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Carbon Brief Staff

24.09.2013 | 10:30am
ScienceMethane reserves: Does size really matter?
SCIENCE | September 24. 2013. 10:30
Methane reserves: Does size really matter?

Size isn’t everything when it comes to the stores of methane locked up in the earth. A recent paper warns a sudden release of these resources could exact a terrible economic and humanitarian cost. But while vast stores of the potent greenhouse gas exist, scientists do not have sufficient evidence to believe they will be suddenly released into the atmosphere, methane expert Dr Vincent Gauci explains.

Stable stores

If all of the carbon locked up in the planet’s soils, plants and permafrost  and even in rocks entered the atmosphere at once, we’d be very, very hot indeed. As climate scientists, we’d be busy measuring and remeasuring the size of these carbon pools while trying to understand the effects of a range of likely environmental changes on their stability. 

But for most carbon stores, such research would be an exercise in futility: A lot of the places where carbon is stored are very stable and show no sign of losing their carbon to the atmosphere over thousands or even millions of years.

So scientists measure the  exchange of greenhouse gases between the atmosphere and stores like  soil or the point of production in Earth’s ecosystems because this is what we ‘know’ is actually contributing to the atmosphere’s radiative balance.   That’s why most scientific attention is focused on measuring exchanges where ecosystems are subject to environmental changes such as land use conversion from forest to farmland, increases in atmospheric carbon dioxide which can affect ecosystem function in a number of ways, or increases in temperature in sensitive areas such as the Arctic. 

Is methane different?

Why then, when it comes to methane, given the publication of a paper several weeks ago by  Gail Whiteman, Chris Hope and Peter Wadhams, is reservoir size seemingly all-important even if there is little evidence that the methane contained within that reservoir might ever actually enter the atmosphere? 

The authors of the paper suggest that a currently stable methane reservoir – east Siberian Arctic shelf methane hydrates – would rapidly destabilise as a consequence of anticipated reduction of Arctic sea ice. They estimate the economic impact of catastrophic methane emission from this currently negligible atmospheric source by employing a scenario of 50 gigatonnes (billion tonnes) emission of methane over a 10 year period, starting as early as 2015. 

To place this in context, if all the sources of methane we currently know about, natural and manmade, were combined, they’d amount to no more than around 5 billion tonnes. Methane is a powerful greenhouse gas at around 25 times the heat trapping power of carbon dioxide and it is produced and emitted to the atmosphere from a number of natural and anthropogenic sources.  

This means that any changes in emissions to the atmosphere can have serious knock-on consequences for Earth’s climate. So the authors are essentially proposing there will be a tenfold increase in methane emissions as a consequence of destabilisation of these ocean sediment methane hydrates. 

They estimate that such an increase in emissions would have an economic impact valued at $60 trillion, or roughly the equivalent to the entire global economy. This is a frightening outcome which would likely result, if true, in massive loss of human life and an erosion of quality of life for remaining inhabitants. 

An implausible scenario

Given such extreme outcomes it is important that the ideas within the paper are subjected to proper scrutiny. Thankfully – I say this with concern for human lives not from lack of scientific objectivity – the central emission scenario used in this study has been roundly criticised as being implausible.

The central criticisms are that there is no historical precedent of an equivalent emission scenario in ice cores and other records of the past, and there is an absence of a stated mechanism to link sea ice loss with increased heat transfer to the buried hydrates over the time frame put forward.

There’s also simply not enough known about observed methane plumes in these arctic waters with too few measurements to state categorically that destabilising methane hydrates are the source. I won’t re-tread the arguments supporting the critique in detail: they are made very well in  contributions by Nasa’s Dr Gavin Schmidt and also in a thorough and well-argued  communication by authors including Dr Euan Nisbet at Royal Holloway University. 

But for me, aside from the details of the argument, the debate raises two important points.  

Stores versus emissions

Firstly, just because there may be large methane reservoirs there should be extreme caution when making the leap to assuming probable emissions to the atmosphere. In the case of the Whiteman et al study, the starting point is that there is a large (just how large is up for debate) reservoir of methane hydrates, not that there are large emissions.

A reservoir or store of methane, is not important climatically if there is no evidence that it will eventually get out. So why should we worry about it? We currently have no real idea of the total amount of methane stored within the largest source of atmospheric methane, wetland soils and sediments.

For studies in these systems, we take measuring the amount of methane emitted from their surface to the atmosphere as the starting point for climate-relevant methane research. Indeed, after a thorough search, I couldn’t find a single reference estimating the size of the total wetland methane store (it’s probably very large).

This in itself is not surprising. Why bother with finding out the size of the total wetland methane store if the vast majority will be oxidised in upper soil layers? Emissions measurements are plentiful and more relevant to the central question: How is earth’s climate influenced by the range of methane sources?   The size of the methane hydrate reserves is not the right thing to focus on. We need more emissions measurements in the region and we need to properly identify their sources.

Reporting and responsibility

This brings me to my second point. When it comes to methane, there seems to be something that encourages erroneous reporting of new and potential sources and emissions of this gas. 

In this respect, reporting on methane research has form. In 2006 the Guardian carried the (admittedly later corrected) overstated front page headline: ‘ Global Warming: Blame the Forests‘ on the reporting of a new source of methane that is produced in all vegetation, aerobically and without involvement from microbes.  

The authors of the study quoted in the Guardian suggested that the source was responsible for contributing up to 240 million tonnes of methane annually to the atmosphere or approaching half of methane emissions from all sources combined.  As with the Whiteman et al paper, the finding prompted much debate. Major critiques at the time focusing on the estimated size of the new source and the manner in which the measurements had been up scaled to provide a global estimate. It was later established that the size of this new source was hugely  overestimated.

Given that it is now well known how powerful a greenhouse gas methane is, it is perhaps understandable that the media have a tendency to leap on a big methane number. They make good headlines and the reporting is made in good faith relying on what scientists report even if the ‘discovery’ is too new for a consensus view supporting that opinion to have emerged. 

This is where the scientific community engaged in this research needs to shoulder some responsibility. To make progress in science we need brave new ideas and we need to test hypotheses and explore more speculative notions. It is crucial to study the methane cycle of the Arctic, a region that is undergoing rapid environmental change and research in this area is being supported. 

Equally, in order to maintain trust and credibility, we need to be upfront when there is uncertainty and, critically, we need to take extreme care to position scenarios within the bounds of reality, especially if the implication is rapid global catastrophe.   

Dr Vincent Gauci is a senior lecturer in Earth Systems and Ecosystems Science at the Open University. He directs the National Environmental Resource Council’s Methane Network. Follow him on Twitter: @GauciGauci

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