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 global climate change is something that we are able to influence. We assess just how big of an impact greenhouse gases such as methane can have on climate change.
How Gases Heat the Earth
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. For us to survive, a certain amount of energy from the sun has to be ‘trapped’ within the troposphere of the Earth.
This is achieved by a mixture of greenhouse gases: CO2, H2O, O3, CH4, N2O, CFCs. Without these, the average temperature of our planet would be somewhere around -18 °C (0 °F)! These molecules are able to somehow absorb infrared radiation and re-emit them in all directions to warm the troposphere.
Bonds in molecules undergo different transitions as they absorb light of different wavelengths. Those that absorb in the infrared region are greenhouse gases. In CO2, its C=O bonds absorb and re-emit infrared radiation due to the presence of vibrational transitions.
Since re-emission occurs in all directions, around 50% of the radiation that would have escaped back to space is re-emitted to the surface by greenhouse gases through differing rotational and vibrational modes.
Methane: The Lesser-Known Greenhouse Gas
Firstly, the point of raising awareness of methane is to look beyond the general consensus that carbon dioxide (CO2) emissions are solely responsible for climate change. 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, all with their own relative contribution to the greenhouse effect. Many of these (methane included) are more capable of affecting environmental changes when compared with the same amount of CO2.
Methane (CH4) is also able to absorb in this range due to the ability of their C-H bonds to absorb infrared radiation. From the table below, we see that methane has a higher ‘global warming potential (GWP)’ when compared to carbon dioxide.
Global Warming Potentials (GWP)
GWP is a measure of radiative forcing, or the contribution of that emission to trap heat. In fact, carbon dioxide has the lowest GWP out of all the other gases!
This is because carbon dioxide is so abundant in the air that it is reaching a saturation limit. Since the bonds in molecules absorb at very specific wavelengths, virtually all infrared radiation in the region corresponding to CO2 bonds reaching the Earth is trapped by it!
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.
Methane Tropospheric Chemistry
Methane in our atmosphere is quickly broken down due to the presence of hydroxyl radicals. Hydroxyl radicals react with a variety of other compounds in our atmosphere, oxidizing them.
Oxidation of methane forms a CH3 radical. This in turn reacts with oxygen (in the presence of a catalyst) to form another radical product (3). It then reacts with the hydroperoxyl radical (formed later in the process) 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 (9+10, 11). CO can then react with a hydroxyl radical to form CO2 as shown in (1):
If you followed through the entire process, you can see that a single molecule of CH4 contributes to the greenhouse effect not just by absorbing IR radiation but also indirectly by the formation of multiple CO2 molecules.
Methane also contributes to tropospheric 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.
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 innocuous like methane can drive climate change.
About the Author
Sean is a consultant for clients in the pharmaceutical industry and is an associate lecturer at La Trobe University, where unfortunate undergrads are subject to his ramblings on chemistry and pharmacology.