Methane Seeps from Arctic Sea-Bed

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BBC News

Methane Seeps from Arctic Sea-Bed

Scientists say they have evidence that the powerful greenhouse gas methane is escaping from the Arctic sea-bed.

by
Judith Burns

Researchers say this could be evidence of a predicted positive feedback effect of climate change.

As
temperatures rise, the sea-bed grows warmer and frozen water crystals
in the sediment break down, allowing methane trapped inside them to
escape.

 

The research team found that more than 250 plumes of methane bubbles are rising from the sea-bed off Norway.

The
joint British and German research team detected the bubbles using a
type of sonar normally used to search for shoals of fish. Once
detected, the bubbles were sampled and tested for methane at a range of
depths.

Writing in Geophysical Research Letters, the team says
the methane was rising from an area of sea-bed off West Spitsbergen,
from depths between 150m and 400m.

The gas is normally trapped as "methane hydrate" in sediment under the ocean floor.

"Methane hydrate" is an ice-like substance composed of water and
methane which is stable under conditions of high pressure and low
temperature.

As temperatures rise, the hydrate breaks down. So
this new evidence shows that methane is stable at water depths greater
than 400m off Spitsbergen.

However, data collected over 30 years shows it was then stable at water depths as shallow as 360m.

Ocean has warmed

Temperature records show that this area of the ocean has warmed by 1C during the same period.

The
research was carried out as part of the International Polar Year
Initiative, funded by Britain's Natural Environment Research Council
(Nerc).

The team says this is the first time that this loss of
stability associated with temperature rise has been observed during the
current geological period.

Professor Tim Minshull of the
National Oceanography Centre at Southampton told BBC News: "We already
knew there was some methane hydrate in the ocean off Spitsbergen and
that's an area where climate change is happening rather faster than
just about anywhere else in the world."

Infographic (BBC)

Methane hydrate is stable below 400m

Nearer the surface the hydrate breaks down as temperatures rise and the methane is released

Gas rises from the sea-bed in plumes of bubbles - most of it dissolves before it reaches the surface

So far scientists haven't detected methane breaking the ocean surface - but they don't rule out the possibility


"There's been an idea for a long time that if the oceans warm,
methane might be released from hydrate beneath the sea floor and
generate a positive greenhouse effect.

"What we're trying to do
is to use lots of different techniques to assess whether this was
something that was likely to happen in a relatively short time scale
off Spitsbergen."

However, methane is already released from
ocean floor hydrates at higher temperatures and lower pressures - so
the team also suggests that some methane release may have been going on
in this area since the last ice age.

Significant discovery

Their
most significant finding is that climate change means the gas is being
released from more and deeper areas of the Arctic Ocean.

Professor
Minshull said: "Our survey was designed to work out how much methane
might be released by future ocean warming; we did not expect to
discover such strong evidence that this process has already started."

"We
were slightly surprised that if there was so much methane rising why no
one had seen it before. But I think the reason is that you have to be
rather dedicated to spot it because these plumes are only perhaps 50m
to 100m across.

"The device we were using is only switched on
during biological cruises. It's not normally used on geophysical or
oceanographic cruises like ours. And of course you've got to monitor it
24 hours a day. In fact, we only spotted the phenomenon half way
through our cruise. We decided to go back and take a closer look."

The
team found that most of the methane is being dissolved into the
seawater and did not detect evidence of the gas breaking the surface of
the ocean and getting into the atmosphere.

The researchers
stress that this does not mean that the gas does not enter the
atmosphere. They point out that the methane seeps are unpredictable and
erratic in quantity, size and duration.

It is possible that
larger seeps at different times and locations might in fact be vigorous
enough to break through the ocean surface.

Most of the methane
reacts with the oxygen in the water to form carbon dioxide, another
greenhouse gas. In sea water, this forms carbonic acid which adds to
ocean acidification, with consequent problems for biodiversity.

Graham
Westbrook, lead author and professor of geophysics at the University of
Birmingham, said: "If this process becomes widespread along Arctic
continental margins, tens of megatonnes of methane a year - equivalent
to 5-10% of the total amount released globally by natural sources,
could be released into the ocean."

The team is planning another
expedition next year to observe the behaviour of the methane plumes
over time. They are also engaged in ongoing research into the amount of
methane hydrate under this area of the ocean floor.

Ultimately, they want to be able to predict how much might be vulnerable to temperature change and in what timescale.

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