Download The Role of Methane in Climate (Change)

Toward Improved Discussions of Methane & Climate
Posted on 1 August 2013 by Chris Colose
Update- See Correction of an error at bottom
Here at Skeptical Science, there is an ongoing effort to combat disinformation from those
who maintain that climate change is a non-issue or non-reality. From time to time,
however, individuals or groups overhype the impacts of climate change beyond the realm
of plausibility. Some of this is well-intentioned but misguided. For those who advocate
climate literacy or for scientists who engage with the public, it is necessary to call out this
stuff in the same manner as one would call out a scientist who doesn’t think that the
modern CO2 rise is due to human activities.
Many overblown scenarios or catastrophes seem to involve methane in the Arctic in some
way. There are even groups out there declaring a planet-wide emergency because of
catastrophic, runaway feedbacks, involving the interplay between high latitude methane
sources and sea ice.
The above two paragraphs set the tone of this discourse. AMEG (Arctic Methane
Emergency Group) is unjustly framed in this introduction as a fringe group using such
terms as “overhype”, “beyond realm of plausibility”, “overblown scenarios or
catastrophes”, “planet-wide emergency”. This is the complete opposite of the truth.
AMEG was founded based on a meeting in October, 2011 in the U.K. and I joined in
December, 2011. We are a group of concerned professionals with a varied background
including climate scientists, engineers, doctors, moviemakers, economists, journalists.
We have studied the Arctic, methane, sea ice, and climate change as a group since that
time, and individually for much longer. We base our work and analysis on
observations, not on models.
The facts on the ground and ocean in the Arctic region speak for themselves. The
PIOMAS work, which has been substantiated independently by Cryosat satellite data
show that the sea ice volume is trending downwards exponentially and if that trend
continued would reach zero around 2015 or 2016. Trending down even faster is the
May and June Arctic snow cover, as measured clearly by Rutgers data. Methane levels
in the Arctic have increased significantly over the last several years. In fact, the
mainstream scientific viewpoint was that the seafloor over the ESAS (Eastern Siberia
Arctic Shelf) was impermeable to methane outgassing. Then Shakhova, Yurganov,
and other Russian scientists measured outgassing plumes tens of meters in diameter
one year expanding to kilometers in diameter the very next year. Flask measurements
in Barrow, Alaska and Svalbaard indicated local levels of >2100 ppb and AIRS satellite
measurements over the last decade have shown greatly increase levels of methane in
the last few years. This is all observation, and not modeled by anybody. In fact, higher
methane emissions have been reported along the Arctic coastlines, presumably from
enhanced wave action due to larger wave action from the increased ice-free ocean.
Also, higher emissions have been measured elsewhere from continental shelves, for
example off the east coast of North America from warm Gulf Stream water that has
shifted eastward over the shelves, warming ocean temperatures several degrees.
Thus, the “radical” or “fringe” or “out-there” view is not from AMEG, quite the
opposite. Based on the precautionary principle, it is imperative that so called “mainstream” science examine this data without preconceptions that it takes centuries or
millennia for methane to outgas. It is unfathomable to AMEG and many others that
main-stream science are behaving like “methane denialists” when the observations are
clearly undermining such out-of-hand rejection, based on inaccurate models that are
clearly missing feedbacks. In fact the situation is so ridiculous that the IPCC is not
even considering methane as a strong feedback in their next report.
People on the street are now recognizing that the weather extremes are moving off the
charts in terms of frequency, severity, and spatial extent (mostly for extensive long
duration droughts, and also torrential rains causing floods). They are starting to
recognize that the collapse in Arctic albedo from declining snow cover and sea ice loss
is greatly amplifying the warming in the Arctic. This obviously lowers the temperature
gradient between the equator and North Pole which via simple physical laws slows the
jet streams making them wavier and stickier. This changing global circulation,
combined with 4% higher water vapor in the atmosphere is causing these weather
extremes.
