Emissions Likely to Increase Significantly

F6 frost polygons_0

26th January 2011.

A major new study published in Reviews of Geophysics* in December 2010 puts the spotlight on methane emissions from natural sources and asks the question, how might they both affect and be affected by future climate change.

The authors conclude, based on a comprehensive review of recent literature, that ‘significant increases in methane emissions are likely, and catastrophic emissions cannot be ruled out’.

Major uncertainties surround the impact and timescale of several important feedback processes in the global methane cycle. How will natural methane emissions from wetlands, permafrost areas and methane hydrate deposits respond to climate change? At the most extreme, one can envisage a scenario in which permafrost melts, the carbon-rich wetland areas increase and the methanogens up their rate of production in response to greater warmth, which in turn promotes release of biogenic volatile organic compounds, these additional BVOCs reduce the potential for atmospheric oxidation of methane by competing for OH radicals, and finally, under further warming, hydrates become unstable and release their vast store of methane, as may have happened at the Palaeocene-Eocene Thermal Maximum.

In reality, the authors acknowledge that this ‘perfect storm’ of methane emissions is likely to be countered by negative feedback processes such as the drying out of some wetlands and reduced BVOC emission efficiency.  But the point remains that the strength and relative timing of both positive and negative feedbacks are currently poorly understood and the potential for significant increases in methane emissions exists.

When asked which gaps in current knowledge he considered the most important, co-author Olivier Boucher of the Met Office listed “…wetland processes and their sensitivity to climate change, permafrost dynamics and its interactions with vegetation, fire and hydrological processes,  the transient aspects of marine hydrate destabilisation and the fate of methane emitted in the ocean”.

*O’Connor, F.M., Boucher, O., Gedney, N., Jones, C.D., Folberth, G.A., Coppell, R., Friedlingstein, P., Collins, W.J., Chappellaz, J., Ridley, J. & Johnson, C.E. 2010, “Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review”, Reviews of Geophysics, vol. 48, no. 4.

Clathrate Gun Shot Down?

Greenland Ice Core_0

23rd July 2010.

Arctic and Antarctic ice cores provide a rich source of evidence that both temperature and atmospheric concentrations of methane and carbon dioxide have fluctuated over the past 800,000 years. These studies have also shown that the fluctuations are cyclic, and that higher temperatures are generally associated with higher levels of methane and carbon dioxide in the atmosphere. The source(s) of the periodic increases in methane concentrations are less clear, and have been the subject of much debate. Two competing hypotheses have been proposed: (1) that the extra methane comes from increased wetlands emissions (the ‘wetland hypothesis’) and (2) that methane hydrate disintegration is responsible for the increase in methane which in turn can trigger a dramatic temperature rise (the ‘clathrate gun hypothesis’).

A new study (Bock et al., 2010) of the hydrogen isotope ratios in methane from North Greenland ice-cores dated from 33,700 – 41,000 years ago, has now tipped the balance in favour of the wetland hypothesis for the series of climate fluctuations known as the Dansgaard-Oeschger events.

The study uses the fact that the hydrogen isotope composition of methane emitted from wetlands falls in the range -300 to -400‰ δD(CH4), distinctly more depleted in deuterium than that in hydrate-sourced methane, which averages ~ -190 δD(CH4). A number of other factors (e.g. precipitation, temperature-related Rayleigh distillation, kinetic fractionation associated with the OH sink) also impact on the hydrogen isotope ratio, but these are secondary effects which only alter the δD(CH4) by a few per mill

Bock et al. observed that over the 7,300 years represented by their ice cores, the methane from the interstadial (warmer temperature) periods was ~ 10 ‰ more depleted in deuterium than that from the cooler periods (stadials), and that this change in δD(CH4) could only be explained by the wetland hypothesis. Modelling indicated the hydrogen isotope measurements in the interstadials were consistent with a six fold increase in high latitude wetland emissions, from ~5 to ~ 32 Tg CH4 year-1, an increase of ~84 to ~118 Tg CH4 year-1 from tropical wetlands, and a constant rate of emission from marine hydrates of ~ 25 Tg CH4 year-1.

