Geologists have discovered underwater deposits of hydrates — icy deposits of
frozen methane gas — at far shallower depths under the ocean floor than
expected. The finding suggests that, in a globally warmed world, the hydrates
could melt suddenly and release their gas into the atmosphere, thus warming the
planet even more.
Hydrates are cage-like structures in which molecules of water surround frozen
molecules of gas. When dug up and brought to the surface, they release fizzy
bubbles of methane into the atmosphere.
As a greenhouse gas, methane has 20 times the global warming potential of carbon
dioxide. Large amounts of the world's carbon is locked up in methane hydrates,
both in polar permafrost regions and buried in marine sediments worldwide. So
scientists have long worried about a potentially catastrophic melting of these
hydrates, triggered by an underwater landslide or warming of the ocean waters
above them, that could send temperatures soaring.
Some researchers have suggested that a great global-warming episode 55 million
years ago could have been caused by a catastrophic release of methane hydrates
from the sea floor.
The deeper the hydrates lie, the greater the pressure above them, and the less
likely they are to destablise and release gas. But now, scientists drilling into
the ocean floor have found hydrates off the coast of Vancouver Island, Canada,
just 60 to 120 metres under the sea floor. That's less than half the depth they
are typically predicted to exist at, given basic equations about the
temperatures and pressures under which hydrates are usually stable.
"This methane can potentially release quickly, in geologic terms. Not in our
lifetime, but definitely faster than people had predicted earlier," says Michael
Riedel of McGill University in Montreal, who co-led the 2005 expedition that
uncovered the shallow hydrates.
The scientists made the discovery last year, while working through the
Integrated Ocean Drilling Program to explore the coast off Vancouver Island,
where hydrates had been found before.
To their surprise, the five cores they drilled, down to 350 metres depth,
contained hydrates very near the surface and distributed across a wide area.
Hydrates have been found to occur at shallow depts to a limited extent, even
outcropping on the sea floor in isolated spots thanks to unusual local
conditions. But the new work indicates that hydrates can be stable in larger
swaths, over larger areas, than known before.
Geologists may now have to recalculate just how much hydrate exists in the
world, says Riedel, who presented the results at a meeting of the American
Geophysical Union in San Francisco this week.
Not everyone sees hydrates as a threat. Oil companies, for instance, have long
talked of harvesting them as an energy resource. India and Japan have initiated
hydrate exploration programmes, says Timothy Collett of the US Geological Survey
in Denver, Colorado. And on Alaska's North Slope, the oil company BP will be
drilling a test well in February to explore the feasibility of mining hydrates