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Atmospheric Methane and the Clathrate-gun Conjecture
Methane, the Clathrate-gun conjecture and
a disturbed Carbon Cycle
A look at recent studies in climate science
Phil Harris
March 2012
This is a longer version of a more concise article by Phil Harris published
by Ugo Bardi at his blog. It is retained in order to provide access to
a wider selection of references to up-to-date climate science
papers. It contains many direct quotes from and introductions to
these papers. The author would value additions, comments and
corrections.
Introduction: a personal quest
When Ugo Bardi wrote recently of atmospheric methane and the potential “clathrate
gun”, I was prompted to dig out my old hard copies of papers that I had collected in
the early 1990s, mostly from the journal Nature. These papers mentioned the potent
greenhouse gas methane deposited as clathrates (gas hydrates), but mostly in those
days with reference to climate changes involving clathrate release in remote
geological ages.
I asked about later updates and raised questions about the age of current methane
gas hydrate deposits and the part they had played, or had not played in altering
atmospheric methane concentrations, or indeed climate, during the Pleistocene.
Glacial conditions increasingly predominated during the last two to three million
years, but these were interspersed with short temperature excursions in addition to
the longer inter-glacial responses to orbital forcing (i.e. the responses to initial forcing
due to Milankovitch cycles; see the last 0.8Ma)1. How stable were these methane
hydrate deposits over these glacial cycles; indeed, how old are the deposits?
Methane is a reactive molecule with a relatively very short residence time in the
atmosphere, so what do we know anyway of the current as well as past variation in
methane (CH4) concentrations in the air? What do we know of the relative volumes
of current sources and sinks, including clathrates?
Ugo suggested that I attempt to collate some up-to-date information. I cannot
legitimately call what follows a scientific review. I will try mostly to use direct quotes. I
have a scientific background but in this case I can only be an interested observer,
and I will not attempt any critique of methodology or draw any conclusions except
perhaps via my last quote at the end.
Are methane gas hydrates potentially more easily mobilised than other sequestered
carbon deposits? Do we know, apart from the more random tectonic events, whether
1 This cooling trend has been reviewed 2011 by Hansen & Sato (NASA); submitted for publication;
FULL PAPER
Atmospheric Methane and the Clathrate-gun Conjecture
there are climate-related positive feedback mechanisms that more frequently trigger
big pulses of CH4 (‘the gun’) from clathrate deposits, and if so, from which deposits?
Can such releases be demonstrated in the geological record?
i
[An interesting set of full papers from 2011 can be accessed from this reference ,
with particular focus on measurement, distribution and mixing of greenhouse gases
in the atmosphere, and on mitigation policies; Greenhouse gases in the Earth
system: setting the agenda to 2030 ]
Opening remarks on the carbon cycle
There has been a planetary carbon-cycle for a very long time. Carbon in the
atmosphere is resident mostly as carbon dioxide (CO2) and this CO2 is in part
exchanged over very short time scales, for example seasonally, with sinks such as
the ocean, and with vegetation during photosynthesis, but for the most part remains
in the atmosphere for many decades and in smaller part for centuries. In remote
geological time carbon became sequestered in very large persistent sinks of
carbonaceous rock and in petroleum and gas deposits. Weathering, tectonic
movement and volcanic activity release carbon from rocks, and seepage occurs from
trapped “fossil fuels” and buried organic material, but since the last 10s of millions of
years, the earlier sequestration has had the net ongoing effect of a reduced carbon
gas level maintained in the atmosphere. Thus, more recent geological ages have
experienced much lower levels of free CO2 and CH4 than those remote epochs when
the largest ancient carbon stores were laid down. 2 Such ancient sink accumulations
have of course included significant quantities of trapped CH 4 gas, such as those
associated with coal and oil that are now being released by human activity, and
those trapped in the form of “meta-stable” gas hydrates. If the total warming effect is
calculated over a century, methane is x25 more potent greenhouse gas than CO 2
molecule for molecule, but is resident in air at very low concentrations for a much
shorter average time of 7.9 years.
In summary, carbon accumulates in living material following photosynthesis. This
carbon is in turn released as CO2 by aerobic respiration or as CH4 during active
anaerobic decomposition by microorganisms, including ruminant digestion, or as
both CO2 and CH4 by wild fires. Ancient carbonaceous rocks release carbon mostly in
the form of CO2 by weathering or other physical action, although CH 4 also seeps
from deposits of ancient trapped gas, or is released from clathrate (gas hydrate)
deposits. Carbon is thus continually being cycled; footnote 3. Human activity adds
2 Ibid 1 Hansen & Sato
3 There is no reason to question the biogenic origin of almost all of the carbon
sequestered geologically on earth or of the fraction participating in the
contemporary carbon cycle, but there is a reference to relatively insignificant abiogenic CH4 reaching the earth’s surface in ref 2, and to a putative mechanism in
ref 20. There are large deposits of methane elsewhere in the solar system
presumably of inorganic origin.
Atmospheric Methane and the Clathrate-gun Conjecture
both CO2 and CH4 to the atmosphere from industrial activity, and CH4 from recent
extension of agriculture.
Sources and sinks of CH4; including gas hydrates (referenceii)
Gas hydrates are deposits of sequestered methane associated with the ocean floor
and sedimentary overburdens and with permafrost, and appear to have had a
continuous but dynamic presence over geological time. Methane continually seeps
from miscellaneous geological sources including gas hydrates as well as from
deeper petroleum sources.
Until additional sources were opened-up during the industrial period, methane in the
air was a result of routine releases from a combination of natural sources, but with a
majority coming from relatively recently photosynthesised material. Soils depending
on type, location, temperature and wetness can be either sources or sinks of CH 4iii.
Methane is cycled very rapidly and needs continuous large-scale release to maintain
any significant level in the atmosphere. The chief agents acting on released methane
are OH’ radicals in the atmosphere during daylight, but soil organisms also make a
significant if much smaller contribution to oxidising CH 4 to CO2 of perhaps 5-10% of
the global sinks iv. There is an interesting relationship with atmospheric ozone:
v
Methane contributes to additional planetary warming through its
involvement in tropospheric photochemistry leading to ozone formation,
and its oxidation in the lower stratosphere generates water vapor that acts
as a highly effective GHG [Green House Gas].
