Looking to the future: Emissions predictions

Happy new year to all from a rather sorry looking, thinly snow covered European Alps!

The low snowfall in the European Alps this winter reminds me of a talk I went to a few weeks ago by James Balog (the man behind the 'Extreme Ice Survey' - http://extremeicesurvey.org for more). 

He has done a really fantastic job of linking science and art through his ongoing study of glaciers in some of the world's most mountainous and polar regions. He poingnently described glaciers as 'the canary in the global coal mine' as he showed clips of glacial retreat the world over. One of his clips was from one of Europe's most significant glaciers the 'Mer de Glace' (The Sea of Ice) about 20 miles away from where I am staying at the moment. I remember visiting this glacier as a young boy, being shocked then to learn about its rapid retreat. As a lover of both photography and of the mountains, spending time exploring the latter whenever I can it was a fantastic talk and I'd recommend his film 'Chasing Ice' which is based on the project to anyone if you haven't already seen it. 

Now back to topic...

This week we look at the complex issue of prediction methane emission into the future and looking at speculative predictions for future anthropogenic methane production as well as the 2 major worries for the future: methane clathrates and the risk of major methane release from defrosting permafrost regions.

Anthropogenic emissions into the future

The period of stabilisation in atmospheric methane levels experienced between 2000 and 2007 sadly appears to be over - growth has been consistent since 2008, passing the 1850ppb mark for the first time in 2014 as the figure below highlights:

Source: NOAA, 2015

Unfortunately predictions for the future aren't optimistic, with Höglund-Isaksson (2012) predicting anthropogenic emissions to break through the 400 Mt/year mark by 2027 and "an expected increase to 414 Mt methane in 2030". The same can be said for natural emissions; "Natural emissions of CH4 are likely to increase in a warmer climate; however, the magnitude and rate of change of future emissions from natural sources are largely unknown." - O'Connor et al (2010).

Source: Höglund-Isaksson, 2012

A sad reality if this is indeed the case, it highlights once again however the growing importance of curbing methane emissions. What though if methane emissions are now out of our control? That anthropogenically stimulated global warming has triggered a tipping point in the arctic in particular and that natural methane emissions from permafrost and methane hydrates are now set to be unleashed? 

References:

Vaks, A., Gutareva, O., Breitenbach, S., Avirmed, E., Mason, A., Thomas, A., Osinzev, A., Kononov, A. and Henderson, G. (2013). Speleothems Reveal 500,000-Year History of Siberian Permafrost. Science, 340(6129), pp.183-186.

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.

Rachold, V., D. Y. Bolshiyanov, M. N. Grigoriev, H.‐W. Hubberten, R. Junker, V. V. Kunitsky, F. Merker, P. Overduin, and W. Schneider (2007), Nearshore Arctic subsea permafrost in transition, Eos Trans. AGU, 88(13), doi:10.1029/2007EO130001. 

O'Connor, F., Boucher, O., Gedney, N., Jones, C., Folberth, G., Coppell, R., Friedlingstein, P., Collins, W., Chappellaz, J., Ridley, J. and Johnson, C. (2010). Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review. Rev. Geophys., 48(4).

Höglund-Isaksson, L.: Global anthropogenic methane emissions 2005–2030: technical mitigation potentials and costs, Atmos. Chem. Phys., 12, 9079-9096, doi:10.5194/acp-12-9079-2012, 2012.

The power of cattle

Cattle and methane production

A teaser from Cowspiracy

The film was for a time free to watch on YouTube but has since been removed, it now costs $5 from the website www.cowspiracy.com and I would definitely say its worth paying for and watching. Cowspiracy (IMDB review here) is an independent documentary looking into cattle based agriculture, its dark impacts and why seemingly nobody wants to discuss it.

So... Cowspiracy aside what are the facts and science behind methane production from cattle agriculture? The following video produced by Harper Adams University gives an insight into how methane is produced and to the importance in feed in determining methane production:

Key points: Cows have large fore-stomach containing millions upon millions of bacteria called methanogens. These are good as they enable cows to create energy from foods we are not able to, the downside is that they create the waste product of methane. This study is looking into the role of changing feed types has on methane production - the focus of intense research at present.

So, with 1.5 billion cows in the world today:

Livestock populations of chickens and cattle

Source: The Economist (available here)

... which are thought on average to create approximately 100kg methane each per year it's clear cows alone are a major source of methane and hence driver of climate change.

To get things into perspective!

Data: NASA Goddard Institute for Space Science, Figure: BBC (available here)

What are the options for the future?