Things are happening that have never been observed before in human history. Like
the rate of decline of sea ice and snow cover, the extensive cracking of sea ice this
March-2013, the “hole” forming near the north pole from relatively weak cycones, the
massive, long duration cyclone at the beginning of August-2012, and the list goes on
and on. AMEG being extreme? Hardly, more like science compartmentalization and
specialization being myopic to the collection of system changes that are screaming out
that the climate system has entered a period of abrupt change that has not been seen
before in human history, but has happened many times in the paleorecords. In fact,
rates of change now are at least 10x higher than any seen in the geologic record.
About a week ago, a Nature article by Gail Whiteman, Chris Hope, and Peter Wadhams
came out analyzing the "Vast Costs of Arctic Change." The Whiteman article is an
honest and thoughtful commentary about the economic impacts of a changing Arctic
climate. I will not comment on their economic modeling here, but rather on a key
scenario assumption that they use which calls for vast increases in Arctic-sourced
methane to the atmosphere. In this case, they have in mind a very rapid pulse of 50
Gigatons of methane emanating from the East Siberian Shelf (see image, including
Laptev and East Siberian sea). Note: 1 GtCH4= 1 Gigaton of methane = 1 billion tons of
methane. Whiteman et al. essentially assume that this "extra methane" will be put in the
atmosphere on timescales of years or a couple decades. This article has been widely
publicized because it calls for an average of 60 trillion dollars on top of all other climate
change costs. Since this was discussed in a prediction context rather than as a thought
experiment, it demands analysis of evidence.
In this article, I will argue that there is no compelling evidence for any looming methane
spike. Other scientists have spoken out against this scenario as well, and I will
encompass some of their arguments into this piece. In summary, the reason a huge
feedback is unlikely is because of the long timescale required for global warming to reach
some of the largest methane hydrate reservoirs (defined later) (no methane was expected
from ESAS since seafloor was thought to be impermeable, until it was measured to
rapidly outgas from one year to the next), and because no evidence exists for such an
extreme methane concentration sensitivity to climate in the past record (methane pulses
released over several years or a few decades is not detectable in ice cores since bubble
closure below firn takes about 50 years or more). Permafrost feedbacks are of concern,
but there is no basis for assuming a dramatic "tipping point" in the atmospheric methane
concentration. (no basis for this statement since observations show large increase in
methane)
The Methane Tour
Methane (CH4) is a greenhouse gas. It absorbs thermal energy that the Earth is trying to
shed into outer space, and can thus warm the surface of the planet. Its concentration in
the modern atmosphere is a little bit shy of 2 parts per million by volume (ppm),
compared to roughly 0.72 ppm in 1750 or 0.38 ppm in typical glacial conditions. Like
CO2, methane has not risen to modern day concentrations during the entirety of the now
~800,000 year long ice core record.
So what about Whiteman's scenario?
For perspective on how big 50 GtCH4 is, I've used data from David Archer's online
methane model to see how atmospheric methane concentrations would change in
response to such a big carbon injection. You can do this as a back-of-envelope
calculation by noting that 1 ppm is about 2.8 GtCH4 if it all stays as methane and isn't
removed, but this model lets you see the decay timescale too. For methane, the decay
back to original concentrations occurs within decades, whereas for CO2 it takes millennia
(CH4 is rapidly oxidized by the hydroxyl radical in the atmosphere). Therefore, CO2
dominates the long-term climate change picture but the methane spike can induce very
large transitory effects. (Keep in mind that the methane lifetime varies greatly
depending on the availability of the hydroxyl radical. On average it is 12 years,
however in dry regions like the Arctic with little water vapor it is longer, while at moist
equatorial regions it is shorter).
I've run two scenarios in which the 50 GtCH4 injection takes 1 year and 10 years to
complete (red and blue lines, respectively). The model starts with pre-industrial CH4
concentrations in years -10 through zero. The modern concentration of methane is shown
as a horizontal orange line.
