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6 Methane
hydrates in the sea floor
Large
quantities of carbon are stored in the sea floor in the form of methane
hydrates, with an order of magnitude comparable to the global occurences
of coal. There are risks associated with methane hydrates due to climate
change as well as ocean mining. There are, however, considerable uncertainties
and gaps in knowledge, so that only a preliminary evaluation of these
risks is possible.
6.1 The
methane hydrate reservoir
Gas
hydrates – such as methane hydrates – are solids composed
of water molecules that have gas molecules enclosed within their crystal
lattices. Carbon dioxide, hydrogen sulphide and methane molecules have
the right size to be trapped inside such a hydrate cage. Methane hydrates
look like dirty ice and are flammable. They store large quantities of
methane within a very small space: in the transition to the gas phase
their volume increases by a factor of 170.
They
are only stable under specific pressure and temperature conditions. The
higher the ambient temperature, the higher the pressure has to be to prevent
the methane hydrate from dissolving. The optimal conditions are typically
found on the sea floor at water depths of at least around 500 m, and in
the Arctic starting already at lower water depth. Here, methane hydrate
can form in the sediments provided sufficient quantities of methane are
produced by the decomposition of organic carbon deposits. The carbon for
the methane hydrate is ultimately derived from the biological production
of the ocean, as dead biomass is deposited in the sediments and bacterially
decomposed on the sea floor (‘biogenic’ methane). The formation
of methane hydrates takes a very long time, so they cannot be considered
as a renewable energy source: the present deposits have probably been
formed over a period of several million years (Davie and Buffett, 2001).
An additional, smaller hydrate source is found in leaking natural gas
formations (‘thermogenic’ methane) from which methane bubbles
rise through the sediments and under favourable conditions (i.e., in the
hydrate stability zone in the cooler upper sediment layers) form hydrates
with water. An example can be found in the Gulf of Mexico.
As
the temperature in the sediment quickly rises with increasing depth due
to the Earth’s heat (at around 30 °C per kilometre) but the
pressure – also increasing – cannot compensate for the temperature
increase, methane hydrates in marine sediments are only stable down to
a certain depth in the sediments. Below the limit of this stability zone,
typically several hundreds of metres thick, methane can again occur as
a gas in the sediments.
Gaining evidence for the presence of methane hydrates, directly by drilling
or indirectly with seismic techniques, is difficult. While the drilling
that has been carried out up to now does not allow broad-area mapping
of its occurrences, seismic methods can only identify the lower limit
of the stability zone. On this basis, no conclusive statement can be made
about the quantity of methane in sediments, because the volume of the
hydrate remains unknown. These measurement problems mean that models must
be used to estimate the global reservoir of methane hydrates. In the 1990s
it was assumed that carbon quantities on the order of 10,000 Gt C were
stored in the form of methane hydrates (that equates to around twice the
entire fossil energy resource: Rogner, 1997), but current estimates suggest
a much lower value (500–3000 Gt C: Buffett and Archer, 2004; Milkov,
2004). Klauda and Sandler (2005) presume that the largest hydrate occurrences
are in the deep-sea basins rather than on the continental margins. They
therefore also report a much higher estimation of 78,000Gt C, but this
is based on unrealistic assumptions of the sedimentation rates of organic
carbon in the deep sea. WBGU considers the estimate of 500–3000
Gt C to be reliable. A comparable amount of methane is present again below
the hydrates in the gaseous state (Archer, 2005). Here are some figures
for comparison: at the end of 2004 the proven coal and natural-gas reserves
amounted to 900 Gt C and, respectively, 92 Gt C (BP, 2005); the atmosphere
contains 805 Gt C of carbon dioxide, of which 210 Gt C stem from anthropogenic
emissions.
6.2 Methane
release due to human intervention
The
stability of methane hydrate deposits can be affected on the one side
by global warming; on the other side, however, there are risks of
an unintentional release of methane associated with the production
of oil, natural gas, and possibly in the future of methane hydrate
itself.