• Reducing cow numbers - and the global population adopting a diet less dependent on beef and dairy products
• Making cow production system more efficient - less time between calving and less time spent fattening cows
• Methane entrapment and use as a fuel

Before too long no longer just a joke? 

(Original source unclear) 

• Diet modification - studies such as that being undertaken at Harper Adams university in the video shown above are investigating feeds to determine variations in methane production. Others are adopting a different approach; one American study found after years of trials that natural oregano is able to reduce methane production by 40% when given to cows as a supplement at the appropriate dosage!

So... It appears our love cheese, milk, steaks, beef burgers and so on should probably be calmed if the biggest culprit of anthropogenic methane emissions is to be tamed. Or further perhaps, by adopting vegetarianism as a whole and cutting out all animal products more broadly...

"Nothing will benefit human health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet" 

(Albert Einstein, Date unknown)

A spotlight on agriculture

It doesn't take much scrutiny of methane emissions data to see the significance of agriculture as the largest component, using Höglund-Isaksson (2012) data for example we see the author has come to a figure of 324Mt CH4 net for anthropogenic sources in 2005. Of this figure 68.7Mt is calculated to come from cattle alone. Pigs were responsible for 5.6Mt and other livestock around the world 22.1Mt. Rice cultivation was responsible for 26.8Mt. This comes to a total of 123Mt before cereals production other than rice is even considered - agriculture is clearly a massive driver of anthropogenic emissions.

The following figure produced with data from the GMI (Global Methane Initiative) portrays the significance of agriculture as a whole to have risen further by 2010 - contributing almost exactly 50% of total anthropogenic methane emissions:

Data: GMI, Figure: SEF (Sustainable Energy Forum), available here

As ruminant animals form over half of agriculture's contribution, of which the majority comes from cattle as highlighted by Höglund-Isaksson cattle will take on the focus of the rest of the next post.

Anthropogenic sources

Anthropogenic emissions

Following up from our post on natural emissions last week this week we turn our attention to the anthropogenic sources. As with natural emissions there is variance just as to the current level of anthropogenic emissions but most figures cite a figure of about 330 Tg/year for 2005 with papers using a more recent reference year being tending to be slightly higher. An extract from the IPCC's AR5 report is shown below:

Anthropogenic methane emissions

Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change

The paper states:

"Anthropogenic CH4 sources are estimated to range between 50% and 65% of the global emissions for the 2000s ... Anthropogenic sources are dominant over natural sources in top-down inversions (~65%) but they are of the same magnitude in bottom-up models and inventories"

This surpassing, or at least equalling in the magnitude of anthropogenic emissions compared to natural emissions is due to the continued growth of emissions in certain sectors in the decades leading up to the turn of the new millennium:

Trends in global emissions of methane (1970-2005)

Source: Emissions Database for Global Atmospheric Research

Significant increases in methane emissions from 'waste' and from 'fugitive emissions from fuel' (which includes the significant component of leaked methane from the production of fossil fuels) were mainly responsible for the continued growth of anthropogenic methane emissions in the last 3 decades of the 20th century. Since 2000 "emissions started increasing again, with an average growth rate of 1.9% per year, which has meant that since 2002, the emissions increased faster than in the last four decades. This led to a global increase of about 20% over the period 2000-2010, driven by increased coal mining by the top methane emitting country China (+50%) and increased cattle numbers in Brazil (+23%)" (IEA, 2012).

The result of these changes since 2000 mean as of 2010 the make-up of anthropogenic emissions is as follows:

Break down of anthropogenic CH4 emissions, 2010

Source: International Energy Agency, available here 

Another interesting way to interrogate anthropogenic sources of methane is geographically - by mapping its emission such as shown here in another graphic by EDGAR (Emissions Database for Global Atmospheric Research):

Mapped anthropogenic methane emissions, 2009

EDGAR, 2009

The blue shading shown in West Bengal and Bangladesh depict the particularly intense 3000-5000 tonnes CH4 emitted per 0.1degree grid cell per year with parts of China being shown to emit 5000 tonnes and above due in part to their intensive coal mining practices and rice agriculture.

So in summary of this week's post we have learnt:

• Anthropogenic methane emissions are at present thought to be approximately 350Tg/year
• This level of emission is at least equal, if not larger than the sum of all natural methane emissions
• Between 2000 and 2010 anthropogenic methane emissions increased by 20%
• Responsible for 43% of methane emissions agriculture is the largest source of anthropogenic methane which we will look into in more detail next week

Any questions or comments please don't hesitate to comment below!

Till next time...

References:

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. 