Everything having to do methane in the ice core record resides below the orange line in
Figure 1 (at least within the resolution of the cores). So we're potentially talking about a
very big change, which the Whiteman article contends is likely to be emitted fairly soon
and should have implications for Arctic policy. (This graph clearly demonstrates that if
glacial ice bubble closure takes 50 years, then the pulse will not be captured. Also, the
molecular weight of CH4 is 16 compared to 30 or so for air (mostly N2) so the methane
does not stay around the surface for long).
For many, the primary concern about “big” abrupt changes in atmospheric CH4 stems
from the large quantity of CH4 stored as methane hydrate or in permafrost in the Arctic
region. These terms are defined below. It should be noted that globally, wetlands are the
largest single methane source to the modern atmosphere. Most of that contribution is
from the tropics and not from high latitudes (even if the Arctic was to start pumping
harder). The Denman et al., 2007 carbon cycle chapter in the last IPCC report is a useful
reference. (methane from wetlands in tropics has short lifetime due to extremely large
quantities of water and thus hydroxyl ions in that region, as opposed to methane from
the Arctic in much drier conditions)
Nonetheless, the Arctic is a region that is quite dynamic and is changing rapidly. The
high latitudes are currently a CO2 sink (this cannot be correct, since CO2
concentrations are higher in the Arctic than the global values measured at Mauna
Loa, for example) and CH4 source in the modern atmosphere, and it’s not implausible
that the effectiveness of the sink could diminish (or reverse) or that the methane source
could enhance in the future, since we expect a transition to a warmer, wetter climate with
an extended thawing season. This makes the carbon budget in the Arctic a “hot” place
for research.
In these discussions, it is important to clarify what sort of methane source we're talking
about.
Methane hydrate is a solid substance that forms at low temperatures / high pressures in
the presence of sufficient methane. It is an ice-like substance of frozen carbon, occurring
in deep permafrost soils, marine continental margins, and also in deeper ocean bottom
sediments. It's also very concentrated (a cubic foot of methane hydrate contains well over
100 times the same volume of methane gas).
On the decade-to-century timescale, the liberation of methane from the marine hydrate
reservoir (or the deep hydrates on land) should be well insulated from anthropogenic
climate change. Deep ocean responses by methane are a very slow response (many
centuries to millennia, Archer et al., 2009). Methane released in deep water also needs to
evacuate the water column and get to the atmosphere in order to have a climate impact,
although much of it should get eaten up by micro-organisms before it gets the chance.
These issues are discussed in a review paper by O’Connor et al., 2010. (Methane
response in deep ocean is not always slow, thus this section is very misleading.
Underwater landslides from slope instability or earthquakes are know to have resulting
in large methane pulses many times in the paleorecords. For example, Storegga off
Norway or off New Zealand, there are extensive pockmarks on the ocean floor
indicating abrupt episodic events. The mainstream view that methane outgassing from
deep water regions does not enter the atmosphere. If release is slow that is correct,
however rapid outbursts overwhelm the micro-organisms and result in large amounts
of methane entering the atmosphere. Even slower releases from deep water off
Svalbaard have been observed recently to enter the atmosphere; another unexpected
development).
There’s also carbon in near-surface permafrost, which is the more vulnerable carbon pool
during this century. Permafrost is frozen soil (perennial sub-0°C ground), and can also
encompass the sub-sea permafrost on the shelves of the Arctic Ocean. This includes the
eastern Siberian shelf, a very shallow shelf region (only ~10-20 m deep, and very broad,
extending a distance of 400– 800 km from the shoreline). This is a bit of a special
case. These subsea deposits formed during glacial times, when sea levels were lower
and the modern-day seafloor was instead exposed to the cold atmosphere. The ground
then became submerged as sea levels rose (going into the warmer Holocene). The rising
seas have been warming the deposits for thousands of years. Because of their exposure
during the Last Glacial Maximum, the shelves may be almost entirely underlain by
permafrost from the coastline all the way down to a water depth of tens or even a hundred
meters (e.g., Rachold et al., 2007 and this USGS page).