Hydrates expert Professor Mark Maslin, Director of the UCL Environment Institute, agrees that the paper “provides good evidence that gas hydrates were not involved in the initial methane rise…” during the Dansgaard-Oeschger events, but believes that there is “still considerable debate about whether tropical wetland, boreal wetlands or flooded shelf due to millennial scale sea level rise are the true source of the methane rise….” The authors of the paper concede that the methane cycle in this period remains underdetermined. However, the ruling out of one important potential methane source represents a significant advance in our understanding.

References

Michael Bock, Jochen Schmitt, Lars Möller, Renato Spahni, Thomas Blunier and Hubertus Fischer (2010), Hydrogen isotopes preclude marine hydrate CH4 emissions at the onset of Dansgaard-Oeschger events, Science, v.328, 1686-1689.

Mark Maslin, Matthew Owen, Richard Betts, Simon Day, Tom Dinkley-Jones and Andrew Ridgwell (2010), Gas hydrates: past and future geohazard? Phil. Trans. R. Soc. A, v.368, 2369-2393

Hydrates and the Deepwater Horizon Disaster

Deepwater horizon

21st May 2010.

‘Methane ice’ has been hitting the headlines in the wake of the Deepwater Horizon drilling rig disaster in the Gulf of Mexico, and the complex issues relating to methane hydrate formation and occurrence have been brought to the attention of a wider public as a result of BP’s effort to contain the oil spill and limit the associated damage.

To researchers interested in methane as a greenhouse gas, methane hydrates are significant for their potential role as agents of catastrophic climate destabilisation. To the oil and gas industry, they represent both challenge and opportunity. Challenge in their capacity to clog-up gas pipelines and other equipment and to endanger drilling operations, and opportunity in that according to some estimates, up to 5 x 1015 m3 of methane (1) could currently be locked up as hydrates in permafrost regions and ocean sediments.

There is no consensus yet (May 2010) as to the events leading to the gas explosion which wrecked the Deepwater Horizon drilling rig and killed eleven men. The main companies involved, BP, Transocean and Halliburton, are refraining from speculation pending investigations. Robert Bea, a civil engineering professor at the University of California, Berkeley and an oil industry consultant, has suggested that methane hydrates in the sub-sea sediments, destabilised as a result of drilling-related operations, could have contributed to conditions which ultimately led to an explosive mixture of high pressure oil and gas surging up the well and triggering the fatal explosion on the drill floor. No-one disputes that methane hydrates are present in the formations through which the well was drilled and that they constitute a serious and recognized drilling hazard. However, given that the hydrate-containing sediments occur at 3,000-5,000 ft* below the sea floor in the region of the well, and the well was drilled much deeper, down to the target oil and gas reservoir at 18,000 ft subsea, the reservoir is the more likely (?) source for the gas which triggered the fatal explosion. The technical complexities involved in drilling a well to these depths in over 5000 ft of ocean mean that the exact sequence of events may take some time to become clear.

The role of methane hydrates in scuppering one of BP’s plans to contain the oil leaking from the well wreckage on the sea floor, is, by contrast, unambiguous. The idea, basically, was to place a large steel container over the main leak, in order to gather up the oil and subsequently pump it into a tanker. However, gas leaking from the well combined with sea-water to form hydrates which blocked the container as it was being manœuvered into place, and prevented it from being successfully deployed due to the hydrate-induced increased buoyancy. To mitigate against further problems with hydrates, BP will be injecting methanol at appropriate points into the equipment used in further attempts to gather up the leaking oil, using the best available information on the imperfectly understood processes of hydrate formation and dissociation.

*All depths are quoted in feet in line with common US/UK oil industry practice.

(1) Milkov A. V. (2004) Global estimates of hydrate-bound gas in marine sediments: how much is really out there. Earth Sci. Rev 66:183–197, (doi:10.1016/j.earscirev.2003.11.002).

Drilling rig image source: Flickr.com cy esp