FROZEN HEAT | Volume 1

3.6 REVIEWOF SENSITIVITY OF GLOBAL GAS HYDRATE INVENTORY TO CLIMATE CHANGE

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.

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