Scientists worry that rising global temperatures accompanied by
melting permafrost in arctic regions will initiate the release of
underground methane into the atmosphere. Once released, that
methane gas would speed up global warming by trapping the Earth's
heat radiation about 20 times more efficiently than does the
better-known greenhouse gas, carbon dioxide.
An MIT paper appearing in the Journal of Geophysical
Research online Aug. 29 elucidates how this underground methane
in frozen regions would escape and also concludes that methane
trapped under the ocean may already be escaping through vents in
the sea floor at a much faster rate than previously believed. Some
scientists have associated the release, both gradual and fast, of
subsurface ocean methane with climate change of the past and
future.
"The sediment conditions under which this mechanism for gas
migration dominates, such as when you have a very fine-grained mud,
are pervasive in much of the ocean as well as in some permafrost
regions," said lead author Ruben Juanes, the ARCO Assistant
Professor in Energy Studies in the Department of Civil and
Environmental Engineering.
"This indicates that we may be greatly underestimating the
methane fluxes presently occurring in the ocean and from
underground into Earth's atmosphere," said Juanes. "This could have
implications for our understanding of the Earth's carbon cycle and
global warming."
Juanes explains that some of the naturally occurring underground
methane exists not as gas but as methane hydrate. In the hydrate
phase, a methane gas molecule is locked inside a crystalline cage
of frozen water molecules. These hydrates exist in a layer of
underground rock or oceanic sediments called the hydrate stability
zone or HSZ. Methane hydrates will remain stable as long as the
external pressure remains high and the temperature low. Beneath the
hydrate stability zone, where the temperatures are higher, methane
is found primarily in the gas phase mixed with water and
sediment.
But the stability of the hydrate stability zone is
climate-dependent.
If atmospheric temperatures rise, the hydrate stability zone
will shift upward, leaving in its stead a layer of methane gas that
has been freed from the hydrate cages. Pressure in that new layer
of free gas would build, forcing the gas to shoot up through the
HSZ to the surface through existing veins and new fractures in the
sediment. A grain-scale computational model developed by Juanes and
recent MIT graduate Antone Jain indicates that the gas would tend
to open up cornflake-shaped fractures in the sediment, and would
flow quickly enough that it could not be trapped into icy hydrate
cages en route.
"Previous studies did not take into account the strong
interaction between the gas-water surface tension and the sediment
mechanics. Our model explains recent experiments of sediment
fracturing during gas flow, and predicts that large amounts of free
methane gas can bypass the HSZ," said Juanes.
Using their model, as well as seismic data and core samples from
a hydrate-bearing area of ocean floor (Hydrate Ridge, off the coast
of Oregon), Juanes and Jain found that methane gas is very likely
spewing out of vents in the sea floor at flow rates up to 1 million
times faster than if it were migrating as a dissolved substance in
water making its way through the oceanic sediment - a process
previously thought to dominate methane transport.
"Our model provides a physical explanation for the recent
striking discovery by the National Oceanic and Atmospheric
Administration of a plume 1,400 meters high at the seafloor off the
Northern California Margin," said Juanes. This plume, which was
recorded for five minutes before disappearing, is believed not to
be hydrothermal vent, but a plume of methane gas bubbles coated
with methane hydrate.
The Jain and Juanes paper in the Journal of Geophysical Research
also explains the short-term consequences of injecting carbon
dioxide into the ocean's subsurface, a method proposed by some
researchers for reducing atmospheric greenhouse gas. Juanes found
that while some of the CO2 would remain trapped as a
hydrate, much would likely spew up through fractures just as
methane does.
"It is important to keep both methane and carbon dioxide either
in the pipeline or underground, because the consequences of escape
can be quite dangerous over time," said Juanes.
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