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How long a warming event takes to affect gas hydrate sta-
bility and whether methane released from gas hydrates is
transported to the atmosphere depend, to a large extent, on
where gas hydrates are located. Based on published distri-
bution estimates, Ruppel (2011) summarizes five general
categories or “zones” of gas hydrate occurrence and the
extent to which predicted climate change would result in
the transport of methane from gas hydrates to the atmos-
phere (Figure 3.10). Following the work of Ruppel (2011),
estimates of gas hydrate sensitivity to warming are given
as percentages of the global gas hydrate store, assuming
99 per cent of the world’s gas hydrates are located in deep-
water marine environments (zones 3-5 in Figure 3.10), and
1 per cent are associated with permafrost, either on land or
submerged in shallow Arctic shelf regions (zones 1 and 2 in
Figure 3.10) (McIver 1981). The percentages given by Rup-
pel (2011) depend on whether future studies uphold the as-
sumed balance between marine and permafrost-associated
gas hydrate volumes. For a sense of scale, even 1 per cent
of the estimated global supply of methane in gas hydrates
(5 000 GtC) is equivalent to 25 times the estimated global
consumption of methane in 2020 (2.15 GtC), based on con-
sumption estimates from the (U.S. Energy Information Ad-
ministration, 2010).
1: Terrestrial Arctic environments
Less than 1 per cent of the world’s gas hydrates are likely to
exist in this environment (Zone 1 in Fig. 3.10). Because the
presence of permafrost dramatically slows the transfer of
heat to the depths at which gas hydrates exist, time scales in
excess of 1 000 years are necessary for atmospheric warm-
ing to begin dissociating gas hydrates at the top of the gas
hydrate stability zone (Ruppel 2011). On an extremely local-
ized scale, thermokarst lakes may provide a conduit for more
rapid delivery of heat into the subsurface to dissociate gas
hydrates. Gas-venting pockmark features beneath delta lakes
and channels at the edge of the Mackenzie Delta have been
attributed to gas hydrate dissociation (Bowen
et al.
2008). As
noted in Ruppel (2011), however, methane seeps in terrestrial
Arctic environments may be carrying methane from deeper
hydrocarbon reservoirs, rather than from gas hydrates break-
ing down due to warming. Identifying the methane source in
this sector is an important research focus.
2: Flooded permafrost environments (<100
metres water depth)
Given the assumption that 1 per cent of the world’s gas hy-
drates exist in polar regions, and much of that 1 per cent
exists below terrestrial permafrost, Ruppel (2011) estimates
less than 0.25 per cent of the global gas hydrate volume is
found in flooded permafrost regions (Zone 2 in Fig. 3.10).
Gas hydrates in Zone 2 are also buried beneath about 200
metres of sediment, and it is not likely that human-activi-
ty-related warming trends are affecting them significantly.
However, this sector has experienced significant warming,
because coastal flooding that occurred about 13 500 years ago
generated up to 17 °C of warming (Shakhova
et al.
2010b) at
the sediment surface. This warming continues to thaw and
degrade both permafrost and underlying gas hydrates (Sem-
iletov
et al.
2004). In these shallow environments, methane
gas released from the sea floor can pass through the water
column and enter the atmosphere (McGinnis
et al.
2006).
This sector is a likely location for gas hydrates to impact the
atmospheric methane concentration over the next few hun-
dred years. However, identifying how much of the methane
release is caused by anthropogenic warming of gas hydrates
requires first distinguishing between methane produced by
gas hydrate dissociation and methane from other sources,
such as organic matter decay or migration from deeper
methane sources.
3.6
REVIEWOF SENSITIVITY OF
GLOBAL GAS HYDRATE INVENTORY
TO CLIMATE CHANGE