[This study actually focuses on ancient ‘greenhouse worlds’ and is referred to again
below.]
Much of the methane released from ocean floors is oxidised by bacteria within the
water column and does not reach the surface. Also, contemporary sequestration is
known to occur via bacterial action next to CH 4 seepages at the ocean floorvi. There
is thus a ‘methane cycle’ as a subset of the ‘carbon cycle’. The natural cycle includes
seepages from petroleum sources but mostly is maintained by both microbial
methanogenesis and methane oxidation under anaerobic/anoxic conditions vii.
There has been a very significant increase in CH4 in the atmosphere since 1850
due to massive releases from fossil fuels and the spread of agriculture.
viii
For example, during the last glacial period the concentration was
w0.35ppm. This amount increased to w0.75ppm by 1850 at which time the
CH4 concentration increased much more rapidly as a result of agricultural
expansion and industrialization (Lelieveld et al., 1998). By 1998, the globally
averaged atmospheric surface abundance of CH 4 was 1.745 ppm,
corresponding to a total burden of w4850 Tg CH4 (ICPP, 2001).
Methane makes a significant contribution to modern greenhouse gas radiative
forcing.
Atmospheric Methane and the Clathrate-gun Conjecture
ix
The global atmospheric methane burden has more than doubled since
pre-industrial times, and this increase is responsible for about 20% of the
estimated change in direct radiative forcing due to anthropogenic
greenhouse-gas emissions.
The above 1998 study discussed the slowing in the 1990s of the previously rapid rise
of atmospheric CH4. That rise has since resumed; see below in “Modern Methane
Trends”.
Snowball Earth
There was, a very long time ago, a Snowball Earth; a period that ended around
635Ma. Gas hydrate releases are mentioned as one of putative positive feedback
mechanisms that brought this phenomenon to an end.
x
Hypotheses accounting for the abruptness of deglaciation include ice
albedo feedback, deep-ocean out-gassing during post-glacial oceanic
overturn or methane hydrate destabilization.
Scientific discussion continues about this interesting period, but for our purposes it is
worth noting the reasons why we do not have a snowball earth.
xi
Ample physical evidence shows that carbon dioxide (CO2) is the single
most important climate-relevant greenhouse gas in Earth's atmosphere. This
is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not
condense and precipitate from the atmosphere at current climate
temperatures, whereas water vapor can and does. Non-condensing
greenhouse gases, which account for 25% of the total terrestrial greenhouse
effect, thus serve to provide the stable temperature structure that sustains the
current levels of atmospheric water vapor and clouds via feedback processes
that account for the remaining 75% of the greenhouse effect. Without the
radiative forcing supplied by CO2 and the other non-condensing
greenhouse gases, the terrestrial greenhouse would collapse, plunging
the global climate into an icebound Earth state (emphasis added).
Past Greenhouse Worlds
There have been past worlds when there was no ice even at the poles, but they were
a long time ago and much has changed since, including configuration of continents
and mountain ranges. That much we know from strong evidence, but past trace gas
concentrations need to be inferred by using recent modelling techniques to match
the conditions we know existed.
xii
Here we report results from a series of three-dimensional Earth system
modeling simulations indicating that the greenhouse worlds of the early
Eocene (55 Ma) and late Cretaceous (90 Ma) maintained high
concentrations of methane, tropospheric ozone, and nitrous oxide.
Modeled methane concentrations were four to five-fold higher than the
preindustrial value [pre-1850].
Atmospheric Methane and the Clathrate-gun Conjecture
[In the final section below there is further discussion of the Palaeocene Eocene
thermal maximum, PETM, 55Ma, and the temperature rise associated possibly with
a singular vast release of methane acting as a trigger event.]
Note that present methane concentrations are maintained 2.5-fold higher than their
1850 preindustrial value, but without a major new sustained chronic source it is
assumed they would reduce rapidly if fossil fuel sources ceased.
More Recent Warm Worlds
An account is available in Hansen & Sato, 2011 4 of the mostly +1-2°C warmer
Pliocene, 5Ma to around 3Ma (million years ago), when sea levels were significantly
higher than present, perhaps 25m higher. [Temperatures are based on deep ocean
core records. Relationships between ocean and ground surface temperature (GST)
are discussed in this reference.] During the increasingly more glacial Pleistocene
(last 2M years) there have been a number of warm interglacial periods culminating in
our own Holocene. Large changes in ‘free’ greenhouse gases are known to have
occurred (there is knowledge from evidence in sediments and ice cores and from
isotope signatures). Carbon dissolved in the ocean, or held in wetlands and peat
deposits, and hydrate deposits, has exchanged with the atmosphere to a greater or
lesser extent during these periods, according to the degree of glaciation, the rise and
fall of sea level, ‘isostatic rebound’ of land mass and greater or lesser precipitation.
A point I shall return to is that none of these partial exchanges of carbon as gases
with the atmosphere over 2M years led to ‘runaway’ positive feedback warming that
could have returned the world to the earlier Pliocene, never mind to the ancient
greenhouse conditions of 55Ma. However, we are in a different context now with an
atmosphere of non-condensing greenhouse gases unparalleled during the
Pleistocene and with CO2 already comparable to or exceeding levels in the
preceding Pliocene. Bear in mind that, in addition to the rising level of the chief noncondensing greenhouse gas CO2, human activity has raised preindustrial CH4 levels
by an even larger 2.5-fold margin:
xiii
The atmospheric concentration of CH4, the second most important
anthropogenic greenhouse gas, is determined by a balance between natural
and anthropogenic CH4 sources and sinks that is still debated.