6.2.1 Response
to pressure and temperature changes
Changes
of pressure and temperature in the hydrate layer lead to changes in
the stability zone, i.e., the depth interval in the sediment where methane
hydrate is stable. Higher pressure stabilizes the methane hydrate, while
warming reduces the thickness of the stability zone. Due to warming,
methane hydrate will normally thaw from below (Fig. 6.1-1). Figure 6.1-1a
uses a phase diagram to illustrate the stability zone in the ocean and
in the underlying sediments. The red curve indicates temperature: in
the ocean it decreases with increasing depth, and in the sediments it
increases again due to the Earth’s internal heat. The black curve
shows the temperature below which methane hydrate is stable, as determined
by the ambient pressure conditions. This means that methane hydrate
can only exist in sediments within the depth interval where the actual
temperature (red) is below the stability temperature (black). So the
point where the two curves cross in the sediment represents the lower
boundary of the stability zone.
Figure
6.1-1
Changes in the methane hydrate layer due to warming.
The black curve describes the stability temperature dependent on depth.
The red curve shows the actual temperature; red dashed lines show schematic
temperature profiles after a warming of 3 °C (stability zone of
hydrates becomes thinner from the bottom) and 8 °C (stability zone
completely disappears), respectively.
Source: WBGU
If
the ocean warms by 3 °C, then the red temperature curve shifts by
the corresponding amount to the right (Fig. 6.1-1b). The new point of
intersection of the temperature and stability-temperature curves defines
the new lower boundary of the stability zone, which has shifted upward.
The amount of gaseous methane below the hydrate layer has also increased
by the corresponding amount.
Figure
6.1-1c assumes that the ocean rapidly warmed by 8 °C, so that the
temperature curve is completely to the right of the stability-temperature
curve, and therefore hydrate is no longer stable at any depth. Whereas
with a 3 °C ocean-temperature increase the total sediment depth
down to the base of the stability zone first has to warm before the
methane hydrate begins to dissolve at all, in the example with an 8
°C increase the destabilization of the hydrate would begin at the
sea floor, i.e., before the total sediment layer has warmed. In the
course of the temperature rise the methane hydrates would dissolve completely
from above.
6.2.2 Effects
of climate change on methane hydrates
Global
warming leads to temperature changes in the ocean as well as to changes
in sea level, and therefore to pressure changes on the sea floor. Figure
6.1-2 provides an overview of the effects this can have on methane hydrate
deposits.
Figure
6.1-2
Causes and effects of methane hydrate destabilization.
The mechanisms are discussed in the text. Numbers above the arrows indicate
the respective time scale of the process in years (no number given =
immediate effect).
Source: WBGU
In
pessimistic IPCC scenarios the average sea-surface temperature increases
by the end of this century to 5 °C above the pre-industrial level.
Regionally, for example in the Arctic, this value could be as great
as 10 °C. The high latitudes are of global importance because it
is here that the cold-water masses originate that fill the deep sea
worldwide. Because of the stable temperature layering and the slow mixing
of the ocean, the warming, as a rule, will only penetrate to the sea
floor very slowly, over the course of several centuries. Similar time
frames are necessary in order to warm the sediment layers down to several
hundreds of metres. Only under very special local conditions –
with hydrate occurrences at shallow sea depths and in well-mixed marine
regions – could hydrates become unstable in the short term (within
this century) due to warming. An escape of hydrates on a large scale
(that is, enough to have a noticeable impact on climate) is not an acute
but a long-term danger. Over a period of centuries a reinforcing feedback
loop with global warming could occur, which over time could become increasingly
difficult to check.
Relatively
rapid and intense local temperature changes could occur when marine
currents are altered, a danger that is commonly discussed with respect
to the northern Atlantic (Section 2.1.3). The development of temperature
at the sea floor seems to depend strongly on how the circulation changes
(Mignot et al., submitted) and is therefore difficult to predict. Simulations
suggest, however, that after a breakdown of the deep-water formation
in the Norwegian Sea the bottom temperature in some regions of the North
Atlantic could quickly rise by over 7 °C. Changes at this order
of magnitude could then also destabilize hydrate reservoirs.
An
additional factor is the rising sea level, which, by increasing the
pressure on the sea floor, could in principle stabilize the hydrate
deposits. Here only the volume of water released by melting land ice
masses is relevant because thermal expansion would not increase the
pressure. The effect, however, is very small: in water depths of 400
m a pressure increase of 0.04 MPa (corresponding to a sea-level increase
of 4 m) results in an increase of the destabilization temperature of
less than 0.1 °C. The long-term sea-level rise can therefore not
compensate for the effect of the long-term warming on hydrate stability.
