Methane – From Cows to Greenhouse Gas
A cow farts, and the earth warms. Atmospheric and environmental chemistry is becoming ever more relevant in this day and age; since the Montreal Protocol in 1989 began to phase out the use of CFCs, people have realized that climate change is something that we humans are able to influence.
But seriously, how much of an impact can cows have on climate change…?
Firstly, I want to raise awareness and zero in on methane because the general consensus is that CO2 emissions are the be all and end all – when the term ‘carbon footprint’ is used it actually only takes into account an organization’s release of carbon dioxide. In reality, there are a whole host of other greenhouse gases – each with their own relative contribution to the greenhouse effect. Truth be told, a study into any one of the greenhouse gases would provide an interesting read.
Let’s begin by taking a look at how gases lead to the heating up of the troposphere (the first layer of the atmosphere closest to the ground) as we embark on this atmospheric chemistry adventure. Of course, a certain amount of energy from the sun has to be ‘trapped’ within the troposphere of the Earth by greenhouse gases (most abundant being CO2, H2O, O3, CH4, N2O, CFCs), or the average temperature would be around -18°C (0°F).
How do simple gases do this? Remember the behavior of molecules when exposed to radiation? Depending on the wavelength, bonds undergo different transitions. This is what makes greenhouse gases such as CO2 special; the ability for its C=O bonds absorb and re-emit infrared radiation due to the presence of vibrational transitions. Methane (CH4) is also able to absorb in this range (~14 µm) due to similarly long C-H bond lengths. In this sense around 50% of radiation that would have escaped back to space is re-emitted to the surface by greenhouse gases, albeit through differing rotational and vibrational modes.
From the GWP table above, you can see that methane has 21 times the GWP of carbon dioxide, a number which would be higher if not for its relatively short atmospheric lifetime. The instability of methane is caused by the presence of hydroxyl radicals (HO•) – which is probably the most important oxidant in the troposphere responsible for:
The CH3• radical reacts with oxygen (in the presence of a catalyst) to form a radical product(3), which then reacts with the hydroperoxyl radical (from reactions 8 and 10) to form CH3OOH:
CH3OOH can undergo several branching reactions:
The CH3O• radical from (7) is first converted to formaldehyde (CH2O) via a reaction with oxygen. Formaldehyde is then converted to CO via two pathways; CO reacts with a hydroxyl radical to form CO2 as shown in (1):
From this, you can see that a single molecule of CH4 contributes to the greenhouse effect not just by absorbing IR radiation but also indirectly by formation of multiple CO2 molecules. Methane also contributes to trophospheric ozone and stratospheric water vapor formation (therefore increasing its GWP) through branching reactions from those shown above, but we won’t go through them here. If you have an unquenchable thirst for knowledge though, you can contact me and I’ll be more than happy to help!
Disclaimer: Livestock is responsible for a large part of CH4 emissions, but of course industrial processes account for a more significant amount of greenhouse gas release. The point of this article isn’t to bash cows and their farting/burping habits, but rather to highlight how something seemingly so insignificant can drive climate change.