Photochemically induced oxidation in the troposphere and stratosphere, and
uptake by methanotrophic bacteria in aerated soils, represent the most
important sinks [refs]. The dominating natural CH 4 sources comprise tropical
and boreal wetlands, ruminants, and biomass burning [refs]. These sources
all differ in their carbon and hydrogen isotopic signature. In addition, a release
of CH4 from marine gas hydrates and emissions from plants under aerobic
conditions are currently debated [refs]. Most probably all those sources and
sinks were subject to palaeoclimatic changes, as reflected by CH 4 being as
low as 360 parts per billion [0.36ppm] [ref] by volume (ppb (v) during the Last
Glacial Maximum (LGM), compared with up to 725ppb (v), [~0.725ppm] in the
preindustrial Holocene epoch3, [refs]. Throughout the glacial period and
4 Ibid footnote page 1, Hansen & Sato;
Atmospheric Methane and the Clathrate-gun Conjecture
during the last transition, CH4 changed by up to 200ppb (v) [0.2ppm] [refs] in
parallel with rapid climate changes.
Glacial and Inter-Glacial Periods over the last 800,000 years
Before our present Holocene interglacial there was the previous warmer Eemian
(+1°C, 125,000 years ago), and before that the warm Holsteinian (400,000 year
ago). Greenhouse gases in the atmosphere rose then to levels similar to recent preindustrial Holocene levels.
Figure 800,000 years of CO2 and CH4 concentrations correspond with timing of
glacial/interglacial temperature fluctuations; from Hansen & Sato, 2011
(Personally, I only get the enormity of what has happened in the last few decades if I
superimpose present CO2 and CH4 concentrations (respectively 392ppm and
approximately 1800ppbxiv) on the end of the above figure (Hansen & Sato,2011).
Methane at the end of the Younger Dryas event was at ~700ppb; dropped to
~600ppb by 5000 years ago; climbed to >700 again by the year 1750.)
There is some debate, but Hansen & Sato (the figure above is from their paper) 5
based mostly on ocean records, make a convincing enough case that the eventual
Eemian global temperature rose to a maximum higher than our current global
average, but not as high as suggested by some previous calculations. Hansen &
Sato conclude:
One conclusion deserving emphasis is that global mean temperatures
[ocean records] in the Eemian and Holsteinian were less than 1°C warmer
than peak Holocene global temperature. Therefore, these interglacial
periods were also less than 1°C warmer than global temperature in year
2000.
Geographical distribution of warmth was somewhat different than experienced in the
Holocene, but the sea level achieved some 5 metres higher than the current sea
level. The different conditions of the Eemian have been ascribed to different patterns
of orbital forcing compared with the current pre-industrial interglacial.
Modern Methane Trends
5 Ibid footnote page 1; Hansen & Sato, FULL PAPER
Atmospheric Methane and the Clathrate-gun Conjecture
To recapitulate: before 1750 natural sources maintained CH 4 levels at ~0.725ppm
(725ppb). The preindustrial value at 1850 has been quoted as 0.75ppm xv . The
following authoritative comment suggested current CH 4 levels appear to be very
close to a steady state, based on the relatively small rise observed since the mid
1990s.
xvi
A factor of 2.5 increase in the global abundance of atmospheric methane
(CH4) since 1750 contributes 0.5 Wm(-2) to total direct radiative forcing by
long-lived greenhouse gases (2.77 Wm(-2) in 2009), while its role in
atmospheric chemistry adds another approximately 0.2 Wm(-2) of indirect
forcing. Since CH4 has a relatively short lifetime and it is very close to a
steady state, reductions in its emissions would quickly benefit climate.
Sensible emission mitigation strategies require quantitative understanding of
CH4's budget of emissions and sinks. Atmospheric observations of CH 4
abundance and its rate of increase, combined with an estimate of the CH 4
lifetime, constrain total global CH4 emissions to between 500 and 600 Tg CH4
yr(-1). While total global emissions are constrained reasonably well,
estimates of emissions by source sector vary by up to a factor of 2
[emphasis added].
The recently resumed yearly rise is measured in a few parts per billion and follows a
levelling off around the turn of the millennium. (See below and footnote 6.)
Elusive methane sources traced (Editor’s summary; Nature; 2011)
Methane is a greenhouse gas with a significant warming effect on climate —
only water vapour and carbon dioxide are more important — yet the factors
influencing its atmospheric concentration are poorly understood. In particular,
a rapid rise in methane levels in the mid-twentieth century gradually (but
temporarily) levelled off around the turn of the millennium; the reasons for this
decline in growth rate are still being debated. Two new studies shed light on
this conundrum — but reach conflicting conclusions. Fuu Ming Kai et al.
measure differences in the concentration and isotopic signature of methane
between the Northern and Southern Hemispheres and conclude that reduced
microbial activity in wetlands was primarily responsible. Changing rice
agricultural practices seem to explain about half of the Northern Hemispheric
trend. By contrast, Murat Aydin et al. combine measurements of ethane
trapped in Antarctic ice with a simple atmospheric model and conclude that
the slow-down was caused by reduced methane emissions from fossil-fuel
production.
The rapid rise in CH4 that followed the rapid increase in the uses of petroleum and
natural gas and in agricultural expansion is unchallenged, but the reasons for the
subsequent decadal pause around the millennium, and the present component
6 Nature Editor's summary with links to abstracts; Elusive methane sources
traced; Nature 2011, August 11; AVAILABLE HERE
Atmospheric Methane and the Clathrate-gun Conjecture
strengths of individual CH4 sources and sinks, are still debated. For example, the
distribution and extent of water precipitation, itself a function of climate variation,
clearly has and will have effects on the rate of release of CH 4 from wetlands.
xvii
Atmospheric methane (CH4) is a major greenhouse gas and plays a key
role in the production of tropospheric ozone (Forster et al., 2007). After a
decade of near stable concentrations, the growth rate of atmospheric
methane has started to increase again (Rigby et al., 2008), with changes of
8.0±0.6 ppb in 2007 and 6.3±0.6 ppb in 2008 (update from Dlugokencky et al.,
2009 … with 60–70% being of anthropogenic origin (Denman et al., 2007). …
the positive emission anomalies from natural wetlands represents _80% of the 2007 global flux
anomaly and _50% in 2008.
