The latest in a series of journal articles based the MAMM project field work has been published today. The article (A cautionary tale: A study of a methane enhancement over the North Sea) is initially behind a paywall, but will become freely available after 6 months. Tweet me at @civiltalker if you want to request a copy.

After many moons of data analysis, and countless graphs and figures, the paper is finally out. It seems a long time ago that I missed out on a flight that circuited a gas field in the North Sea, looking for any methane that might has escaped the gas rigs or pipes. Nearly 4 years on, the work has been published in a peer-reviewed publication, and we can all relax bask in its glory!

As it’s paywalled for the first 6 months, I thought I would bash out a quick post about it here, even though it’s technically not about Arctic methane, as we sampled off the east coast of the UK. But it was a flight on the way back from the Arctic, at the end of our 2013 field campaign, during which we took advantage of the weather to get some good science in.

The idea was this. The wind was blowing in from the North Atlantic, over the UK, and then out over the North Sea. We (as in, the rest of the team except me) flew round and round the Leman gas field at low altitude, to see if we could see methane escaping from the gas field. This is what we saw…

North Sea gas rig

Measuring methane during a flight around a North Sea gas rig. Photo credit: John Pyle.

The flight track from Aberdeen to Cranfield, with a detour around the North Sea. Colours show methane measured on board the aircraft.

The flight track from Aberdeen to Cranfield, with a detour around the North Sea. Colours show methane measured on board the aircraft.

 

Methane observed around the Leman Gas field

Methane observed during the flight, zoomed in around the Leman Gas field. Wind barbs show the wind direction. The thin blue line represents the coast of East Anglia.

The wind barbs show that the wind was coming from the North West. The colours show the amount of methane we measured in the air, during this flight at about 80m above sea level, and the red box is the rough location of the gas field area. On first glance, you can see more yellow (ie more methane) can be seen downwind of the gas field. But as you may have guessed (the paper is called “A cautionary tale” after all), this isn’t the whole story.

We had a three-pronged approach to analysing the data.

  1. We worked out theoretically how much methane would have to be emitted from the gas field region to explain our results by a gas field source alone. It was quite a lot! However, we have no idea if this was a one off pulse of emissions, or a continuous release. In the former case, a large but short pulse isn’t too surprising. As a continuous release though, it would be a huge amount in total!
  2. We modelled the scenario, using the UK’s emissions inventory, which is a map which tells you how much methane is emitted in a year. So we assume it’s emitted at the same rate throughout the year (not a perfect assumption, but we have nothing better to go on), and use a meteorological model to blow the methane through the atmosphere by the wind. Then we see how much methane arrives at the location of the aircraft. The answer was “not enough”. We had to add an additional source of methane to the North Sea gas field area to get the model to look like the observations.
  3. Then we worked out the amount of carbon-13 in the methane source. The amount of carbon-13 in methane varies according to its source. Natural gas has a lot of it, and is described as heavy compared to average (the atmospheric average has more carbon-12). Biogenic sources (wetlands, agriculture) has less, and so is described as light. To our surprise, the carbon-13 isotopes showed that our methane did not look like natural gas from the North Sea rigs. It was too light!

How to reconcile these somewhat contradictory analyses?

We have two possibilities. One is that the methane source is not from the gas rigs, but is from the land. Perhaps some kind of industrial process that emits a short pulse of methane. Maybe our model is not quite good enough to capture the local winds, and that explains why we don’t reproduce the right pattern if we include an extra source on land. In this case, it is just by chance that we see the methane downwind of a gas field, as it’s coming from the land.

Or, the other is that maybe we have a combination of methane sources. Some additional methane (75%) is coming from the land, above what is in the inventory. And the remaining 25% is coming from the gas fields. This is consistent with the carbon-13 isotopes, and perhaps feels intuitively a little more plausible. However I don’t think there’s evidence that tips the balance either way, so both options may be possible.

So why is this a cautionary tale? Surely not just that science is hard and data is messy! No, the key message is that if we had not measured the isotopes, we could easily have assumed that the methane was indeed all coming from the gas rigs. We could have concluded that there was a large fugitive emission from this gas field on this day. It’s a reminder to ourselves, and hopefully others, to refrain from jumping to conclusions, and to make sure you collect as much relevant data as you can to explain your results thoroughly.

–Dr Michelle Cain, University of Cambridge

The paper is:

Cain, M., et al. (2017), A cautionary tale: A study of a methane enhancement over the North SeaJ. Geophys. Res. Atmos.122, doi:10.1002/2017JD026626.

More info on methane isotopes in this blog post.

Advertisements