All GHGs are equal, but CO2 is more equal than others

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In a previous post, I discussed how an increase in the concentration of some Green House Gases (GHGs) is leading to an increase in global temperatures, leading to climate change. While the previous post deal generally within a wide ambit, this post focuses on one particular concept - the Green House Gases.

Most discussions of climate change revolve around CO2 concentrations. Why?

CO2 is not the only GHG

There are at least 20 known GHGs. These include water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), chlorofluorocarbons (CFCs), among others. All of these gases have the ability to absorb radiation in that particular band of wavelength which causes them to trap heat in the Earth's atmosphere. Scientists monitor the concentrations of all these gases in the atmosphere.

CO2 is not the most abundant GHG

Water vapour is! However, water vapour concentrations are (a) natural, (b) difficult to "control" and (c) critical in the grand scheme of things (the hydrological cycle). Controlling water vapour is hence, not the central focus in the climate crisis. This is important to note since I often come across conspiracy theories where scientists are "hiding" the role of water vapour as a GHG. This is a pretty basic fact in climate science, and if it's being "hidden", I wonder why no one thought of erasing it from all the textbooks. Water vapour is an important GHG, it's just not what is leading to the current climate crisis.

CO2 is not the most potent GHG

SF6 is.

GHGs can be discussed in terms of their Global Warming Potential. This index basically tells us how much global warming one molecule of that gas would cause over a period of time, compared to CO2. Naturally, since everything is being normalised to CO2, CO2 has a Global Warming Potential of 1. Methane has a Global Warming Potential of 23, which means that one molecule of methane is 23 times worse than one molecule of CO2, in terms of the global warming it would cause. SF6 has a Global Warming Potential of 23900. Whew!

CO2 concentrations do not have the greatest anthropogenic contribution

Compared to the chlorofluorocarbons (CFCs) or the halogenated gases or SF6, CO2 concentrations have a smaller contribution by man. While 100% of the CFCs etc come from anthropogenic emissions (i.e. they are not naturally found in the atmosphere), about 28% of methane and 3% of CO2 comes from anthropogenic sources.

So why do we keep talking of CO2?

Let's answer this question in 2 ways.

First, why do we not make the other known GHGs the central focus?

Water vapour is the most abundant GHG but it also has a very short residence time. This means that while there is a lot of water vapour in the air, it is naturally cycled quite quickly through precipitation. We also know that water vapour is not the culprit in the modern climate crisis. As scientists put it, it is a feedback, not a forcing (more on this in a later post). Lastly, any attempts at altering the water vapour concentrations come with considerable risk of unpredictably altering precipitation patterns, which could lead to disastrous effects (including economic effects). All of this does not mean that scientists aren't studying water vapour for it's GHG effect; it means water vapour is not the central focus in climate solutions.

There are several far more potent gases than CO2  The artificial CFCs are several thousand times more potent than CO2  for example. Such gases are often called high-GWP (Global Warming Potential) gases. They are being studied and monitored quite closely and some of them are already regulated by international treaties. High GWP remain a cause for concern, however, they are currently in very small concentrations in the atmosphere. For example, the concentration of SF6 is in the order of parts per trillion, while the concentration of CO2 is in the order of parts per million. (CO2 is  ~106 times more abundant.)

Methane concentrations have a greater anthropogenic contribution than CO2  This means that if we compare methane and the CO2 concentrations in the atmosphere, a larger proportion of the methane has been put into the atmosphere by us, compared to CO2. There are many natural sources of CO2. However, methane has a much shorter residence time in the atmosphere. After emission, an average molecule of methane takes roughly 12 years to move out of the atmosphere. Any decrease in methane emissions will show a rapid result, and methane remains a GHG in focus.

Second, what's special about CO2?

Perhaps the greatest cause of concern regarding CO2 is its residence time. A molecule of CO2  once emitted, may remain in the air for several centuries. Natural processes take a chunk of emitted CO2 out of the atmosphere fairly quickly (in the order of decades) but a large proportion of CO2 remains in the atmosphere remains beyond that time and may stay in the atmosphere even for millennia. It is difficult to arrive at an exact value of residence time for CO2  which makes the effects of emissions unpredictable and far more lasting than for other gases.

This is a rather unnerving thought. This means that any policy changes around methane , for eg., may have a quick impact, but any policy changes around CO2 will affect many, many future generations. As with all GHGs with a large residence time, the quicker we stop emitting CO2, the better it is for all of us, and our children, and our grandchildren, and our great-grand children ... you get the gist.

CO2 also forms the chunk of anthropogenic GHG emissions. Of all the GHGs we emit, CO2 takes the first spot. If we had to stop reducing our GHG emissions, it makes sense to start with the greatest culprit.

Source: IPCC AR5

Another reason for the greater focus on CO2 is the availability of alternatives. A sizeable chunk of CO2 emissions comes from fossil fuel emissions. Due to the development of renewable energy, we now have a variety of alternatives to fossil fuels. Renewable energy also helps on other fronts - for eg., shifting to renewables leads to a dip in pollution levels.

Conclusion

There are at least 20 known GHGs. There are many parameters used to study GHGs. CO2 may not be at the top when we look at a particular parameter, but the current emphasis on CO2 is justified. Most comprehensive analyses talk of CO2, CH4 and N2O. For non-scientists, keeping up with the CO2 levels is an adequate way of keeping up with how far we are from solutions to the climate crisis.

In short, all GHGs are equal, but CO2 is more equal than the others. So far.
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