There's actually no good evidence of shallow hydrate on the Siberian shelves, even
though there are substantial quantities of subsea permafrost. Hydrate may exist deeper
down however, more than 50 meters below the seafloor. The stability of these hydrates is
sustained by the existence of permafrost, and it's not quite clear to what extent hydrate
can also be stored within the permafrost layer. (Permafrost people have an over-reliance
on uniform slab models which examine time taken for heat to propogate through the
slabs to melt the deep permafrost. They severely underestimate the fracturing and
nonuniform nature of the permafrost, presence of taliks, etc. All that is needed is one
weak spot or fracture region and heat can transfer downward much faster and further
than the models suggest. Similar slab models are used to estimate glacial ice melting
and they have clearly been incorrect and completely underestimate the rates of melting
from dynamic effects and Moulin pathways, for example.)
The estimates of the amount of methane in these various Arctic reservoirs are very
uncertain. Ballpark numbers are a couple thousand gigatons of carbon (GtC) stored in
hydrates in global marine sediments (e.g., Archer et al., 2009) of which a couple hundred
gigatons of carbon are in the Arctic Ocean basin, and between 1000-2000 GtC in
permafrost soil carbon stocks (e.g., Tarnocai et al., 2009) after you include the deeper
deposits. For comparison, there is a bit over 800 GtC in the atmosphere, of which about
5 Gt is in the form of methane, and estimated ~5000 GtC in the remaining fossil fuel
reserve. These numbers seem big compared to the atmosphere, but for methane direct
comparison isn't too relevant unless you put it in rapidly, since it has such a short lifetime
in the atmosphere. Large amounts of CO2, in contrast, last much longer.
A couple years ago, Shakhova et al. (2010a) reported extensive methane venting in the
eastern Siberian shelf and suggested that the subsea permafrost could become unstable in
a future warmer Arctic. Shakhova et al (2010b) cite ~1400 Gt in the East Siberian Arctic
Shelf, which comprises ~25% of the Arctic continental shelf and most of the subsea
permafrost. Shakhova et al (2010c) ran through a few different pathways in which they
argued for 50 GtCH4 release to the atmosphere either in a 1-5 year belch or over a 50-yr
smooth emission growth, which they suggest, “significantly increases the probability of a
climate catastrophe.” This assessment was the foundation for the concern in the recent
Whiteman Nature article, linked at the top.
The physical mechanism outlined by some of these authors is related to the rapid
reduction in Arctic summer sea ice observed over the last few decades, which allows for
greater amounts of solar radiation to penetrate the waters around the Arctic
shelf. Warming water propagates down in the well-mixed layers tens of meters to the
seabed, and might melt frozen sediments underneath. Because the shelf in this region is
shallow (compared to other regions), one doesn't need to wait a long time for the seafloor
to feel the atmosphere-surface forcing, and methane leakage might have an easier escape
path to the atmosphere. Allegedly, this has been leading to an acceleration of methane
flux.
Responses from Scientists
As a response to the first paper from Shakhova on enhanced methane fluxes, Petrenko et
al (2010) criticized the authors for misunderstanding several of their references and
primarily for the logical implications of their conclusions. For example,
“A newly discovered CH4 source is not necessarily a changing source, much less a
source that is changing in response to Arctic warming. Shakhova et al. do acknowledge
these distinctions, but in these times of enhanced scrutiny of climate change science, it is
important to communicate all evidence to the scientific community and the public clearly
and accurately” (Examination of the methane concentrations in the atmosphere in the
Arctic region from AIRS satellite data over a decade or so shows an obvious large
increase in the amount of methane, and has been corroborated with flask
measurements at locations across the Arctic, namely Barrow, Alaska and Svalbaard.
How is this not a changing source?)
