Here, Prof. Euan Nisbet provides us with an explainer on methane hyrdates, which could potentially release methane into the atmosphere as the ocean warms, and how we are going to target our measurements to find out more about them.
The MAMM team board the ARA after refuelling at Longyearbyen, Spitsbergen, during the field campaign in 2012. The ARA will soon be sniffing out the methane in the Arctic air. Photo credit: Michelle Cain.
Our flights across the Barents Sea (like the one being attempted today) are designed to assess the methane releases into the Arctic atmosphere. Using knowledge about the wind, the atmospheric research aircraft can ‘sniff’ methane sources thousands of miles upwind from its flight path (note: methane has no smell to humans, but our laser-based instrument is a very sensitive methane-sniffer). The “cavity ring-down spectroscopic” system operated by FAAM (Facility for Airborne Atmospheric Measurements) and the University of Manchester is so sensitive that it can sniff at a precision better than one molecule in every one billion molecules in air – as sensitive than the keenest hound.
There have been many news stories recently about the problem of methane release from the huge amount stored a few hundred metres under the surface in the Arctic, within the enormous deposits of a water-methane structure called methane hydrate. In the past few years, several papers in scientific journals have suggested that huge amounts of methane are already being released from hydrate, and in July this year an article in the prestigious journal Nature modelled the enormous impact this might have (Whiteman et al., Vast costs of Arctic change, Nature, 25th July 2013, 401-3).
To look for methane in the high Arctic, the aircraft flies high and low across the Arctic Ocean. The high flights can measure the methane in the middle of the troposphere (the main part of the atmosphere), while the low flights, close to the sea surface, measure the methane in the “boundary layer” (or near-surface), where surface sources mix up into the winds. By back-tracking the winds, we can sniff methane sources many thousands of miles away and determine where they have come from. For example, if there is an easterly wind, a flight north of Norway can measure methane released by sources far away in northern Russia. This is a sort of methane telescope — or, to use the Greek for nose, a “methane telerhino” — is used just in the way a fox sniffs a chicken coop upwind. Using even more sophisticated methods such as carbon isotopic analysis of the carbon atoms in methane by the Royal Holloway University team, we can tell what “type” of methane it is – for example, whether it is methane that has been emitted from microbes in wetlands or natural fossil gas. Using measurements of other gases from instruments such as a mass spectrometer, we can also say from where those gases originated with greater confidence as we know that certain processes should emit specific gases together (e.g. formic acid and methane in forest fires).
What is hydrate, and why might it be important?
Gas Hydrates (also known as clathrates) are ice-like materials made by gases such as methane and CO2, and water. Think of freezing Coca-Cola. They are stable under pressure and cold temperatures. There can be an enormous amount of gas stored in them, and when they are heated up they release this gas. They exist all round the world where methane gas seeps up into wet sediment in the right pressure and temperature conditions. They occur in the Amazon delta, in the Gulf of Mexico deep oilfields, and very widely in the tropics, stabilised under the pressure of a fairly thick sediment load. In the Arctic the cold temperatures help stabilise hydrate at much shallower depths. Offshore, the sea water a few hundred metres down is close to 0oC and methane hydrates are stable near the seabed under about 300 to 400m of water. Near-shore, in shallow salty water that can be as cold as -2oC, hydrate is stable under a few hundred metres of sediment or less. Onshore, in extremely cold areas where the mean annual temperature can be as low as -10oC, hydrates can be stable quite close to surface.
As hydrates warm, they can potentially release great quantities of methane. This is a powerful greenhouse gas – the warming can then feed the warming. Long ago, in the 1980s, Gordon MacDonald and Euan Nisbet independently worried that there might be a link between warming hydrates and climate. An old but more-or-less still valid figure from Nisbet’s 1989 paper (below) shows where the hydrates occur and how they respond to warming.
This figure is ancient, but still more-or-less relevant. The curves show the stability of the hydrates 0.5 to 100 years after a surface warming to +5C. From Nisbet, E. G. (1989).
Nisbet, E. G. (1989), Some northern sources of atmospheric methane – production, history and future implications, Can. J. Earth Sci., 26(8), 1603-1611.
MacDonald, G. J. (1990) Role of clathrates in past and future climate change. Climate Change
16, 247-82. See also text of MacDonald’s 1983 comment in http://www.killerinourmidst.com/MH and global climate.html.
Bubbles seen by ships – is the Arctic shelf degassing?
