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Drop the Methane Bomb

  • 16 Dec 2011, 11:40
  • Christian Hunt

Source: NOAA Ocean Explorer

There's a lot of methane in the Arctic deep freeze, where ice formations called 'clathrates' trap huge amounts of the powerful greenhouse gas in the Arctic Ocean sea bed. For the intelligent apes currently experimenting with the Earth's atmosphere (that would be us), that methane is in exactly the right place - kept safely out of the atmosphere.

The question is, with the Arctic warming about twice as fast as the rest of the planet, do we need to worry about it thawing out? And so, every now and again, you'll see newspaper articles about a '  methane time bomb' in the Arctic. The latest comes from the Independent, who ran a story headlined '  Shock as retreat of Arctic sea ice releases deadly greenhouse gas'.

Based on an interview with Igor Semiletov, a professor at the International Arctic Research Centre in Alaska, the article details how his research has found large 'plumes' of methane bubbling up from the sea bed of the Arctic ocean:

"This is the first time that we've found continuous, powerful and impressive seeping structures, more than 1,000 metres in diameter. It's amazing," Dr Semiletov said. "I was most impressed by the sheer scale and high density of the plumes. Over a relatively small area we found more than 100, but over a wider area there should be thousands of them."

The Independent then gives us the background:

"...scientists estimate that there are hundreds of millions of tonnes of methane gas locked away beneath the Arctic permafrost, which extends from the mainland into the seabed of the relatively shallow sea of the East Siberian Arctic Shelf. One of the greatest fears is that with the disappearance of the Arctic sea-ice in summer, and rapidly rising temperatures across the entire region, which are already melting the Siberian permafrost, the trapped methane could be suddenly released into the atmosphere leading to rapid and severe climate change."

'Clathrate gun'

This is what has been called the 'clathrate gun' hypothesis - that by warming the Arctic ocean, we might kick-start a process where methane clathrates melt, releasing huge amounts of the gas into the atmosphere - a process that, like a gunshot, would be impossible to stop.

This concern about 'catastrophic' methane release has been fuelled by examinations of the planet's climate past. Over the past 800,000 years, temperature, carbon dioxide and methane levels have all gone up and down, sometimes very quickly. Some of the fastest changes saw sharp rises in temperature and methane at the same time.

Hence the interest in how much methane is coming out of the Arctic sea bed.

However, scientists are usually wary of putting too much weight on one piece of research, particularly when there's a complicated story to tell.

I talked with Dr Vincent Gauci, a researcher at the Open University and director of the website MethaneNet.org. I asked him about the idea that big shifts in temperature in the past have been driven by a 'clathrate gun' process. Other research, he said, suggests the methane increases associated with warming in the past are more likely to have come from changes in the planet's wetlands, rather than methane clathrates.

Methane contains variants of its constituent elements known as isotopes. Scientists can use measurements of isotopes from methane trapped in ice cores to work out where the methane has come from, letting them build up a picture of past atmospheres:

"When we've seen big methane emissions over the past 30-40 thousand years ... the isotopes just don't support big hydrate emissions. They do support changes in wetland extent and productivity having a bigger control over the amount of methane in the atmosphere."

If it's been warmer in the past without massive Arctic methane release from clathrates, it may be unlikely that higher temperatures will lead to methane release from clathrates now:

"The clathrates just respond to what conditions they get - temperature and pressure. If we know certain conditions occurred and we didn't see a response in the isotope record, we can tell that it's not likely to be a driver and lead to destabilisation on a large scale in the future. The problem is knowing how much hydrate is shallow and potentially vulnerable, and how much is deep and relatively safe."

Another recent scientific paper (picked up by bloggers Christopher Mims and Andy Revkin) suggests that the methane release from the Arctic sea-floor may be more about the planet's ongoing and very slow response to coming out of an ice age than man-made climate change.

Much of the methane gas currently bubbling up into the ocean is thought to dissolve in the surrounding seawater, so only a tiny fraction of the methane released this way currently reaches the atmosphere. (It's worth noting this dissolving methane is likely to increase ocean acidity, which could be bad news for the local marine life.)

The Semiletov research is important because it's worth paying close attention to how the planet is behaving on this one. But we probably shouldn't conclude that 'methane plumes' equals 'methane bomb going off' just yet.

Methane in context

In 2010 scientist and methane researcher David Archer wrote an interesting post on the website Realclimate about methane release from the Arctic ocean:

… so far it is at most a very small feedback … Most of the methane in the atmosphere comes from wetlands, natural and artificial associated with rice agriculture. The ocean is small potatoes...

It's useful to get a sense of how Arctic methane emissions compare with other sources. The Independent article cites another study by Dr Semiletov's team published in 2010, which it says estimated that methane emissions from this region were about eight million tonnes a year - but suggests that this may not be a significant underestimate of the phenomenon.

The most recent IPCC report sets out the best estimates of how much methane makes it into the atmosphere each year. Overall, methane emissions are between 500-600 million tonnes per year. Methane from fossil fuel burning makes up around 80 million tonnes. Waste decomposition, from landfills, is 35-69. Rice agriculture 31-112, and cows and sheep produce 76-189.

In this context, the 8 million tonnes per year that the Semiletov study cites is significant - methane is a powerful greenhouse gas, and any extra released because of climate change will make it harder to stop global warming - but hardly catastrophic.

Don't hit the bomb shelter just yet

Meanwhile, scientists are busy measuring what's actually happening in the atmosphere. While carbon dioxide levels continue their steady march upwards, there don't seem to be any signs of 'catastrophic' methane release.

NASA, who maintain a record of the amount of atmospheric Methane as recorded by their observatory in Hawaii, note that

"Methane was steadily increasing in the 1980's, it's growth rate slowed in the 1990's, and it has had a near-zero growth rate for the last few years."

When it comes to methane, clathrates and the Arctic researchers are still assembling the pieces of the scientific puzzle. New research can help understand what's going on, but it shouldn't be overplayed. A recent review of the scientific literature on the subject concluded

"Our current estimates of gas hydrate storage in the Arctic region are ... extremely poor. It is still unknown whether future ocean warming could lead to significant methane release..."

That doesn't mean clathrates should be ignored. By burning fossil fuels we're conducting a grand experiment with the planet's atmosphere, and it would be foolish to shrug off concerns about how the planet might react.

But it's clear that more research is needed to fill in the picture, and so for the moment warnings about Arctic methane clathrates causing abrupt and severe climate change need to be treated cautiously.

 

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