Another paper, Dmitrenko et al (2011) reinforced this statement and came to the
conclusion that there is currently no evidence that Arctic shelf hydrate emissions have
increased due to global warming. This is also discussed in the review article by
O'Connor et al (2010, linked above). (Again, does one trust a direct observation or a
conclusion from a paper? Obviously the direct observation.)
The work done by the Dmitrenko paper shows that although the changing Arctic
atmosphere has led to warmer temperatures throughout the water column (over the
eastern Siberian shelf coastal zone), it takes a very long time for the permafrost feedback
at the bed to respond to this signal. They noted that the deepening of the permafrost table
should only have been on the order of 1 meter over the last several decades, which does
not permit a rapid destabilization of methane hydrate. (Deepening of the permafrost
table of 1 meter over several decades is based on a slab model and let to the erroneous
mainstream view that the seafloor over the ESAS was impermeable to methane release.
Measurements show otherwise.)
It is important to emphasize that simple point source emission estimates are not often
suitable for determining changed sources and sinks over the last few decades, and thus
don't tell you how that translates into atmospheric concentration. This should be kept in
mind when seeing dramatic videos of methane venting from a shelf or exploding lake,
which might not actually have much to do with global warming. (This is a very alarming
view, and would fit in fine on any of numerous climate denial websites. Rapid methane
emissions in the Arctic are what they are. Call a spade a spade.)
In 2008, there was a comprehensive report on Abrupt Climate Change from the U.S.
Climate Change Science Program, which is a bit dated but nonetheless makes a statement
reflecting most of current scientific thinking. Quoting Ch. 5 Brook et al (2008):
"Destabilization of hydrates in permafrost by global warming is unlikely over the next
few centuries (Harvey and Huang, 1995). No mechanisms have been proposed for the
abrupt release of significant quantities of methane from terrestrial hydrates (Archer,
2007). Slow and perhaps sustained release from permafrost regions may occur over
decades to centuries from mining extraction of methane from terrestrial hydrates in the
Arctic (Boswell, 2007), over decades to centuries from continued erosion of coastal
permafrost in Eurasia (Shakova [sic] et al., 2005), and over centuries to millennia from
the propagation of any warming 100 to 1,000 meters down into permafrost hydrates
(Harvey and Huang, 1995)" (Again, slab model thinking. Episodic events like
landslides negate these claims, as does fractures and other weakspots in the slabs
which allow pathways for huge heatflow. A good analogy is polyanas in sea ice that
allow for enormous heat flow between the ocean and the atmosphere in a sea ice field.)
Paleo-Analogs
One of the primary reasons we don't think there's as much methane sensitivity to
warming as has been proposed by Shakhova, and argued for in the Whiteman Nature
article, is because there's no evidence for it in the paleoclimate record. This has been a
point made by Gavin Schmidt on Twitter (a compilation of his many tweets on the topic
here) but the objections to the Nature assumptions have been further echoed in recent
days by other scientists working on the Arctic methane issue (e.g., here, here).
One can argue from a process-based and observations-based approach that we don't
understand everything about Arctic methane feedback dynamics, which is
fair. Nonetheless, the methane changes on the scale being argued by Whiteman et al.
should have been seen in the early Holocene (when Summer Northern Hemispheric solar
radiation was about 40 W/m2 higher than today at 60 degrees North, 7000-9000 years
ago). (Earth tilt was larger, so Winter Northern Hemispheric solar radiation was about
40 W/m2 lower than today at 60 degrees North. Thus, the ice formed much more
quickly and much thicker in the winter back then. Also, at night much more heat was
radiated out to space in the lower GHG world then as compared to our 400 ppm levels
today). Even larger anomalies occurred during the Last Interglacial period between
130,000 to 120,000 years ago, though with complicated regional evolution (Bakker et al.,
2013).
Both of these times were marked by warmer Arctic regions in summer without a methane
spike. It's also known pretty well (see here) that summertime Arctic sea ice was probably
reduced in extent or seasonally free compared to the modern during the early Holocene,
offering a suitable test case for the hypothesis of rapid, looming methane release.
