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

Welcome

Hello and welcome to this perhaps seminal blog on methane.

BBC, April 2014

Although it may be easy to dismiss Peer Simon's suggestion about flatulence caused by baked beans as  being completely ridiculous his attempt to bring the public's attention to the role of flatulance, and in the broader sense, the role of methane as a contributor to climate change perhaps is not as ridiculous as it first sounds!

To many 'Methane' may just conjure up associations of a dodgy smell, for others perhaps too they may think of its use as a common gas used for heating and cooking. There are others though who preach of its importance in arena of global warming and to its grave suggested global warming potential in the near future.

This is a blog investigating the significance of methane in the atmosphere and its influence on environmental change today and into the future.

Blogging is a first for me and please bear this in mind in the first formative few weeks as I find my feet... Alright! So... It's an odourless gas and a hydrocarbon fuel but what other basics do we need to know before we get started? It's composition maybe? Methane is a simple hydrocarbon; it comprises of one carbon atom bonded to four hydrogen atoms. The smell commonly associated with it comes from other gases often accompanying it in gas supplies.

Is it really a big deal though? Is it an important GHG? Is this a substantiable claim? (and a worthwhile topic for a blog!?). Well, we will find out. False assumptions will be quashed, statistics scrutinised and the real truth about Methane as a GHG unearthed over the coming months.

Methane is a less dense gas than air meaning if it is released into the atmosphere at surface level it is likely to make its way up into the upper atmosphere where the air is thin and water vapour levels low. It is likely to persist there for about 12 years, a period much less it should be noted than the (open to debate) atmospheric residence time of 'rival GHG' CO2. Over these 12 years though its ability as a gas to absorb solar radiation far outstrips CO2 as it can be said to be about 100 times more powerful a GHG than CO2 on a pound for pound basis. 

Statistics like this sounds quite grave really but surely there's not much of it to make a big impact, is there? It's all about CO2 in the grand scheme of things is it not?

Sam Carana, 2013

Well... Figures such as these highlighting the recent changes in atmospheric concentration of CO2 and of methane over the past 400,000 years suggest perhaps methane is due some more attention in light of atmospheric changes seen since the rise of man as the dominant species on earth. When CO2 is running at about 65% above its mean level for the past 400,000 years methane is currently running at a level 230% above its mean level from the last 400,000 years.

IPCC AR5, 2013

Clearly today's levels are unprecedented for many hundreds of thousands of years but the extent of variance from the historical mean is 3.5 times greater for methane than it is for CO2 and a contributing reason for the focus of this blog as methane's significance as a major GHG continues to rise.

Let's take a look at following figure to consider its recent increases a little further:

Although both CO2 and methane are clearly rising fast the level of methane can be said to have been at approximately 1,800 parts per billion in 2000. Studying the CO2 figure, for fair comparison, it could be said that in 2000 the CO2 level was at about 370 parts per million, or 370,000 per billion. In 2000 it could hence be said that atmospheric CO2 concentrations were about 200 times greater than atmospheric methane levels, which is a number regularly cited when comparing relative atmospheric presences today.

Considering this ratio of 1-200 and that methane is 100 times more a powerful GHG than CO2 it can easily be seen that methane's radiative forcing contribution is about 50% as great of that of CO2's. In fact, it is even more significant than this, according to the IPCC AR5 report the RF of CO2 today when published in 2013 was 1.82W/m2, for methane it was said to be 0.97W/m2 - 53% as great as the radiative forcing resulting from atmospheric CO2.

Clearly then methane is an important GHG and as the lesser discussed sibling as the perennially discussed CO2 it will form the focus of this blog going forward in the coming weeks.

I hope you enjoy reading and following the developments over this time and consider following my blog/engaging below on any topics, if you wish to.