This note on measurement of atmospheric CH 4 illustrates problems in identifying rate
changes in any one source.
xviii
The temporal and spatial behavior of the isotopic ratio of atmospheric
methane can be quite different from that of total methane. Changes in the
isotopic ratio are the result of shifts between isotopically different
methane sources, as well as changes in the total amount of methane into
which methane from sources is being diluted.
Methane pulses and the ‘Clathrate–Gun’ Conjecture
The end of ‘Snowball Earth’ 635Ma (noted above) is believed to have involved vast
releases from clathrates (methane hydrates).
There is a large literature centred on studies of the Santa Barbara sedimentary
basin, including a book by Prof. J.P. Kennett 7 that suggests the ‘Clathrate-Gun’ was
significant for warming events in recent ice-ages, including events that occurred
during the last glacial period and its termination. However, I have chosen rather to
quote extensively, if selectively, from Nisbet 2002,xix who was also one of the first to
set out the ‘Clathrate Gun’ hypothesis. He reviewed methane hydrates over the ages
and discussed in detail the more recent geological records since the last glacial
maximum.
My own original questions, sketched above in my Introduction, also appear in
Nisbet’s 2002 review, but are only in part answered. We do see though that
persistence of gas hydrate deposits over time has not been an ‘all-or-nothing affair’;
huge reserves seem to have persisted as such over millions of years. [We need to
distinguish, however, between hydrate reserves associated with sediments
containing buried organic material and stabilised by deep ocean pressure, and those
shallower deposits maintained by pressure/temperature conditions found at high
latitudes. See Archer, referred to below, for discussion of different deposits. There is
potential for abrupt release from deep ocean ‘structural’ deposits, but this potential
appears unrelated to anthropogenic climate change.]
7 ‘Methane Hydrates in Quaternary Climate Change: The Clathrate Gun
Hypothesis’; James P. Kennett (Editor), 2002
Atmospheric Methane and the Clathrate-gun Conjecture
The impact on the climate from CH4 release depends not so much on reserves but
on rate of release. This point is usefully discussed with regard to the next century
and beyond, also in Archer, 2007; quote:
xx
[‘Exceptions’ are hydrates not insulated from anthropogenic climate
change.] The exceptions are hydrate in permafrost soils, especially those
coastal areas, and in shallow ocean sediments where methane gas is focused
by subsurface migration. The most likely response of these deposits to
anthropogenic climate change is an increased background rate of chronic
methane release, rather than an abrupt release.
While the vast reserves hypothetically have the potential to take the climate back to
a catastrophically hotter world, it is the rate of release that matters for such rapidly
oxidised molecules.
[Both Nisbet and the later Archer paper can be accessed online 8. Nisbet sets the
scene for understanding the context of much of the clathrate research since 2002
and Archer makes a detailed ‘cool’ extensive 2007 summary.]
Nisbet in 2002 (ref xix) set out the original case for the ‘Clathrate-Gun’ as a trigger of
positive feedback climate responses, both past and future: I quote the following:
Methane is the third greenhouse gas, in order of importance (IPCC 2001), but
arguably the most likely to cause catastrophic change because the mass
needed to change climate is smaller than for carbon dioxide, and the mass
available is so large. Water vapour, by far the principal greenhouse gas, is a
‘consequential’ greenhouse force: its abundance is in consequence of the
changes in carbon dioxide and methane, though in turn it also affects both
carbon gases, by sustaining plant growth and so managing carbon dioxide,
and by controlling OH and hence methane.
… In the present-day air, methane is a much stronger greenhouse-warmer,
but after ca. 10 yr is oxidized to carbon dioxide. Thus, if a time-scale of, say,
100 yr is taken, for 90 of those years the carbon atom, formerly in the
methane, is in CO2, and for only 10 yr is it in methane. If, however, 20 yr is
taken as the reference time, then for half that time the carbon atom is in
methane. Broadly, on a weight-for-weight basis, the 20-year GWP of methane
is about 62 times that of carbon dioxide, while the 100 yr GWP is about 23
times that of carbon dioxide (IPCC 2001).
Thus a pulse of CH4 could be a potential trigger for further climate change:
The relative quantities needed for the same short-term climate forcing are
important. Geologically, it may be easier to release a ton of methane than 60 t
of carbon dioxide, if the time-scale of change is short.
Methane reserves as Clathrates are broadly in two compartments:
8 Nisbet 2002 Reference xix FULL PAPER; Archer 2007 Reference xx FULL PAPER
Atmospheric Methane and the Clathrate-gun Conjecture
The total methane hydrate reserves of the Earth are unknown. Estimates
(Kvenvolden
1988, 1998) have been made, though they vary hugely. There are two
principal types of hydrate: Arctic hydrate, stabilized mainly by low
temperatures in permafrost, on land and in the shallow Arctic Ocean; and
marine hydrates, stabilized mainly by pressure under sea water and some
hundreds of metres of sediment.
Thermal shocks at the surface of sediments produce inevitable warming at depth:
Thermal shocks, which occur when surface temperatures change, take time to
penetrate into sediment (Nisbet 1989). Thus, if the surface temperature
changes, say by 10◦C (as is expected in the high Arctic in this century), it may
take 100 yr for significant heating to penetrate 100m, and 1000yr to penetrate
500m into permafrost sediment. If the hydrate is 200 m down, then the
warming effect may take 100–500yr to have impact. But, though slow, the
warming is inevitable. . [Subsequent surface cooling will not prevent the
thermal penetration.] … warming cannot be halted. If the warming effect is
large enough, the hydrate will break down into water and free methane.
[However, there are recently developed numerical models that factor-in both icebound permafrost (IBP) and latent heat: see ‘Gas Hydrate Formation and Dissipation
Histories in the Northern Margin of Canada’; Majorowicz et al., 2012, reference xxvi;
discussed below. These recent Canadian studies appear to support Archer’s
contention quoted above: “The most likely response of these deposits to
anthropogenic climate change is an increased background rate of chronic methane
release, rather than an abrupt release.” I have even more recently read this paper
“On carbon transport and fate in the East Siberian Arctic land–shelf–atmosphere
system”, 2012, which makes a strong case for future monitoring of these processes.
