Rising Temperatures will Release Methane Hydrates into the Ocean and Atmosphere

There are large deposits of methane hydrates stored in sediments in shallow Arctic waters along continental shelves.  Methane is a greenhouse gas that can cause rapid global warming; scientists estimate that its effects are 25 times greater in magnitude than CO2.  Additionally, methane can affect Arctic Ocean water pH and oxygen content.  When the methane hydrates escape from the benthic sediments, they turn into methane gas.  As global temperatures warm, the bottom water in the Arctic could correspondingly rise, which would trigger the release of these methane hydrates.  25 % of hydrates are in shallow and mid-depth waters.  Rupke et al. used models to predict both the current temperatures of Arctic bottom water and the future temperatures 100 years from now.  Looking at the gas hydrate stability zone (GHSZ), where hydrostatic water pressure is greater than temperature and salinity dependent dissociation pressure, they determined that changes it its thickness will cause the release of both structure one and structure two hydrates.  Structure one and two hydrates have different molecular structure and therefore act differently and are unstable under different conditions.  12% of the total estimated 100 Gt C of methane at a sulfate reduction zone thickness of 5m is predicted to be released into the ocean and atmosphere.  This will cause little effect on the climate but could raise the pH and hasten oxygen depletion in the Arctic Ocean .Katherine Recinos
Rupke, L., Biastoch, A., Treude, T., Riebesell, U., Roth, C., Burwicz, E., Park, W., Latif, M., Boning, C., Wallmann, K., 2011. Rising arctic ocean temperatures cause gas hydrate destabilization and ocean acidification. Geophysical Research Letters 38.

The authors investigated how the temperature of Arctic bottom waters would change in relation to overall warming.  They used a hindcast with the ocean/sea-ice NEMO by the DRAKKAR collaboration.  This was compounded with a global simulation at ½ degree resolution (ORCA05) and 46 levels.  A repeated-year forcing scenario was subtracted from this model.  This gave the researchers a map of current water temperatures with deeper oceanic strata and exposed shelves having colder water.  A coupled climate model called KCM was then used at a 2 degree ORCA2 31 level resolution.  An atmospheric model, ECHAM5 [15] was used to model changes in the atmosphere.  These models generated eight 100 year global warming simulations and a 430 year control experiment.  They used a 1% increase in CO2 and present day CO2 levels respectively.  It took around 50 years for steady trends to develop, but the results showed a pan-Arctic increase of around 2.5 °C per century with the greatest changes along the continental slopes and on the shelves.     

As previously mentioned, there are two types of hydrates found in the Arctic; structure I and structure II.  Most of the effects considered in this paper are on structure one hydrates, however both types of hydrates are predicted to be affected, especially in shelf regions.  Rupke et al. then estimated the amount of hydrates present in Arctic sediments; 900 Gt carbon found north of 60° latitude under a 5m hydrate free zone.  The amount of hydrates found globally is estimated at 500-64,000 Gt C.  The total methane hydrates that would be released by a greater than or equal to 20 meter decrease in the gas hydrate stability zone would be 100 Gt C, but in the next 100 years only about 12% of that amount will actually be released.  Some of the released methane hydrates could become a sediment based carbon sink through microbial anaerobic oxidation (AOM).  The remaining methane will travel up through the water column and pass into the atmosphere.  On its way some will be transformed into CO2 and will lower ocean water pH by as much as 0.25 units.  Combined with acidification from increased CO2 in the atmosphere, oceanic pH could decrease by around 0.6 units.