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How likely is a huge Arctic methane pulse? We find disagreement among scientists

  • 24 Jul 2013, 17:45
  • Freya Roberts

Rising temperatures in the Arctic Ocean could see 50 billion tonnes of methane that's currently frozen in the seabed released into the atmosphere, a comment piece published in the journal Nature argues.

According to the modelling, the resulting climate change impacts could cost the globe an extra $60 trillion. But the scenario is unlikely, other scientists say.

A warming Arctic

The shallow seabeds off the coast of Eastern Siberia contain what scientists believe is a huge store of frozen methane, which is a powerful greenhouse gas.

The Arctic is warming about twice as fast as the global average, in part because the sea ice is in long term decline, meaning the Arctic ocean absorbs more heat.

If ocean waters warm too much, deposits of frozen methane in the seabed could start to thaw and escape into the atmosphere, accelerating climate change, the authors of the piece argue.

Their paper uses an economic model and a scenario proposed in one scientific study to calculate that if 50 billion tonnes of methane currently trapped in the East Siberian sea bed was to thaw and be released to the atmosphere over a period of ten years, the resulting climate change impacts from this pulse of methane into the atmosphere could cost 60 trillion dollars.  

Methane pulse

Expressing the consequences of huge and sudden methane release as an economic cost is new, and that makes this first-of-a-kind study an interesting thought experiment. But the idea that such a sudden methane burst could occur as a result of melting permafrost (frozen ground) has been around for a while.

The important question is how likely such a scenario is. In particular, there is scientific disagreement about how quickly methane could be released. The scientists we spoke to suggested the authors have chosen a scenario that's either implausible, or very much at the upper limit of what we can reasonably expect. Dr Vincent Gauci, a researcher at the Open University and director of the MethaneNet research network explained to Carbon Brief:

"It's not a given all the methane will end up in the atmosphere. Some could be oxidised [broken down] in the water by bacteria, and some could remain in the sediments on the seafloor."

Dr Gauci told us that the authors had made an "enormous leap" assuming that the entire 50 billion tonnes of frozen methane trapped in ocean sediments would end up in the atmosphere over a ten-year period.

Those sentiments were mirrored by Professor David Archer from the University of Chicago, who researches ocean sediments and methane. He told us even if the ocean warms, most of the methane released by thawing permafrost could stay in the seabed or dissolve in seawater. Professor Archer, who blogs at Realclimate , described the scenario as "totally unjustified", saying:

"No one has proposed any mechanism for releasing methane that wouldn't take centuries, not just a few years."

Speaking on twitter , NASA Scientist Dr Gavin Schmidt, who runs Realclimate, said the study assumed a "highly unlikely" scenario of "50 Gt increase in emissions in short time" and that there was no evidence in ice cores of these fast intense blips in past warm periods.

ESAS
The location of the East Siberian Arctic Shelf containing frozen methane (Adapted from Shakhova et al., 2010)

The thickness of the permafrost layer in question is one reason scientists might disagree about the length of time needed to release 50 billion tonnes of methane. Dr Julian Merton from the University of Sussex explained to us that permafrost doesn't respond quickly to rising temperatures:

"Permafrost hundreds of metres thick simply doesn't warm or thaw much in ten years on account of its thermal inertia."

But Professor Wadhams told us the layer of undersea frozen methane off the coast of East Siberia may only be 20 metres thick, and so more susceptible to Arctic warming. He agreed there is a question about how much methane will be released and how quickly, but he told us the process could happen sooner rather than later:

"Summer [Arctic sea] ice retreat now is enough to expose the shallow sea beds containing permafrost, so we are already in danger."

He has also expanded on his views on the Guardian website.

While scientists are often keen to warn about 'unknown unknowns' and tipping points in the climate system, the likelihood of such methane pulses is hotly contested among experts who work in this area. While there is research suggesting the possibility, other scientists think undersea frozen methane could be relatively stable for the time being.

Slower release

Although the new paper uses 50 billion tonnes of methane over ten years as the basis of its calculations, the authors also looked at the effects of a smaller or slower release.

The modelling showed that economic costs and methane emissions increased at a similar rate. So if only half the methane emissions materialise, the economic impact would also be halved. As Wadhams explains:

"If a better estimate of likely [methane] release comes up, you can adjust the figures down (or up) accordingly."

The economics modelling also showed that if methane was released more slowly, over 30 years rather than ten, the costs would stay largely the same.

But the modelling doesn't examine whether much slower methane release, say over centuries, would lead to the same sorts of costs.

Methane is a powerful greenhouse gas, causing lots of warming while it's present in the atmosphere. But it breaks down much faster than other greenhouse gases, so very slow steady release could be "massively less costly", according to Prof Archer.

Apocalypse when?

The paper has a core message that climate impacts will lead to economic costs. It considers a particularly scary scenario so it's perhaps not surprising that the costs are large.

However there isn't agreement on the likelihood of such extreme scenarios among the scientific community. The $60 trillion topline gives no sense of how likely the release of that much methane is over such a short time period.

But if this scenario is at the extreme end of what's possible, that isn't a reason to dismiss the risks of methane release altogether. Any extra methane release from the region as it warms reduces the remaining carbon budget to stay within a certain level of global warming - making the task of decarbonising harder.

Even if less methane is released into the atmosphere than this scenario suggests, and even if that process occurs more slowly than one best estimate suggests, the effects of thawing methane can't be ignored.  But while carbon dioxide emissions continue to rise as they are, the possibility of methane burps like this are probably not the thing to be worrying about.

Whiteman et al., (2013) Vast costs of Arctic change. Nature 499; p401

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