The same is true for short-term relative changes in sea level resulting
from circulation changes (Levermann et al., 2005), the results of which
cannot compensate for the abrupt temperature changes they also cause.
If
the methane hydrate stability zone is reduced, then methane gas forms
below the hydrate layer. This gas can either penetrate through the hydrate
layer and escape out of the sea floor through small channels or permeable
sediment layers, or it can blast through the hydrate layer if sufficient
quantities of gas collect below a continuously thinning layer. In such
a blowout large amounts of methane gas are abruptly released. Because
the shattered blocks of methane hydrate released are less dense than
water, they rise to the surface and dissolve there.
The
quantity of methane gas that would escape from the hydrate layers by
one of these mechanisms in the future can presently only be roughly
estimated, because the stability and permeability of sediment layers
are dependent on highly variable local conditions.
6.2.3 Mining
of methane hydrates
Methane
hydrates represent a source of fossil fuel and can therefore be of interest
for commercial exploitation. The economic feasibility of their recovery
depends greatly on the available methane concentration in the hydrate.
The few examples of practical experience obtained in exploiting methane
from hydrate deposits are from the Messoyakha gas field (Siberia) and
the Mallik (Alaska) research project. The Russian Messoyakha gas field
is an occurrence below permafrost that was discovered as early as the
1960s. Not only were the mining costs here extremely high, but it has
also come into question whether the methane recovered here in the 1970s
really was, as claimed, retrieved from hydrate deposits (EIA, 1998;
Schindler and Zittel, 2000a). Mallik 2002 is a drilling project on the
Arctic coast of Canada, where the methane concentration of the hydrate
is rated similar to that found in Japanese coastal waters. The project
included gas hydrate production tests and is part of an international
research consortium in which states (incl. USA, Japan, India and Germany)
and companies are participating.
In
principle, the mining of methane hydrates on the high seas would be
possible. It is considered technically feasible to drill into the sea
floor in water depths up to four kilometres. The technical and especially
the economic practicability of potential recovery mining methods is
a subject of research in which Japan and the USA are playing particularly
important roles. The Japanese programme for methane hydrate mining (National
Methane Hydrate Exploitation Program, MH21), among other aspects of
methane hydrate research, is expressly pursuing the ambitious goal of
beginning production tests in 2007 and is aiming to have the technology
for commercial large-scale production by 2012 (MH21, 2005). Financing
for the US American methane hydrate research programme (Methane Hydrate
Research and Development Act of 2000) was extended through 2010 by the
Energy Policy Act of 2005. Commercial mining of methane hydrate in US
American waters is deemed possible by 2015 and large-scale mining by
2020 (DOE-NETL, 2005; Ray, 2005).
These
expectations are compatible with the estimation that methane hydrate
mining will be economically feasible in some regions within the next
5–10 years, while it would take 30–50 years before worldwide
massive mining is possible (Methane Hydrate Advisory Committee, 2002;
Collett, 2005). Methane hydrate exploitation in permafrost areas on
land could reach industrial proportions more quickly than the exploitation
from the sea (Johnson, 2004). That is because progress in the identification
and evaluation of occurrences feasible for exploitation on land is ahead
of that for occurrences beneath the sea. In addition, there has already
been extensive experience gained in recovery and production technology
on land (Mallik research drilling, Messoyakha gas field). The more favourable
recovery conditions compared to the sea also make it likely that mining
will first be carried out on land. In combination with economies of
scale and learning effects, there could therefore be cost advantages.
Overall, this means that there is an initial advantage for methane hydrate
exploitation on land over that at sea. The predicted technological feasibility
as well as the economic and energy-strategic potential of this kind
of energy production, however, is critically questioned and considered
to be clearly overestimated (Schindler and Zittel, 2000b).
Targeted research into the production of marine methane hydrate has
been limited so far to a few pilot studies. They probably will not go
beyond the stage of feasibility studies during this decade.
6.3 Possible
results of methane release
The
consequences of a release of methane gas from hydrates depend on the
mechanism – ‘diffusion’ or ‘blowout’ –
as well as the time scale of the release.
