The natural sources

The estimates vary for natural sources

Due to the discrete and variable nature of natural methane emissions estimates of the total natural methane budget vary.

"Our ability to quantify the global methane budget is poor"  (Dlugokencky et al., 2011)

Unsurprisingly we hence see discrepancies between publications as to what natural (and anthropogenic) methane emission levels are. What we also see though is quite striking differences for the methane budget when working from a bottom up compared to a top down approach. Kirschke et al's peer assessed paper Three decades of global methane sources and sinks published in Nature in 2013 provides a good example of this:

For their 2000-2009 emissions calculations the two figures varied by a hardly negligible 60%:

218Tg/year by top down calculation compared to 347Tg/year by bottom up. I have chosen to reference this paper in particular as it is widely seen as drawing on findings from the most independent studies and was consequentially the one employed in the IPCC AR5 report. 

I have created the following diagram highlighting the relevant importances of the natural sources:

Source: Authors own

The significance of wetlands' is clearly huge, despite losses to wetland areas over the last century. Something to consider over the coming century is the potential for wetland areas to increase globally, as sea levels rise. Another interesting point to note is the current relative insignificance of both hydrates and permafrost. To see where this natural methane is coming from geographically the following digram of surface methane, produced by the NOAA is highly useful:

Source: NOAA 2010, available here

Natural breakdown of emissions

On the note of natural sources a note on the breakdown of emissions is fitting. We've heard in previous posts as to the significance of the OH radical in breaking down methane - this figure makes reference to the other, less significant but still important sinks. This paper suggests OH is responsible for breaking down 528Tg/year (83% - less than the figure of about 90% most papers suggest). Other sinks are stratospheric loss (cited at 8%), tropospheric Cl breakdown (4%) and breakdown by soils (at 4.5%).

In summary of this week's post:

1. Natural methane emissions are about 250Tg/year
2. Wetlands, producing 62% of natural methane emissions, are by far the most significant natural contributor
3. The role of OH in breaking down methane is perhaps slightly lower than the often cited 90%, according to Kirschke et al's 2013 data it is 83%, despite Kirschke et al citing the popular 90% figure in the introduction of the same paper.

References:

Dlugokencky, E., Nisbet, E., Fisher, R. and Lowry, D. (2011). Global atmospheric methane: budget, changes and dangers. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 369(1943), pp.2058-2072.

Kirschke, S., Bousquet, P., Ciais, P., Saunois, M., Canadell, J., Dlugokencky, E., Bergamaschi, P., Bergmann, D., Blake, D., Bruhwiler, L., Cameron-Smith, P., Castaldi, S., Chevallier, F., Feng, L., Fraser, A., Heimann, M., Hodson, E., Houweling, S., Josse, B., Fraser, P., Krummel, P., Lamarque, J., Langenfelds, R., Le Quéré, C., Naik, V., O'Doherty, S., Palmer, P., Pison, I., Plummer, D., Poulter, B., Prinn, R., Rigby, M., Ringeval, B., Santini, M., Schmidt, M., Shindell, D., Simpson, I., Spahni, R., Steele, L., Strode, S., Sudo, K., Szopa, S., van der Werf, G., Voulgarakis, A., van Weele, M., Weiss, R., Williams, J. and Zeng, G. (2013). Three decades of global methane sources and sinks. Nature Geoscience, 6(10), pp.813-823.

Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton, 2013: Carbon and Other Biogeochemical Cycles. In: Cli- mate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 

OH...

So we closed last week on the subject of '^•OH' - atmospheric ^•OH unlike the molecule OH- is a radical molecule created in small quantities in the troposphere. Its concentrations are very low and it has a life of less than 1 second but it plays an incredibly important role in digesting methane and other atmospheric pollutants and GHGs it is exposed to. A summary of this courtesy of the University of Leeds' School of Chemistry is given below:

"OH is primarily produced by the photolysis of ozone followed by reaction with H₂O. It is the primary daytime oxidising species responsible for the removal of CO, CH₄ (and higher hydrocarbons), H₂, NO₂, H₂S, (CH₃)₂S, NH₃, the hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). The concentration of OH defines the oxidising capacity of the atmosphere and hence the ability to control levels of species that contribute to global warming, acid rain, and photochemical smog."

Clearly then it is quite valuable little molecule with an important function, especially for methane with "The primary sink [for CH4 being by] hydroxyl radicals (OH), mostly in the troposphere, which accounts for around 90% of the global CH4 sink. Additional oxidation sinks include methanotrophic bacteria in aerated soils27,28 (~4%)" (Kirschke et al., 2013).

 The following schematic is a simplified diagram showing the main sources and sinks of •OH in the troposphere (Fiore, 2014).  The key ingredients to •OH are: 03, UV light and H20.

