The Gulf Stream is defrosting undersea methane: What does it mean for climate change?
- 25 Oct 2012, 16:30
- Roz Pidcock
A Nature study released yesterday says that warming of the Gulf
Stream in the past 5000 years has triggered the release of methane
into the ocean that was once locked up in the seafloor. Since
methane is a potent greenhouse gas, we ask how the release could
affect global warming.
Under high pressure and low temperature in the sea bed, methane
combines with water to form frozen methane
hydrate. Some scientists have raised
concern that rising ocean
temperatures could thaw hydrates, potentially releasing methane
to the atmosphere. Scientists have already found that
ten times more methane is escaping from melting permafrost in
the Arctic than previously thought.
Since methane is a powerful greenhouse gas, around 30 times
more potent than carbon dioxide, researchers fear that methane
in large enough quantities in the atmosphere could accelerate
global warming. This has led some media outlets to talk about
gas hydrates as
"the methane time bomb". We have discussed the appropriateness
- or otherwise - of this phrase before.
Gas bubbles
The new study used pulses of sound to produce an image of the
seabed on the eastern US continental shelf. Ocean circulation in
this area is dominated by the Gulf
Stream, a major ocean current transporting warm water towards
the North Atlantic.
Satellite image showing how the Gulf Stream transports warm
water (red) from the Gulf of Mexico to the northeast Atlantic
ocean.
The researchers looked at the depth at which the frozen hydrate in
the seabed melts and becomes gaseous methane. Since the conversion
to gaseous methane is affected by temperature, the depth of this
boundary is an indicator of the water temperature at the sea
bed.
However, heat is very slow to penetrate the sea floor so where
the boundary sits now is actually a reflection on how warm or cool
the overlying water was about 5000 years ago. With this in mind,
the scientists deduced that the Gulf Stream must have been around
eight degrees cooler 5000 years ago.
This temperature rise, although it has happened slowly, has
destabilised the methane hydrate in the sea bed, allowing methane
gas to rise up through the water in plumes of bubbles.
Methane released from gas hydrates forms plumes of bubbles
that travel up through the water column.
The researchers estimated that 2.5 gigatonnes of methane is
currently destabilising in an area 10,000 square kilometres in
size. This may continue for centuries unless the Gulf Stream shifts
location or the temperature cools by several degrees.
It is unlikely that the North Atlantic Margin is the only place
experiencing changes in the temperature of deep ocean currents.
Lead author Benjamin Phrampus from the Southern Methodist
University in Dallas, explains in the paper:
"Our estimate of 2.5 gigatonnes of
destabilizing methane hydrate may therefore represent only a
fraction of the methane hydrate currently destabilizing
globally".
Vast reserves
Scientists have estimated global stocks
of methane hydrate at around 74 400 Gt, which is three orders
of magnitude larger than conventional natural gas reserves.
Professor David Archer, expert on the impact of methane hydrates on
global climate at the University of Chicago, explained to Carbon
Brief yesterday:
"There is a lot of carbon frozen
into methane hydrate deposits, so over the coming centuries and
millennia the hydrates (along with permafrost soil carbon) have a
large potential to amplify global warming by releasing
carbon".
Warming events in geological history
can provide clues about the link between methane release from
hydrates and climate change. But as professor of geology at the
University of Tromso, Juergen Mienert, explained in a commentary
accompanying the Nature article, predicting the stability of
methane hydrates is complicated. He says:
"[T]he big unknowns regarding this
century's ocean-temperature shifts are to what extent, and how
rapidly, such shifts will reduce the stability of methane hydrate
in ocean margins".
As Euan Nisbet, professor of Earth Sciences at the University of
London, told Carbon Brief:
"Methane hydrate destabilisation by
warming seawater is the great monster-under-the-bed of climate
change. We don't know if it is an important danger, and the
evidence of its possible role in major past changes is still a
matter of debate, but it could be a major problem."
Looking ahead
Even if rising water temperatures thaw a significant fraction of
methane hydrates, in water more than about 100 metres deep most of
the methane dissolves or is oxidised by microbes and
never reaches the atmosphere. There are exceptions, such as in
the shallow East
Siberian Arctic Shelf (ESAS) where a significant amount is
being released to the atmosphere.
But even in shallow water, methane release from hydrates and
permafrost is likely to happen over timescales
of thousands of years rather than decades. As Archer
explains:
"[T]he release rate of carbon from
both of these sources [is] fairly slow, both in comparison with
present-day methane emissions from tropical wetlands, and relative
to fossil fuel carbon release as CO2."
This implies that methane released from hydrates is likely to
have a much smaller impact on climate in the coming centuries than
carbon dioxide released from burning fossil fuels. Or as Archer
puts it:
"Don't worry about the methane,
worry about the CO2. If CO2 emissions aren't stopped, methane will
just be a thin bit of frosting on the cake, and if CO2 is fixed,
methane won't be a problem."
Ocean acidification
There are likely to be consequences of methane release from
hydrates besides long term climate change. Methane oxidises to
carbon dioxide after about a decade, which dissolves in seawater,
making it more acidic, which has consequences for marine
life.
Assessing the role of methane in future climate change is still a
relatively young science and there is still uncertainty about how
big and how sensitive the global inventory of methane hydrates
might be. So while it seems that methane is not quite the ticking
time bomb that it has been made out to be, each bit of new research
adds a valuable piece of the puzzle.