As a ‘lay person’ I heartily endorse the authors’ case.]
At the recent glacial termination and subsequently there must have been, in total,
over time, large releases of methane from hydrate reserves both in the arctic and
elsewhere. Nisbet continues:
Arctic and marine hydrates need to be considered separately. There is very
abundant geological evidence for massive release from Arctic hydrate in the
Late Glacial/Early Holocene (Hovland & Judd 1988). Tropical offshore shelf
methane hydrate reserves are also huge. There is evidence for releases from
this reservoir also in the Early Holocene and Late Glacial (Maslin et al . 1998)
[but see later conclusions by others, below.]
According to Nisbet, persistent glaciation apparently primed methane hydrate
reservoirs for later release by two distinct modes, and at different geographical
locations.
Firstly, particularly in the Northern Hemisphere, GHs accumulate in the glaciated
area:
Atmospheric Methane and the Clathrate-gun Conjecture
Ibid
Hydrates tend to accumulate in glacial episodes. Methane sources include
both gas seeping upwards from the vast deeper-level organic-rich formations,
as they mature slowly, and gas from the decay of millions of km 2 of thick
interglacial peats and soil carbon that would have been extensively trapped
under ice. During glacial periods, especially after prolonged events, very
large-scale hydrate formation would occur.
And secondly:
Ibid
During glacial times, the sea level fell by up to 125m. This drop in water
overburden steadily reduced pressure on the hydrate layers in the marine
sediment. By the Last Glacial Maximum, marine hydrate layers worldwide
may have become critically destabilized by the pressure reduction.
There is much of interest discussed Nisbet, ref. xix, but the take-home point just now
might be that although past thermal shocks must have gradually de-stabilised some
CH4 gas hydrates, thus increasing chronic methane release and adding to warming
events during de-glaciations, these shocks did not cause sustained runaway
temperatures during the subsequent inter-glacial periods. Further methane-induced
positive feedback did not happen. Vast reserves of CH4 and other near-surface
carbon still remained. For example; the previous Eemian inter-glacial 125,000 years
ago achieved a greater global warmth (about +1°C with reference to year 2000,
according to ocean cores, see Hansen & Sato above), high enough to entail a 5m
higher sea level than at present, but did not provoke a self-stoking methane/CO 2
release sufficient to prevent later re-glaciation. In the last very few decades,
however, humanity is administering a powerful thermal shock to a still warm interglacial by inducing concentrations of non-condensing greenhouse gases that are
higher by a margin not seen in the past 2 - 5M or more years.
Methane related to temperature ‘shocks’ in both ‘cold’ and in ‘warming’ worlds
Sudden ‘warm shocks’ occurred both during glacial periods and at their termination;
and are discussed in this US Government source without reference to methane. The
events are referred to as abrupt warmth, in some cases followed by even deeper
cold:
9
Two different types of climate changes, called Heinrich and DansgaardOeschger events, occurred repeatedly throughout most of this time [last
glacial period]. Dansgaard-Oeschger (D-O) events were first reported in
Greenland ice cores by scientists Willi Dansgaard and Hans Oeschger. Each
of the 25 observed D-O events consist of an abrupt warming to nearinterglacial conditions that occurred in a matter of decades, and was followed
by a gradual cooling. … Related to some of the coldest intervals between D-O
events were six distinctive events, named after paleoclimatologist Hartmut
Heinrich,
9 NOAA Paleoclimatology reviews these events without discussing methane.
Atmospheric Methane and the Clathrate-gun Conjecture
Increased methane concentration associated with these warm periods has been
recorded trapped in ice-cores, but abrupt outbursts from marine hydrates seem to
have made little difference to the overall rise in methane during glacial termination
20,000 to 10,000 BP.
xxi
Throughout the glacial period and during the last transition, CH 4 changed
by up to 200 ppb(v) in parallel with rapid climate changes. Using the interhemispheric CH4 gradient in ice cores, an increase of high-latitude CH 4
sources in the Northern Hemisphere was derived for warm periods. However,
a more detailed quantitative source attribution is still missing.
…
[This] clearly shows that a CH4 contribution from deuterium-enriched marine
gas hydrates is not supported by the observed isotope changes. However,
quantification of a short CH4 outburst from marine hydrates will require more
high-resolution isotope data around rapid climate warming(s) together with
time-resolved isotope modelling of the atmospheric CH4 cycle and diffusion
effects in the firn [snow pack] column.
It was still possible until these more recent studies to point to ‘abrupt outbursts’ from
marine hydrates as the putative cause of abrupt warm periods. As late as 2004, the
‘clathrate-gun’ was mentioned in a study of sudden CH 4 emissions from deep-ocean
clathrate in a sedimentary basin associated with the outflow of a river system at
lower latitude in the Pacific. This study suggested there could be evidence of
instability of these deep marine gas-hydrates in a glacial world 39ka and 55ka, and
also referred to another 45ka warming:
xxii
Thus, although the area of the Fly River shelf [Gulf of Papua] is small, it
produced a significant deep-sea methane hydrate source for the global
methane cycle. … If such events were widespread and more common during
MIS 3, [marine isotope stage 3 at 45ka] this source could explain a large part
of the methane fluctuation recorded in ice cores as hypothesized in the
clathrate gun hypothesis.
Whatever combination of factors caused their abrupt onset these were warm
excursions in what remained a refrigerated world with a low sea level. In general,
most deep ocean marine hydrates, despite the lower sea level and pressure, appear
to have been relatively stable during climate change during the Pleistocene. See
below, and also in Sowers 2006xxiii and footnote 14.