Sources and sinks of atmospheric OH 

(Fiore, A. Nature 513)

All 3 are needed to produce OH and if one is lacking the rate of OH production is reduced accordingly. We read last week about suggestions that in times of large scale methane releases atmospheric OH may be inundated so to speak by the amount of methane present in air -  that the OH produced is unable to process the methane and break it down at a rate that permits its current classification of a GHG with a 100 year GWP of 34 that it is known to have today.

As OH is so short lived and only found in the upper atmosphere we can't look at any direct OH record to find this out. We have to study methane records, the flux of which through time can be used to infer whether OH was in such short supply in the atmosphere that methane could not be broken down at the rate it does at present which we consider to be normal.

There are though two significant limitations to deducing results from methane records in this manner:

1. It only really works when the duration and magnitude of methane release is known
2. It only really works when the methane record has a respectable resolution

The latter of these is touched upon in Bock et al., 2012 - we learn:

"Air bubbles trapped in polar ice provide an almost direct record of atmospheric methane over the last 800kyr... Before being trapped in bubbles, the air slowly diffuses in the firn, from the surface down to the close-off zone. The bubble enclosure also takes place progressively. Hence, fast variations of the atmospheric signal are partly smoothed out."

It's clear then the difficulties of drawing conclusions from past methane degassing events when even large emissions form smoothed atmospheric signals in our best long term records - bubbles in ice cores. This hasn't stopped scientists from looking to the future and speculating (with the aid of climate models) what methane releases could cause if they were to take place.  

Isaksen et al.'s 2011 is a paper focused specifically on the modelling of atmospheric chemistry feedbacks in response to methane emissions, it comes to several conclusions of note that mirror those in similar recent papers:

1. Assuming several hypothetical scenarios of CH4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry 
2. In particular, the impact of CH4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO2 as a result of atmospheric chemical processes. 
3. "Additional studies linking CH4 emissions to the possibilities for large future warming in the Arctic are needed." 

To wrap this lengthy (but important) post up I would say despite recent increases in interest in the abundance of OH and whether it is becoming increasingly depleted the very scarcity, longevity and residence location of OH makes it a tricky one to pin down and study. As many scientists are saying it clearly should be the focus of further study in coming years, especially as polar regions (with significant frozen methane reservoirs) appear to be responding to GW at a rate far greater than in the mid latitudes.


References:

Bock, J., Martinerie, P., Witrant, E. and Chappellaz, J. (2012). Atmospheric impacts and ice core imprints of a methane pulse from clathrates. Earth and Planetary Science Letters, 349-350, pp.98-108.

Fiore, A. (2014). Atmospheric chemistry: No equatorial divide for a cleansing radical. Nature, 513(7517), pp.176-178.

Kirschke, S., Bousquet, P., Ciais, P., Saunois, M., Canadell, J., Dlugokencky, E., Bergamaschi, P., Bergmann, D., Blake, D., Bruhwiler, L., Cameron-Smith, P., Castaldi, S., Chevallier, F., Feng, L., Fraser, A., Heimann, M., Hodson, E., Houweling, S., Josse, B., Fraser, P., Krummel, P., Lamarque, J., Langenfelds, R., Le Quéré, C., Naik, V., O'Doherty, S., Palmer, P., Pison, I., Plummer, D., Poulter, B., Prinn, R., Rigby, M., Ringeval, B., Santini, M., Schmidt, M., Shindell, D., Simpson, I., Spahni, R., Steele, L., Strode, S., Sudo, K., Szopa, S., van der Werf, G., Voulgarakis, A., van Weele, M., Weiss, R., Williams, J. and Zeng, G. (2013). Three decades of global methane sources and sinks. Nature Geoscience, 6(10), pp.813-823.

Isaksen, I., Gauss, M., Myhre, G., Walter Anthony, K. and Ruppel, C. (2011). Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions. Global Biogeochem. Cycles, 25(2), p.n/a-n/a.

Methane; a turbulent past?

"Dinosaurs may be partly to blame for a change in climate because they created so much flatulence, according to leading scientists... It is even possible that the climate change was so catastrophic that it caused the dinosaurs eventual demise"

The opening paragraph to an article by the well known powerhouse in scientific journalism that is the Daily Mail. The article stated a paper was about to be published suggesting the above.

Quite a headline claim! I had to investigate further and find cette paper - the paper in question was published by David Wilkinson, Euane Nisbet and Graeme Ruxton in Current Biology. It focuses in particular on sauropods, the largest of the terrestrial dinosaurs. By applying assumptions on metabolism from large present day mammals to estimate the likely density of these dinosaurs per kilometre in their known geographical habitable extent and then methane production values based on the production values of the largest modern day ruminants Wilkinson et al deduced that saurapods alone could have created comparable methane to all anthropogenic sources today, as their figure here suggests:

Wilkinson et al., 2012

Comment: Interesting that they suggest this level of methane emission may have been enough to lead to the dinosaurs eventual demise mentioned... If we're emitting comparable amounts now to back then are we pushing ourselves towards a similar timely demise(!?).

