Submarine Methane Hydrate: A threat under anthropogenic climate change?
by Stephen Hunter and Alan Haywood
Gas hydrates are ice-like crystalline cage-structures that confine low-molecular weight gasses, primarily methane. They exist within marine sediments particularly in the continental margins and within relic subsea permafrost of the Arctic margins.
Methane hydrate forms over millions of years and because methane is not stored as a free-gas, it has the potential to act as a geographically-widespread concentrated store of methane.Given the potential impact of hydrate dissasociation in response to anthropogenic climate change it is imperative to understand how the global inventory of methane hydrate will evolve.
The sensitivity of gas hydrate stability to changes in local pressure-temperature conditions and their existence beneath relatively shallow marine environments mean that submarine hydrates are vulnerable to changes in bottom water conditions (i.e. changes in sea level and bottom water temperatures). Following dissociation of hydrates, sediments can become unconsolidated, and structural failure of the sediment column has the potential to trigger submarine landslides and further breakdown of hydrate. The potential geohazard presented to coastal regions by tsunami is obvious. The fate of the methane following dissociation of sedimentary hydrate is complex. In shallower waters, methane from these events has the greatest chance of propagating through the water-column into the atmosphere. Once in the atmosphere the radiative impact of methane is far greater than CO2, which could provide a positive-feedback mechanism, where warming drives more hydrate dissasociation.
A number of previous studies have considered the global steady-state hydrate inventory, forcing hydrate models with modern bottom-water conditions derived from modern. Our previous work supplemented and expanded upon these studies by modelling how the global oceanic hydrate inventory evolved through glacial-interglacial cycles. In our current work we are using the CMIP5 model runs to conduct a multi-model evaluation of global bottom water conditions. We will then use this climate envelope to constrain the global hydrate inventory under pre-industrial conditions and IPCC future emissions scenarios. This work will address the following questions: What is the pre-industrial hydrate inventory? How will bottom waters respond to future anthropogenic climate change? How and on what timescale does the hydrate stability zone and methane hydrate inventory respond to these predicted changes in bottom water conditions? How will subsea-permafrost hosted hydrates potentially respond to these anthropogenic changes in bottom water conditions?
Image : Modelled pre-industrial hydrate inventory. We predict between 4700 and 5030 Pg (Gt) of Carbon is locked up within subsea hydrate within the continental margins. Our model does not account for subsea permafrost-hosted hydrates and so those of the shallow Arctic margin (<~300m) were not considered.
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