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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.

Gulfstream

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.

Gasbubbles

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.

 

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