(Incorrect, the summertime Arctic is not believed to be seasonally ice free during these
periods. The last time this happened was likely 2 or 3 million years ago.)
It should be noted that Peter Wadhams did offer a response recently to the criticisms of
the Whitehead Nature piece (Wadham is a co-author) but did not address why this idea
has not been borne out paleoclimatically.
Yesterday, an objection to the paleoclimate comparison cropped up in the Guardian
suggesting that the early Holocene or Last Interglacial analogs are not suitable pieces of
evidence against rapid methane release. They aren't perfect analogs, but the argument
does not seem compelling. (Colder winters in the early Holocene and Last Interglacial
and much colder nights (in summers and winters then) meant much thicker and
extensive ice formation in winters, and slower melting at night, respectively.
Compelling arguments.)The Northeast Siberian shelf regions have been exposed many
times to the atmosphere during the Pleistocene when sea levels were lower (and not
covered by an ice sheet since at least the Late Saalian, before 130,000 years ago, e.g.,
here). As mentioned before, when areas such as the Laptev shelf and adjacent lowlands
were exposed, ice-rich permafrost sediments were deposited. The deposits become
degraded after they are submerged (when sea levels increase again), resulting in local
flooding and seabed temperature changes an order of magnitude greater than what is
currently happening. Moreover, the permafrost responses have a lag time and are still
responding to early Holocene forcing (some overviews in e.g., Romanovskii and
Hubberten, 2001; Romanovskii et al., 2004; Nicolsky et al., 2012). A book chapter by
Overduin et al., 2007 overviews the history of this region since the Last Glacial
Maximum. These texts also suggest that large amounts of submarine permafrost may
have existed going back at least 400,000 years. It therefore does not seem likely that the
seafloor deposits will be exposed to anything in the coming decades that they haven't
seen before. (What is unique now is the extremely high concentration levels of CO2
(400ppm) and CH4 (>1900ppb). These high concentrations trap the heat in the
troposphere 24/7. Thus, at night heat loss is limited by the GHG blanket. At all
previous times the GHG blanket was much weaker, with CO2 ranging from 180 to 280
ppm and CH4 ranging from 350 to 700 ppb, or so. This makes an enormous
difference.)
What about other times in the past? Fairly fast methane changes did occur during the
abrupt climate change events embedded within the last deglaciation (e.g., Younger
Dryas), just before the Holocene when the climate was still fluctuating around a state
colder than today. These CH4 changes were slower than the abrupt climate changes
themselves, and have been largely attributed to tropical and boreal wetland responses
rather than high latitude hydrate anomalies. Marine hydrate destabilization as a major
driver of glacial-interglacial CH4 variations has also been ruled out through the interhemispheric gradient in methane and hydrogen isotopes (e.g., Sowers, 2006) (Episodic
events like landslides, as mentioned before, cannot be discounted. In fact geological
events like landslides occur at much higher frequencies when there is a rapid
temperature transition, as covered extensively in Bill McGuire’s new textbook. Also,
the text on “The Clathrate Gun hypothesis” cannot be completely discounted.)
To be fair, we don't have good atmospheric methane estimates during warmer climates
that prevailed beyond the ice core record, going back tens of millions of years. Methane
is brought up a lot in the context of the Paleocene-Eocene Thermal Maximum (PETM, 55
million years ago). During this time, proxy records show global warming at the PETM
(similar to what modern models would give for a quadrupling of CO2), extending to the
deep ocean and lasting for thousands of years. In addition, there were substantial amounts
of carbon released. It may very well be that isotopically light carbon came from a release
of some 3,000 GtC of land-based organic carbon, rather than a destabilization of methane
hydrates, although this is a topic of debate and ongoing research (see e.g., Zeebe et al.,
2009; Dickens et al., 2011).
