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Clathrates
by -- February 2nd, 2016

Water exists as a vapor, liquid, and solid—which we know as ice. Similarly, methane is found as a gas, liquid, and a solid, known as methane clathrate or hydrate. We mostly think of methane as the gas we burn to cook or heat our homes. Methane can be liquefied under pressure and cold temperatures to form Liquified Natural Gas (LNG), which is the form that is most often carried in ships to transport natural gas to distant places. When it is very cold and under pressure, methane can freeze into the solid clathrate form.  Brought to the surface, methane clathrate skips the liquid phase and goes right from solid to gas, which can be burned.

Methane clathrates form naturally in deep, cold ocean sediments under lots of pressure. The methane accumulates as a result of the decomposition of organic materials in the absence of oxygen. The Arctic regions are thought to have vast stores of methane clathrate, which would dwarf the pool of methane in Earth’s atmosphere if the clathrates were ever to vaporize.   So, there is some fear among global change ecologists that warming of the Arctic ocean and its underlying sediments might decompose the methane clathrates and release large amounts of methane in burps to the atmosphere. Such catastrophic releases of methane would escape the natural destruction of methane by marine microbes, as the methane rises rapidly to the ocean’s surface.

Like carbon dioxide, methane is a greenhouse gas, which absorbs infra-red radiation trying to leave the Earth’s surface. All atmospheric chemists agree that methane is more effective than CO2 as a greenhouse gas, but the exact amount of the warming from an individual methane molecule is somewhat controversial—between 20 and 80 times the effect of a CO2 molecule. There is no doubt that large amounts of methane in the atmosphere would warm the earth. This has apparently happened a few times in Earth’s geologic past, so it is not unreasonable to be worried that it could happen again.

Already, chemical oceanographers have measured methane bubbling up from the sediments of the Arctic ocean off Siberia. There is some indication that methane release has occurred during warming and cooling of the Arctic seawaters during the past 3000 years. This should give us some pause: large releases might occur in response to the unidirectional warming that now appears in the north polar waters.

The size of the estimated pool of methane in Arctic sediments, 500 to 1000 billion metric tons of carbon, and the potential for additional methane clathrates to occur in Antarctic regions rivals the emissions of greenhouse gases that are possible with continued use of fossil fuels. In a very real sense, global warming may beget drastically more global warming, if we ever destabilize the pool of methane clathrates that are buried in polar regions.

 

 

References

Berndt, C. et al. 2014. Temporal constraints on hydrate-controlled methane seepage off Svalbard.  Science 343: 284-287.

Burwicz, E.B., L.H. Ruepke, and K. Wallmann. 2011. Estimation of the global amount of submarine gas hydrates formed via microbial methane formation based on numerical reaction-transport modeling and a novel parameterization of Holocene sedimentation.  Geochimica et Cosmochimica Acta 75: 4562-4576.

Darnell, K.N. and P.B. Fleming. 2015. Transient seafloor venting on continental slopes from warming-induced methane hydrate dissociation. Geophysical Research Letters doi: 10.1002/2015GL067012

Martin-Moreno, H., T.A. Minshull, G.K. Westbrook, B. Sinha, and S. Sarkar. 2013. The response of methane hydrate beneath the seabed offshore Svalbard to ocean warming during the next three centuries.  Geophysical Research Letters  doi: 10.1002/grl.50985

Pinero, E., M. Marquardt, C. Hensen, M. Haeckel. and K. Wallmann. 2013. Estimation of the global inventory of methane hydrates in marine sediments using transfer functions.  Biogeosciences 10: 959-975.

Wadham, J.L. et al. 2012. Potential methane reservoirs beneath Antarctica.  Nature 488: 633-637.

Zhang, X., K.C. Hester, W. ussler, P.M. Walz, E.T. Peltzer, and P.G. Brewer. 2011. In situ Raman-based measurements of high dissolved methane concentrations in hydrate-rich ocean sediments.  Geophysical Research Letters 38: doi: 10.1029/2011GL047141

1 Comment

  1. Jenny Goldie
    Feb 2, 2016

    Thank you for expressing it so clearly. Inaction on fossil fuels could lead to 5-9 degrees C warming as occurred 55 million years ago during the Palaeocene-Eocene Thermal Maximum or PETM, associated with a major wipe-out of ocean life. The likely cause for the extra warming was the release of millions of tonnes of frozen methane from the tundra and seabed floor, as also occurred 250 million years ago in the Great Permian Extinction. According to author Julian Cribb, this ‘double whammy’ of initial warming from the burning of fossil fuels, followed by the release of even more methane from the tundra and ocean floor, is the principal threat to civilisation in the 21st Century.

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