For some years, there has been an extremely interesting annual voyage across the east Siberian Arctic Shelf led by Nathalia Shakhova and Igor Semiletov of the Univ. of Alaska at Fairbanks. This is remarkable scientific work in a very important and little studied region. In 2010, Shakhova et al. reported major methane emissions from the eastern Siberian shelf and suggested the annual outgassing might be as much as 8 million tons, enough to be globally significant (the world emits somewhat over 500 million tons annually from all sources, human and natural). However, in the scientific discussion following the publication, the quantification of the emission was disputed. Much of this shelf is flooded peatland, which can also release methane when the permafrost melts.
Shakhova, et al. (2010) Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf. Science 327, 1246
Methane emissions from seabed hydrates were also found off Spitsbergen by a major NERC study from the ship RRS James Clark Ross. About 250 plumes of gas bubbles were seen. The plumes were discovered by ‘fish-finder’ sonar (more commonly used to search for cod than methane!). Bubble trains came from the edge of the gas hydrate stability zone, and some reached nearly to the surface. Rebecca Fisher and Mathias Lanoisellé from the Royal Holloway group were on the ship (they’ve now taken wings on the FAAM aircraft). The plumes were reported by Westbrook et al (2009):
Westbrook et al. (2009) Escape of methane gas from the seabed along the West Spitsbergen continental margin. Geophys. Res. Lett. 36, L15608.
The emissions were in response to seawater warmth, but this was off W. Spitsbergen where the water is at the northerly end of the Gulf Stream, where the warmth is brought north from the Gulf of Mexico.
Is Godzilla about to arise? Is there a methane monster?
In the 25th July issue of Nature this year, Whiteman et al. suggested a monster methane release is about to occur in the Arctic. They modelled a release of 50 Giga-tons of methane from Arctic hydrate, at 5 Gt a year over 10 years from 2015 to 2025. One Giga-ton is 1000 million tons, or 1015 grams. To put this in context, the total amount of methane in the world’s air now is about 5 Gt, and the annual input is about 0.5 Gt, so this would double the methane in the air within the first year. They based this number on a ‘single stage blowout’ scenario from another paper by Shakhova et al, (2010). The Whiteman et al. paper had immediate press interest, from newspapers as prestigious the Guardian and the New York Times to a wide range of blogs.
Contrary voices were also heard, in particular from researchers on methane and hydrates (including the present author). They were widely sceptical of such large releases. Responses were both published later in Nature, and also a posted comment that is accessible by scrolling far down the page on:
The full text is on:
There’s clearly a great deal of methane hydrate in the Arctic, and much of it is likely to be destabilised by Arctic warming. But is it going to come out as a great sudden burst in a few years? Or is it going to dribble out as a chronic release, as suggested in 2008 by David Archer, a recognised hydrate expert? Remember also that the northern wetland methane emissions respond very fast to warming. There’s much evidence that at the end of the last glaciation it was not primarily the hydrates but the wetland response that drove the very rapid increase in methane.
Archer, et al (2008) Ocean methane hydrates as a slow tipping point in the global carbon cycle, Proc. Natl. Acad. Sci. 106, 20596–20601
Nisbet, E.G. and Chappellaz, J., (2009) Shifting gear, quickly. Science 324, 477-8
The scepticism of Arctic researchers about the 50 Gt blowout scenario was initially dismissed by an influential Guardian blog as “narrow arguments of scientists out of touch with cutting edge developments in the Arctic.” http://www.theguardian.com/environment/earth-insight/2013/aug/05/7-facts-need-to-know-arctic-methane-time-bomb
However, later the comment was modified:
The answers to these puzzles is what we’re trying to find out….
MAMM Flights to the High Arctic
Our MAMM flights are designed to measure the Arctic winds. If the 8 million ton per year methane emission inferred by Shakhova et al (2010) is already happening, this outpouring will produce an excess of methane in the polar air above the regional temperate background. If the winds are suitable, we should be able to detect that as we fly north in the polar air.
The ARA flying over Spitsbergen for MAMM in 2012.
Of course it all depends on what the winds bring us, but we can go searching. This is what Michelle Cain and colleagues at the UK Met Office do – they use a weather model to predict where the air will come from. In the same way that the modellers predict how clouds of volcanic ash will travel to disrupt airline flights, so they can work out where the air is coming from. That means we can move the aircraft flight path, and go up and down in altitude, to seek out air masses that have come from the Siberian shelf, or from the vast Russian and Siberian wetlands.
And that is exactly what the flight today on the ARA is aiming to do. Follow this blog to find out how it went.
–Prof. Euan Nisbet, Royal Holloway University of London