It's also important to emphasize that any destabilization of oceanic methane hydrates at
the PETM, or any other time period, would imply that the carbon release is a feedback to
some ocean warming that occurred first- perhaps on the order of 1000 years
beforehand. Furthermore, once methane was in the atmosphere, it would oxidize to CO2
on timescales significantly shorter than the PETM itself (decades.) Unfortunately, there
is no bullet-proof answer right now for what caused the PETM, but rather several
hypotheses that are consistent with proxy interpretation. However, methane cannot be
the only story.
The Role of Methane in Climate (Change)
To be clear, CH4 is important as we go forward, and is already a key climate forcing
agent behind CO2 (coming in at ~0.5 W/m2 radiative forcing since pre-industrial
times). Additionally, methane is quite reactive in the atmosphere, and the effect of other
things like tropospheric ozone, aerosols, or stratospheric water vapor are partly slaved to
whatever is happening to methane (Shindell et al., 2009). This means methane emitted
has a bigger collective impact on climate than if you just do the radiative forcing
calculation by comparing methane concentration changes to what it was in 1750. (It is
important to point out an enormous misconception in public and scientific reports on
methane regarding the Global Warming Potential (GWP). A number in the low 20s is
almost always reported (22x, 25x…) and is based on a 100 year timescale. On a 20
year timescale, methane GWP is around 70x, and on a 1 or 2 year timescale the GWP
is >150x. Clearly, in terms of methane in the Arctic sourced from marine or terrestrial
permafrost the number of significance to sea ice and localized warming is 150x.)
Permafrost thawing is also going to be important in the coming century (this is a good
paper), and the uncertainties pretty much go one way on this. There's not much wiggle
room to argue that permafrost will reduce CH4/CO2 concentrations in the future. This is
also likely to be a sustained release rather than one big catastrophic event. For example,
permafrost was not included in Lenton (2008) as a "tipping point" for precisely the reason
that there's no evidence for any "switch" of rapid behavior change. (Exclusion of
methane as a “tipping element” in this paper by the “experts” in 2008 was based on
rates of change based on slab models, which recent observations of emissions has
clearly invalidated). Much of the carbon is also likely to be in the form of CO2 to the
atmosphere, and even implausible thought experiments of catastrophic methane release
(see David Archer's post at RealClimate) give you comparable results in the short-term as
to what CO2 is going to do for a long time.
Conclusion
The observed methane venting from the East Siberian shelf sea-floor to the atmosphere is
probably not a new component of the Arctic methane budget. Furthermore, warming of
the Arctic waters and sea ice decline will likely impact subsea permafrost on longer
timescales, rather than the short term. (Is this author so sure of this as to be willing to
stake the stability/instability of the entire global circulation system on this?)
Methane feedbacks in the Arctic are going to be important for future climate change, just
like the direct emissions from humans. This includes substantial regions of shallow
permafrost in the Arctic, which is already going appreciable change. Much larger
changes involving hydrate may be important longer-term. Nonetheless, these feedbacks
need to be kept in context and should be thought of as one of the many other carbon cycle
feedbacks, and dynamic responses, that supplement the increasing anthropogenic CO2
burden to the atmosphere. There is no evidence that methane will run out of control and
initiate any sudden, catastrophic effects. (There is not evidence that methane will not
run out of control, in light of large increases of concentrations in recent years). There's
certainly no runaway greenhouse. Instead, chronic methane releases will supplement the
primary role of CO2. Eventually some of this methane oxidizes into CO2, so if the
injection is large enough, it can add extra CO2 forcing onto the very long term evolution
of global climate, over hundreds to thousands of years.
Errata Update: Gavin Schmidt let me know that in the first version of this post, I used
gigatons of carbon instead of gigatons of methane. I mistakingly read the Shakhova paper
as an injection of carbon. Since the molecular weight of carbon is 12 g/mol, and CH4 is
16 g/mol, then 1 GtC=1.33 GtCH4. The figure in the post has been revised accordingly
and doesn't impact the argument here.