The most recent ‘cold’ event was the Younger-Dryas (YD) that interrupted the last
de-glaciation, (footnote10) before sea levels rose by100-120m about 10,000 years
ago. In the view of these authors Whiticar & Schaefer, 2007, clathrate emissions,
have an isotope ‘signature’ deriving from the action of Archaeal bacteria; (but see
10 See the Younger Dryas Cold Event ca. 12.9–11.6, ka during the last
termination and a putative cold event during termination III, in ‘What Caused the
Younger Dryas Cold Event?’, Carlson A, 2010, Geology; doi:
10.1130/focus042010.1 v. 38 no. 4 p. 383-384; FULL PAPER
Atmospheric Methane and the Clathrate-gun Conjecture
footnote with implied query11), and in this case there is apparently no evidence in the
ice-core record of a ‘clathrate-gun’ restoring warming at the end of the YD 11.6ka,
despite the recorded rapid rise in CH4:
xxiv
The d13CH4–dD-CH4 signatures in segments 1 to 3 show that a massive
Archaeal hydrate release, i.e. ‘Clathrate Gun Hypothesis’ as proposed by
Nisbet (1990, 2002) and Kennett et al. (2002) is not responsible for the
methane increases at the YD–PB [Preboreal] transition period.
Thereafter, CH4 levels decreased by 100ppb from the end of the YD to 5,000 years
ago, before rising gradually to recent preindustrial levels.
Leaving aside the marine hydrates in deep-ocean sedimentary basins that are
probably, as Archerxxv puts it, “insulated from anthropogenic climate change”; we
should consider Arctic regions and their almost certainly more dynamic
accumulations of gas hydrate (GH).
Gas Hydrate Formation and Dissipation Histories in the Northern Margin of
Canada: referencexxvi
[I consider the 2012 paper discussed below to be the basis for any future update;
providing as it does, tools for future studies of arctic methane, which gas will
continue to contribute to the sudden climate change that humanity has ‘triggered’ in
our “Anthropocene”. The full paper is available online, but is highly technical.]
Canadian gas hydrates (GHs) have been studied intensively, though data is still
lacking for much of the Arctic continental margin. This study looks back to the past
14M years and presents numerical models based on the long term cooling of global
surface temperature (GST) when this ancient trend led to the onset of ice-age
oscillations nearly 3M years ago (41ky cycles) and thence to the deeper 100ky
cycles starting 1M years ago, which were a significant amplification of the climate
response to orbital forcing (i.e., Milankovitch cycles).
11 Archaeal bacteria, as both methanogenic (CH 4 generating) and methanotrophic
(CH4 oxidising) organisms, are responsible for a ‘Methane Cycle’ as a subset of the
Carbon Cycle and are agents also for sequestering carbon as carbonaceous
methane next to CH4 seeps from the ocean-floor. But see Majorwicz et al., 2012,
reference xxvi, who suggest that rich sources of gas hydrates in the arctic are derived
from thermogenic petroleum systems.
Atmospheric Methane and the Clathrate-gun Conjecture
Figure Detailed history of the last 5.5Myr of surface [temperature] forcing. From
Majorowicz et al. 2012 (previously modified from Paleoceanography, vol. 20, no. 1, pp.
1–17, 2005)
Numerical models based on past reconstructions for ice-bonded permafrost, (IBP),
and for gas hydrates, are compared with existing field observations. The study also
looks at a future world where doubled CO2 is imposed on present conditions.
The study in effect ‘samples’ the region by using data from representative different
deposits; each having different geological histories. Both ancient and relatively
modern gas hydrates exist under a wide range of dynamic stability conditions. Arctic
GH accumulated during glacial times from seepage from deeper petroleum sources
and has the signature of thermogenic natural gas (NG) derivation 12.
Ibid
Natural petroleum gas hydrates (GHs) are a significant feature of Canada’s
continental margins and Arctic regions [ref]. Two major types of gas hydrate
occurrences are found on Canada’s continental margins at a large range of
pressure-temperature [P-T] stability conditions [refs]. The first are essentially
terrestrial GHs that [originally] formed in Arctic regions on, or adjacent to the,
continental shelf in response to low ground surface temperatures during
glacial intervals … Part of both areas were transgressed [by encroaching
relatively warmer sea] in response to Holocene sea level rise and,
additionally, part of the [QEIG] coastline has emerged [into harsh relatively
low temperature surface conditions13 ] more recently as a result of isostatic
uplift following ice sheet removal [refs]. The second type [marine gashydrate] occurs on sub-Arctic continental margins, along the Pacific and
Atlantic-Baffin continental shelves, where water column pressure and
temperature [both] contribute to gas hydrate stability [refs]. The second type
of GHs probably [also] exist on the margins of the Arctic Ocean Basin, but a
lack of data, … precludes current description of much of the Arctic continental
margin. [Emphasis added]
12 Gas hydrates can derive from natural gas (NG) associated with petroleum sources, or
from biogenic gas, which is unrelated to the processes that form oil. DESCRIPTION
13 Majorowicz 2012 ref xxvi: “This process of temperature change from subsea, −1◦C, to harsh
terrestrial arctic temperature conditions, −20◦C, controls, to a large extent, the subsequent IBP and
GH formation.”
Atmospheric Methane and the Clathrate-gun Conjecture
Note that other marine hydrates elsewhere world-wide can be considered separately
from arctic marine hydrates; see Archerxxvii and Sowersxxviii. .
Different numerical models within Majorowicz et al. 2012 provide separate ‘cases’ in
order to account for the complex geographical distribution and geological histories of
different clathrate GH deposits, and to account for latent heat release when
modelling both GH formation and stability. For example, methane trapped below a
pre-existing impermeable seal does not form GH unless it is reached by the
propagating glacial cold front.
Ibid
[Case 2 numerical model] … illustrates well the interplay of the downward
propagating surface cooling and the latent heat release during permafrost and
gas hydrate formation.
Their models further suggest that propagating warm fronts do not completely
destabilise the GH layer, for example:
Ibid
[Models] show clearly that GHs have persisted through interglacial
episodes. Lower surface temperatures in the more northerly QEIG result in an
earlier appearance of GH stability that persists through glacial-interglacial
intervals, although the base of GH base stability varies up to 0.2 km during the
100 ka cycles.
Majorowicz et al. suggest that their modelled historical reconstructions appear
consistent with findings by Sowers14 who suggested marine hydrates had been
stable during previous abrupt temperature oscillations; the YD, OD, and one stadial.