International Energy Agency, I. (2012). PART III: GREENHOUSE-GAS EMISSIONS. [online] Emissions Database for Global Atmospheric Research. Available at: http://edgar.jrc.ec.europa.eu/docs/IEA_PARTIII.pdf [Accessed 04 Dec. 2015].

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...

Just how does it work?

Although it's easy to label a gas as a GHG (green house gas) it may also be easy to forget just what this means and how it acts, this post highlight's what methane does to earn its label as a powerful GHG.

Okay so GHG as the name suggests means it behaves a little like the glass in a green house does - it is a layer that acts as a blanket, making it harder for energy to leave the body beneath yet still able to absorb incoming external radiation.

Methane absorbs radiation most effectively in 2 frequency bands seen below. Although the first absorption band is in a quiet part of the spectrum which doesn't see large amounts of incoming or outgoing radiation the second band is in a significant part of the outgoing, infrared spectrum very close to the peak emission wavelength of 10µm.

Figure 1

(adapted from Virginia, 2012)

So to clarify what does this mean exactly? It means methane is very good at absorbing radiation the earth has emitted. This energy is then trapped in the atmosphere, rather than being emitted to space and hence warming the earth. 

This really though is only half the story, visit back soon to find out more about the lesser known secondary warming effects of this interesting GHG.

Clarifying power and potency

I mentioned last week that methane is "100 times more powerful a GHG than CO2 on a pound for pound basis", There are a wide range of different statistics for it's potency as a GHG and this post hopes to summarise the main ones and explain where this suggestion of '100 times' came from.

There are many ways of looking at how effective a gas is as a GHG, the two most known methods being:

- A straight analysis of the gas' physical properties; how it behaves in the atmosphere, how much radiation of different wavelengths it is able to absorb etc. This method has its uses, but makes comparison with other gases GHGs which may behave in different ways difficult.

- A second, frequently used approach is a standardised measure, comparing all gases to the behaviour of benchmark gas CO2. This can be defined as "an index used to compare the relative radiative forcing of different gases without directly calculating the changes in atmospheric concentrations. GWPs  are calculated as the ratio of the radiative forcing that would result from the emission of one kilogram of a greenhouse gas to that from the emission of one kilogram of carbon dioxide over a fixed period of time, such as 100 years." (US Energy Administration). Still a bit unsure about GWPs? Further explanation is available here.

The first working group on climate change created the GWP for their first report in 1995, they suggested then that methane had 21 times the GWP as CO2 over a 100 year time horizon, a figure which has been widely cited since. Their 1995 GWP statistics can be seen in Figure 1.

Figure 1

Source: IPCC AR2

Although the 1995 figures are still regularly cited today the understanding of methane's atmospheric residency time and its direct and associated indirect radiative forcings have improved since then. This has led to improvements in accuracy of the gas' GWP figures. Figure 2 shows the figures the IPCC Working Group 5 relied up in their most recent report (AR5) published in 2013.

Figure 2
Source: IPCC AR5, 2013

Whether cc fb (climate carbon feedback) is incorporated or not the more recent figures suggest that methane is in fact 50% more powerful a GHG as it was first thought to be in 1995, at least over the 20 and 100 year time horizons cited. The revisions in its strength as have been continuous and are shown in Figure 3 below:

Figure 3
Source: Author's own

Now returning to last week's claim that methane is '100 times more powerful than CO2 as a GHG'. 

A bit of backward engineering of the data above hopefully should yield (simplified of course) a ball park figure which hopefully should suggest where figures such as the '100 times' are deducible from:

Working with the most recent AR5 data shown above...

-The lifetime of methane is said to be 12.4 years

-It's GWP over a 20 year horizon is said to be 86

-12.4 as a fraction of 20 years is 0.62

-86 / 0.62 is 138, suggesting that potency of methane is actually 138 times that of CO2.

Despite this being a highly simplified approach to working out the direct, immediate potency of methane compared to CO2 it does yield a very sizeable figure, and greater in fact than the '100' mentioned last week.

Perhaps though we should look to the climate experts for a more reliable, scientifically developed figure rather than one reliant on my simple maths! The AR5 report suggests that it is actually perhaps closer to a figure of about 120, this can be seen in Figure 4 by noting the intersect of the CH4 plot with the x-axis origin, depicting a time horizon of 0.

Figure 4
Source: IPCC AR5, 2013

So in short, a summary of this week's post:

• Data for the potency of methane as a GHG is improving, as is understanding into associated impacts of the gas in the atmosphere
• The GWP of methane on a 20 year horizon is now thought to be about 86 times that of CO2 and 34 times that on a 100 year horizon
• Considering the immediate potency of a molecule of methane as a GHG and its radiative absorption properties it can be said to be 120 times as potent as CO2