When methane gas diffuses through the hydrate layer and slowly escapes
in small bubbles from the sea floor, a large portion of it will probably
be dissolved in the water column as it rises. A new study shows, however,
that methane bubbles could also possibly rise through the upper water
layers and escape into the atmosphere (Sauter et al., 2006). Dissolved
methane in the ocean has a lifetime of about 50 years before it oxidizes
to H2O and CO2. A large portion of the released methane would therefore
be released to the atmosphere before it oxidizes. Firstly, the remaining
oxidized portion would increase the concentration of dissolved inorganic
carbon in the ocean, which contributes to further acidification (Section
4.1). Secondly, an equivalent decrease in oxygen concentration would
occur. For comparison: in order to exhaust all of the 2 · 1017
mol of oxygen contained in the ocean, it would have to react with 1000
Gt of methane (Archer, 2005). Thirdly, in the long term, a new carbon-equilibrium
state would be established between the atmosphere and ocean, over the
course of about 1000 years, and about one-fifth of the carbon incorporated
in the ocean released into the atmosphere. The concentration of CO2
in the atmosphere would thereby increase, strengthening the greenhouse
effect. Hence, over the long term, this effect would come about in any
case: the result is the same whether methane escapes directly into the
atmosphere and oxidizes there to CO2, four-fifths of which is gradually
taken up by the ocean, or if it is first released in the ocean, oxidized
there, and one-fifth is given off to the atmosphere.
When
large quantities of methane are suddenly released, most of it will reach
the water surface and abruptly increase the methane concentration in
the atmosphere. Because methane is a considerably more effective greenhouse
gas than CO2 (around 25 times stronger per molecule) due to its much
lower concentration and therefore less saturated absorption bands, the
effect of comparatively low amounts of methane is significant. But atmospheric
methane quickly oxidizes (with an average residence time of eight years),
to CO2, which accumulates in the atmosphere due to its long life expectancy,
so that in the long term the escaped methane after its oxidation to
CO2 has an even greater impact on climate than before.
Figure
6.2-1 shows how anthropogenic CO2 emissions can lead to methane emissions
from hydrate deposits over the coming millennia. A total emission of
1000 Gt CO2 is assumed. Figure 6.2-1a reveals how strongly this could
cause the atmospheric methane concentration to increase, whereby the
uncertainty of the time scale of the release is taken into account with
three different assumptions.
Figure
6.2-1
Atmospheric methane concentration for a scenario
with a total quantity of 1000 Gt of anthropogenic CO2 emissions (a).
The curves describe the resulting methane release over different time
frames (1, 10, and 100 thousand years). Climate-impacting radiative
forcing for the case of the shortest release period of 1000 years (b).
This is a combination of the forcing due to methane itself (green; it
gradually oxidizes to CO2 and thus disappears), that due to anthropospheric
CO2 emissions (black), and CO2 from the oxidation of methane. The last
two together yield the radiative forcing due to the total increase of
CO2 (red).
Source: Archer und Buffet, 2005
Figure 6.2-1b illustrates the climatic consequences of the methane emissions
for the 1000 Gt of CO2 scenario for the case of a methane release within
1000 years. The results are caused both directly through the increase
in atmospheric methane concentration (green), as well as on a longer
time scale by the increase of the CO2 concentration (red). Although
the direct methane effect is lower than that of the original anthropogenic
CO2 emission, the subsequent increase in CO2 concentration through oxidation
of the methane leads, over the long term, to a near doubling of the
greenhouse effect.
Methane eruptions can also present other dangers. They can destabilize
continental slopes and trigger large submarine landslides, which can
then possibly result in the break-up of additional hydrates. Evidence
of such slides can be found on the sea floor. For example, in the Storegga
landslide off the coast of Norway around 8000 years ago, an average
of 250 m of the continental slope with a width of 100 km were transported
downslope (Archer, 2005). This event triggered a tsunami that was at
least 25 m high off the Shetland Islands and at least 5 m high along
the British coast (Smith et al., 2004). The amount of methane released
by this landslide is estimated at 0.8 Gt C (Archer, 2005). When this
amount of methane directly enters the atmosphere, it can alter the radiative
forcing by as much as 0.2 W per m2 (for comparison, today’s radiative
forcing due to anthropogenic greenhouse gases is 2.7 W per m2). This
example illustrates that an abrupt release of methane, even in the case
of a large catastrophic slide of the continental slope, would only have
a relatively minor impact on climate.