In addition they predict that in future warmed arctic climates, thinning of the arctic
GH layers might be very small (although the absence of thick existing IBP layers
appears to govern the extent of ‘thinning’ of some GH layers, as in “non-IBP GHs”,
see below):
Ibid
… likewise, the impact of future climate change attending a doubling of
atmospheric CO2 does not completely destabilize the GH layer. Majorowicz et
al., [ref] predicted that future GH layer thinning is very small, and within the
range of previous natural cycle variations, in spite of the accelerated surface
warming accompanying climate change. This model results appear consistent
with the implications of recent observations of methane isotopic compositions
from ice cores for the “clathrate gun” hypothesis of Sowers et al., [see
footnote 14 below].
Nevertheless, they add the caveat:
14 Quote from Sowers, ref xxiii: “Because marine clathrates have a distinct deuterium/hydrogen (D/H)
isotope ratio … values during the end of the Younger and Older Dryas periods and one stadial period,
[suggest] that marine clathrates were stable during these abrupt warming episodes.” FULL PAPER
Atmospheric Methane and the Clathrate-gun Conjecture
Ibid
However, the GHs can destabilize rapidly in response to environmental
change in the marine non-IBP GHs [Emphasis added; the authors then
supply the reference to Nisbet discussed already above xxix].
They have acknowledged also the earlier warm PETM world 55Ma and the
underlying power and threat of marine hydrates:
Ibid
The Paleocene-Eocene Thermal Maximum (PETM) could have been
caused by abrupt releases of methane hydrates under the bottom of the
ocean [refs].
The real interest though in events that led to the PETM actually might lie more with a
quantitative comparison of these ancient events with the totality of human-induced
radiative forcing that has already begun, and whose sustained continuation is
inevitable for centuries. See the final section below.
.
----------------------Crossing a boundary: Carbon Isotope Excursions (CIE)
Interest has centred on the disrupted carbon cycle of the PETM as a possible
appropriate analogue for future events and ecologies in our human-disturbed world.
This more recent 2009 paper by Euan G. Nisbet et al. compares ‘trigger’ sources of
methane and CO2, and asks whether they were large enough to lead to the PETM:
xxx
The warm period known as the Palaeocene-Eocene Thermal Maximum
(PETM) was a period of climatic turmoil that lasted more than 100,000 years
[ref]. Ocean temperatures increased by 3-10°C and atmospheric
concentrations of methane and carbon dioxide rose sharply [ref]. Meanwhile,
the oceans became more acidic and the carbon isotope values of marine
carbonates declined significantly, a sign of a disrupted carbon cycle …
More data on the environmental conditions of the PETM are becoming
available, and it is now clear that the warming began several thousand years
before the main disruption to the carbon cycle that is documented in the
worldwide decrease in carbon isotope values [ref] often used to define the
event. [Emphasis added.]
Very importantly:
A recurrent release of greenhouse gases is therefore required to explain the
much longer-term warming in the PETM. …
The period between gas release events (repeat time) needs to be comparable
to, or shorter than, the atmospheric residence time of the warming gas,
otherwise the warming effect of one release event will fade before the
next event occurs. [Emphasis added.]
The above paper compares quantities of radiative forcing in the present day with
those that both led to and then sustained the PETM disorder.
Atmospheric Methane and the Clathrate-gun Conjecture
In particular, single events of methane release are examined as trigger events for a
cascade leading to sustained high levels of atmospheric non-condensing gases.
Single releases from sources such as ocean floor hydrates were/are not, individually,
sufficiently large, nor did they recur frequently enough, to act as trigger events for
subsequent self-sustaining high atmospheric concentrations, and these sources are
rejected as explanations for the PETM ‘trigger’. The authors, though, identify one
possible singular source of methane, the geologically brief Kilda Basin 55Ma. This
basin apparently has no large modern parallel although some modern Rift Valley
conditions provide qualitative parallels. The ancient Kilda Basin could have provided
a single source large enough to overwhelm the atmospheric OH’ oxidising sink and
thus prolong the atmospheric residence time of a massive methane release for many
decades. Hence, the release could have been big enough to promote a subsequent
very prolonged period of high CO2 concentration. (It is possible that the Kilda Basin
might have produced recurrent exhalations). Plausibly the trajectory to the inevitable
PETM was begun in this way. The authors speculate:
Unlike other suggested triggers, bursts of methane and carbon dioxide from
Kilda could have been large enough, and could have been repeated
frequently enough, to initiate the persistent global warming throughout the
PETM. Could the comparable injection of modern anthropogenic
emissions induce the same response from the planet? [Emphasis added.]
Thus, for now, my remaining query will be: Are ‘we’ the modern ‘Kilda Basin’?
We have signs these days of a disrupted carbon cycle. Given the ongoing
experiment we call the Anthropocene, scientists can take seriously the parallels with
the PETM and other ancient worlds.
There was a symposium in London at the Royal Society of Chemistry, Burlington
House, November 2-3, 2010, and abstracts are available on-line 15. Presentations
reviewed past CIE’s, particularly the Palaeocene Eocene thermal maximum (PETM,
55Ma), and discussion centred on these past ‘greenhouse worlds’ and mass
extinction events as analogues for future events and ecologies. One hopes these
conjectures eventually might be ruled out for our descendents, but as a matter of fact
one must recognise, as one author puts it:
Temperatures can fluctuate greatly from year to year but have risen
sharply since the 1980s. However, the change already experienced is
likely to be modest relative to warming predicted for the 21st century.
(Sparks)
Personally I do not wish to even think about a future PETM equivalent, even if it is
not imminent for a thousand or more years. The current human induced mass
extinction of biota and the emergence of a ‘New Climate’ are bad enough to
contemplate, even with scientific caveats about uncertainty.
