrial Holocene were 22 to 56 teragrams of methane per year (95% confidence interval), which is comparable to today

perspective: Joshua F. Dean, Science 21 Feb 2020: Vol. 367, Issue 6480, pp. 846-848 DOI: 10.1126/science.aba8518 Old methane and modern climate change


 * Researchers at the University of Rochester—including Michael Dyonisius, a graduate student in the lab of Vasilii Petrenko, professor of earth and environmental sciences—and their collaborators studied methane emissions from a period in Earth’s history partly analogous to the warming of Earth today. Their research, published in Science, indicates that even if methane is released from these large natural stores in response to warming, very little actually reaches the atmosphere.


 * Dyonisius et al. (3) describe their search for signals of old methane released to the atmosphere during the last deglaciation about 18,000 to 8000 years ago. This was when Earth last showed warming similar to what is predicted for our immediate future. They found that methane emissions from old carbon sources during this time were small, suggesting that substantial emissions of old methane may not be triggered in response to current and near-future climate change.

Past mass extinction events
At one point it was thought that runaway methane clathrate breakdown might also have acted over longer timescales of tens of thousands of years during the Paleocene–Eocene Thermal Maximum 56 million years ago, and most notably the Permian–Triassic extinction event, when up to 96% of all marine species became extinct, 252 million years ago. It was thought to have caused drastic alteration of the ocean environment (such as ocean acidification and ocean stratification) and of the atmosphere.

However, the pattern of isotope shifts expected to result from a massive release of methane does not match the patterns seen there. First, the isotope shift is too large for this hypothesis, as it would require five times as much methane as is postulated for the PETM, and then, it would have to be reburied at an unrealistically high rate to account for the rapid increases in the 13C/12C ratio throughout the early Triassic before it was released again several times.

One of the hypotheses being considered in its place is that the temperature increase of the PETM was due to the roasting of carbonate sediments such as coal beds by volcanism. Potentially this may have released more than 3 trillion tons of carbon

Effects thousands of years into our future
Although significant effects are effectively ruled out at present, the oceans would continue to warm by several degrees under the "Business as usual" scenario. This would lead to the clathrates warming and eventually dissociating, and some of this could contribute to the long tail of CO2, helping to keep CO2 levels in the atmosphere higher for longer, as it gradually is removed from the atmosphere by natural processes. David Archer, author of many papers on gas hydrates, put it like this :

"On the other hand, the deep ocean could ultimately (after a thousand years or so) warm up by several degrees in a business-as-usual scenario, which would make it warmer than it has been in millions of years. Since it takes millions of years to grow the hydrates, they have had time to grow in response to Earth’s relative cold of the past 10 million years or so. Also, the climate forcing from CO2 release is stronger now than it was millions of years ago when CO2 levels were higher, because of the band saturation effect of CO2 as a greenhouse gas. In short, if there was ever a good time to provoke a hydrate meltdown it would be now. But “now” in a geological sense, over thousands of years in the future, not really “now” in a human sense. The methane hydrates in the ocean, in cahoots with permafrost peats (which never get enough respect), could be a significant multiplier of the long tail of the CO2, but will probably not be a huge player in climate change in the coming century."

In fiction

 * The science fiction novel Mother of Storms by John Barnes offers a fictional example of catastrophic climate change caused by methane clathrate release.
 * In The Life Lottery by Ian Irvine unprecedented seismic activity triggers a release of methane hydrate, reversing global cooling.
 * The hypothesis is the basis of an experiment in the PlayStation 2 game Death By Degrees.
 * In Transcendent by Stephen Baxter, averting such a crisis is a major plotline.
 * The novel The Black Silent by author David Dun features this idea as a key scientific point.
 * In the anime Ergo Proxy, a string of explosions in the methane hydrate reserves wipes out 85% of human life on Earth.
 * The novel The Far Shore of Time by Frederik Pohl features an alien race attempting to destroy humanity by bombing the methane clathrate reserves, thus releasing the gas into the atmosphere.
 * The novel The Swarm by Frank Schätzing features what first appear to be freak events related to the world's oceans.
 * In Charles Stross' Laundry Files universe, an intentionally triggered clathrate gun scenario is viewed as a possible retaliatory strategy that could be utilized by Blue Hades in response to terminal violation of the Benthic Treaty.

Video interview with Carolyn Ruppel (USGS Gas Hydrates Project)
Interview (excerpts) with Carolyn Ruppel, PhD, Chief, USGS Gas Hydrates Project, Woods Hole Coastal and Marine Science Center. Includes notes of some of the points to make it easier to locate a video of interest:

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 * .(Part 1 of 6) - About one sixth of all the methane on the planet is trapped in the clathrates. There is concern that as we modify the pressure or temperature they may or may not remain stable. They formed associated with permafrost before the last major glaciation on Earth - the sea has risen and covered some of the deposits by 100 to 125 meters. That increases the pressure which in theory would make it more stable. But this is terrestrial permafrost that got inundated, and it is warmer than it was during the ice age. So these are breaking down now with the warming over the last 15,000 years. If all these methanes are in hydrates and breaking down and in shallow water can reach the atmosphere - the concern is whether they can catastrophically break down. The terrestrial geology is complex, and they are going to concentrate in places where there is highly permeable sand. It doesn't mean it is everywhere, only in certain areas. Not necessarily as huge or as thick as some people have been estimating.

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(Part 2 of 6) - another type that has come to the fore, with lots of papers particularly from the Scandinavian region. These are pock marks in the Berings sea, associated with methane plumes. They are found in the Svalbard margin and a few other places where originally the gas hydrate formed beneath ice sheets. The US Arctic didn't have a glaciated shelf. In these places in the Bering sea, though, the ice sheet itself had hydrate beneath it. As a result of the unloading and inundation, it's a new environment but that doesn't mean we need to panic about the amount of methane coming up.

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 * (Part 3 of 6) - Worldwide, clathrates generally are associated with methane produced by microbes. In the Arctic the permafrost clathrates are associated with conventional oil and gas basins - and methane leaking up from those places. So you want to intersect where these conventional sources are with the permafrost, not ubiqitous. Again that means it's not as much as people would think there was if they don't think about the vagaries of gas hydrates.

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 * (Part 4 of 6) - methane doesn't go directly into the atmosphere. Any methane emitted more than approximately 100 meters (depends on other parameters) by the time they reach the sea surface don't retain methane because the ocean is very undersaturated in methane.

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 * Part 5 of 6) - Another misconception. People not too aware of the thermodynamics might think that once you trigger breakdown of the clathrates, that you can't stop it. But the nature of the reaction is that it is endothermic [needs heat constantly supplied to keep it going]. This is a problem when we try to produce methane. It keeps shutting itself down. It's not a situation where you trigger methane and the whole deposit breaks down. It's not a scientifically sound worry as it is not how these deposits function.

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 * (Part 6 of 6) - In my own research I am now working on the atlantic margin. Methane seeps clearly happening in a lot of places. Not new. Tools changed a lot in the last few decades. Now can use a "fish finder" to find methane and it is because we now have the tools that we keep finding it. So the processes are probably pretty long lived. Inappropriate to say that it is new, probably been happening all along. There are rocks in some of these places associated with methanogenic processes that must have taken a long time to form.

Video interview with Michael Mann
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 * Michael Mann of Penn State University on the arctic "methane bomb".