Maybe this paper is a touch overzealous in their estimate for the amount of methane sauropods in particular produced, or perhaps too as to the sheer number of these massive animals that the earth was home to. The paper is not alone though in this area of discussion and there are many more papers suggesting similar relationships between atmospheric methane abundance and the status of life on earth. It highlights an important point I believe that methane is clearly a key facilitator, but also an inhibitor to life on earth depending on its levels of presence. 

Taking a step slightly further back now to 200 million years ago, what caused the rapid extinction of half the earth's species and the dawn of the Jurassic? This paper is a very interesting read albeit slightly technical...

Ruhl and co propose that yes, carbon dioxide was the instigator to a preliminary global warming. It was this (relatively) minor preliminary warming though that then instigated a slightly subsequent release of isotopically depleted carbon (which they suggest is indicative of a methane release) into the atmosphere which caused the massive and rapid further warming. They form this deduction based on the otherwise inexplicable disruption of the carbon-cycle in which 12x10^3 gigatons of isotopically depleted carbon was injected into the atmosphere shortly after the initial warming - they suggest this nature and scale of release could only really be answered to by methane releases from methane clathrates (more to come on clathrates in the near future, worry not!). [For those wanting to know a little more but not to read the whole paper a brief overview of this paper's findings can be found in the fourth and fifth paragraph of this article in Science Magazine]

Looking back further still to the end of the Permian Period 252 million years ago 'The Great Dying' (the greatest extinction event in the earth's history) could too it has now been suggested have been caused by a runaway methane warming induced feedback loop caused by marine methane producing microbes.

So this brings me nicely onto next week's topic (the last in this group of posts focusing on the physicalities of methane, past and present) - next week's blog will be looking at OH.

Don't worry if this means nothing to you now, it is significant though, I promise, both in relation to what we've discussed in this blog and just maybe in relation to our future on earth too...

A teaser: 
-GWP's, to which we are so accustomed, are reliant on residency times. 
-Residency times are reliant on the metabolism and removal of GHGs after a standard period of time... 
-But what happens if what's needed to remove a GHG; the ingredients to its removal are no longer available or all priorly used up?

Till next time... 

The sting in the tail

The sting in the tail...

As mentioned last time, methane is a powerful GHG and highly able at trapping radiation. We've also learnt that methane has a relatively short residence time in the atmosphere of about 8.9 years (according to the AR4 report). Its warming ability however is not just linked to the time methane exists as methane within the atmosphere - the majority of methane metabolises into other harmful GHGs (including CO2) and consumes free radical OH. This is something the AR4 report as with others did not fully recognise...

"We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in the current carbon-trading schemes or in the Kyoto Protocol.

and

"...studies including the Inter-Governmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), provide estimates of RF [radiative forcing] and GWPs ... however, except for the indirect effect of NOx emissions on nitrate aerosols, gas-aerosol interactions were not included." [...] ...any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions." 

 [both: Shindell et al, Improved Attribution of Climate Forcing to Emissions, 2006]

Why is this? Because when these interactions are not considered the overall image for the warming potential of the GHG is underestimated, particularly so with methane. Let's look into this with the following figure again published by Shindell et al:

When looking at the emissions based diagram, methane's contribution to radiative forcing as methane (the yellow brick within the methane column) is little over half the total radiative forcing of methane when considering the effect its metabolites and interactions with other aerosols has on radiative forcing.

When these are not considered the ease to underestimate methane's total contribution to radiative forcing is significant: this can be seen clearly below - look how great the difference between AR4's figure for methane's 100-year GWP is and Shindell et al's calculation, incorporating the gas's direct and indirect interaction with aerosols is...

We've talked in previous weeks about how the GWP of methane has been edged up through the years:

The IPCC published in 1995 that the 100 year GWP for methane was 21. In the AR4 report as shown in these diagrams it was published as being nearer to 25. In the AR5 report it was published as being either 28 or 34 depending on whether climate-carbon feedback is included or not. 

Shindell et al's 2009 paper did not incorporate climate carbon feedbacks and they calculate as the diagram shows methane to have a 100 year GWP of 33 when direct and indirect aerosols are considered. If they considered climate-carbon feedbacks as well (which AR5 showed to raise the figure by 6 the 100 year GWP of methane would be close to 40, possibly greater if the climate carbon feedback is non-linear (something that could quite reasonably be assumed?). 

Is then even now the real extent and importance of methane in climate change still being underestimated? Or masked even in major climate reports and in carbon trading schemes? That's up to you to decide...