15 Past CIEs and future ecologies; Burlington House, London, 2-3 November 2010
ABSTRACTS HERE
Atmospheric Methane and the Clathrate-gun Conjecture
1) Climate change is already projected to have a major impact on species,
with on average 15% to 37% of species committed to extinction by
2050 and between 1% and 43% of endemic species by 2100. (Berry)
2) Analysis of past abrupt climate changes show that some did carry early
warning signals in the form of „critical slowing down‟, whereas others
were purely „noise induced‟ (i.e. triggered by large natural variability)
with no early warning. Our analysis of recent climate data suggests
that a new climate state is already beginning to emerge. (Lenton)
I was much taken by the safety instructions for Burlington House displayed
prominently at the end of the programme ’flyer’;
If you hear the Alarm
Alarm Bells are situated throughout the building and will ring
continuously for an evacuation. Do not stop to collect your
personal belongings.
iREFERENCES
Greenhouse gases in the Earth system: setting the agenda to 2030; Manning et al.,
2011, Phil. Trans. R. Soc. A 28 May 2011 vol. 369 no. 1943 1885-1890 FULL PAPER
ii Gaia’s breath—global methane exhalations; K. A. Kvenvolden, B.W. Rogers; Marine and Petroleum
Geology 22 (2005) 579–590 FULL PAPER
iii (1) Ibid ref ii Gaia’s breath—global methane exhalations
(2) Detection and classification of atmospheric methane oxidizing bacteria in soil.;
Nature, 405, 175-178 (11 May 2000) Ian D. Bull. et al.
iv Ibid ref iii (2); Detection and classification of atmospheric methane oxidizing bacteria
in soil; Bull et al. 2000
v Enhanced chemistry-climate feedbacks in past greenhouse worlds; Proc Natl Acad
Sci U S A. 2011 Jun 14;108(24):9770-5. Beerling DJ, Fox A, Stevenson DS, Valdes PJ.
vi Concretionary methane-seep carbonates and associated microbial communities in
Black Sea sediments; J. Reitne et al; 2005; Palaeogeography, Palaeoclimatology,
Palaeoecology
Volume 227, Issues 1–3, 28 October 2005, Pages 18–30 FULL PAPER
vii The global methane cycle: recent advances in understanding the microbial
processes involved. Environmental Microbiology Reports, 1: 285–292.
doi: 10.1111/j.1758-2229.2009.00038.x ABSTRACT
viii
Ibid ref ii Gaia’s breath—global methane exhalations
ix Continuing decline in the growth rate of the atmospheric methane burden; Nature
393, 447-450 (4 June 1998) E. J. Dlugokencky, K. A. Masarie, P. M. Lang & P. P. Tans;
SEE ALSO BELOW Ref. xiv Dlugokencky et al., 2011
x Snowball Earth termination by destabilization of equatorial permafrost methane
clathrate;
Kennedy M, Mrofka D, von der Borch C. Nature, 2008 May 29; 453(7195):642-5.
xi Atmospheric CO2: principal control knob governing Earth's temperature; Lacis A.A.
et al. Science. 2010 Oct 15;330 (6002):356-9.
xii Ibid ref v Enhanced chemistry-climate feedbacks in past greenhouse worlds,
Beerling et al. 2011
xiii Changing boreal methane sources and constant biomass burning during the last
termination; Fischer et al., Nature, 2008, 17 April; Vol. 452; doi:10.1038/nature06825 FULL
PAPER
xiv Global atmospheric methane: budget, changes and dangers; Dlugokencky EJ, et al. Philos Transact A
Math Phys Eng Sci. 2011 May 28;369(1943):2058-72.
xv
Ibid ref ii Gaia’s breath—global methane exhalations
xvi Ibid ref xiv Global atmospheric methane: budget, changes and dangers;
Dlugokencky EJ, et al 2011
xvii Source attribution of the changes in atmospheric methane for 2006–2008; P.
Bousquet et al, Atmospheric Chemistry and Physics, 11, 3689–3700, 2011, FULL PAPER
xviii A note on isotopic ratios and the global atmospheric methane budget; P.P. Tans,
GLOBAL BIOGEOCHEMICAL CYCLES, 11, (1), 77-81, 1997
xix Have sudden large releases of methane from geological reservoirs occurred since the
Last Glacial Maximum, and could such releases occur again? E. G. Nisbet; Phil. Trans. R.
Soc. Lond. A 2002 360, 581-607 FULL PAPER
xx Methane hydrate stability and anthropogenic climate change; D. Archer,
Biogeosciences Discuss., 4, 993–1057, 2007, FULL PAPER
xxi Ibid ref xiii Changing boreal methane sources and constant biomass burning during the
last termination; Fischer et al., 2008
xxii Evidence for large methane releases to the atmosphere from deep-sea gas-hydrate
dissociation during the last glacial episode Thibault de Garidel-Thoron et al., 2004; PNAS _
June 22, 2004; 101; 25, 9187–9192 FULL PAPER
xxiii Late Quaternary Atmospheric CH4 Isotope Record Suggests Marine Clathrates Are
Stable; T. Sowers; Science 10 February 2006: Vol. 311 no. 5762 pp. 838-840 FULL
PAPER
xxiv Constraining past global tropospheric methane budgets with carbon and
hydrogen isotope ratios in ice; Michael Whiticar and Hinrich Schaefer; Phil. Trans. R.
Soc. A 2007, 365; 1793-1828
xxv Ibid ref
xx
Methane hydrate stability and anthropogenic climate change; D. Archer, 2007
xxvi Gas Hydrate Formation and Dissipation Histories in the Northern Margin of
Canada: Beaufort-Mackenzie and the Sverdrup Basins; J. Majorowicz et al.; Journal of
Geological Research Volume 2012 (2012), Article ID 879393 FULL PAPER
xxvii Ibid ref xx Methane hydrate stability and anthropogenic climate change; D. Archer, 2007
xxviii Ibid ref xxii Late Quaternary Atmospheric CH4 Isotope Record Suggests Marine Clathrates Are Stable;
T. Sowers, 2006
xxix Ibid ref xix Have sudden large releases of methane from geological reservoirs occurred since the Last
Glacial Maximum, and could such releases occur again? E. G. Nisbet
xxx Kick-starting ancient warming; E. G. Nisbet et al.; 2009, Nature Geoscience 2, 156
- 159 (2009)