6.4 Recommendations
for action: Preventing methane release
Through
the warming of seawater, anthropogenic climate change can lead to a
destabilization of methane hydrate deposits on the sea floor. According
to the present state of knowledge, however, the danger of a sudden release
of large, climate-influencing quantities within this century is very
small. Of much greater importance is the probability of a continuous
methane release over many centuries to millennia due to the slow intrusion
of global warming into the deeper ocean layers and sediments. The consequences
of human actions persist in this respect not just over centuries, but
could influence the Earth’s climate over tens of thousands of
years.
Limiting
global warming follows once more here as a recommendation for action,
because methane release from hydrates could further amplify climate
change in the long term. This feedback effect presents the danger that
humankind could lose control of the greenhouse-gas concentration in
the atmosphere, as the outgassing of methane from the sea floor cannot
be controlled or limited.
There
is already a need today for institutional action with regard to marine
methane hydrate deposits. This is with respect to, for one, the targeted
mining of marine methane hydrates, and for another to the unintentional
release of methane that could occur during sea-floor mining.
Theoretically, efforts to recover methane from hydrates could unintentionally
trigger their release into the environment, in the worst case as a sudden
eruption. The risks of this have not yet been sufficiently investigated
(Archer, 2005). A leak of methane into the environment during mining
would unnecessarily amplify global warming. In the worst case even a
slope slide could be caused that could trigger a tsunami.
The
risks associated with mining are very variable depending on the geological
conditions. The risks of methane mining therefore have to be carefully
reviewed for each individual case. An environmental impact assessment
along with monitoring according to universal standards is necessary
for every case.
The
International Seabed Authority, an institution of the international
United Nations Convention on the Law of the Sea (UNCLOS), is responsible
for methane hydrate deposits as well as for other resources on the sea
floor outside the exclusive economic zone. The Authority grants mining
licenses and monitors mining operations. Its regulations adopted in
2000 for the exploration of deep-sea mineral resources contain various
environmental aspects. This is a starting point for agreement on concrete
standards for mining marine methane hydrate on the high seas. In the
opinion of WBGU it is furthermore necessary to improve and expand the
monitoring system. It is, however, important to note here that so far
‘only’ about 150 countries have ratified UNCLOS, and of
those only about 120 countries have ratified the rules governing seabed
resources (those who have not signed include, for example, Iran and
the USA). A
framework within which more countries can be persuaded to accede to
the agreements for maintaining universal standards in hydrate mining
still needs to be worked out. Also needed are agreements binding under
international law for the mining of methane hydrates in marine regions
that lie within the territorial sovereign rights of coastal nations
(Box 2.6-1). This is necessary considering that both the above-mentioned
Japanese pilot project and American plans target future commercial methane
production from hydrate deposits in national coastal waters.
The
danger of methane hydrate release also exists in principle in other
sea-floor mining activities. If methane were to be destabilized and
unintentionally released in the mining of resources, these emissions
would hardly be measurable, and therefore not accounted for in the emissions
inventory of a country, or only insufficiently so. The applicable IPCC
guidelines of 1996 for national emissions inventories do not include
methane that is unintentionally emitted at sea. WBGU therefore recommends
for the upcoming reworking of the guidelines in 2006 that this omission
be corrected despite the difficulties in measurement. But at the very
least a reporting obligation should be introduced for such releases
of methane.
6.5 Research
recommendations
Because
estimates of the risks of methane release are still hampered by large
uncertainties and gaps in knowledge, there is a significant need for
research. To begin with, the methane occurrences need to be more extensively
mapped and quantities estimated. The primary focus here should not be
on the potential workable deposits, but on occurrences that could possibly
become destabilized by climate change, and on the danger of slope slides.
Furthermore, modelling studies should be employed to investigate which
regions of the ocean show the greatest risk for hydrates to become destabilized
through global warming.
While research on the long-term stability of marine methane hydrates
and climate protection implications should continue to be strengthened,
WBGU sees no need for government subsidies for applied research for
the mining of marine methane hydrates. Public funding of such projects
does not seem purposeful because mining poses considerable risks and
methane hydrates do not represent a sustainable energy source.
There would, however, be a need for targeted natural science research
if appropriate standards for the mining of marine methane hydrates need
to be defined. Natural science investigations should be supplemented
by social science and legal studies of the prospects for worldwide implementation
of such standards.
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