Why we talk more about CO2 even if methane is 80 time more potent — GWP and CO2 equivalents explained.
Short answer:
The relevance of a green house gas is the product of it warming potential, how much of it is in the atmosphere and how long it stays in the atmosphere.
Relevance = Quantity * Hang Time * Warming Potential
Long answer:
When I was starting to read more and more about the climate crisis, I was sometimes confused when I heard that methane is 80 times more effective in warming the planet than CO2. I was wondering, why are we talking about CO2 so much then?
So this blog post aims to answer that question. To do so, we will briefly look into the greenhouse-effect, go through the most important greenhouse gases, global warming potential and carbon-equivalents.
Some basics:
First of all, it is important to understand that almost all energy consumed on earth originates from the sun.
Secondly, it is important to state that our atmosphere is crucial for the survival of humans and most other forms of life on this planet. The 0.0006% of Ozone in our atmosphere, for example, protects us from the damaging UV-radiation that sunlight entails. Before the ozone layer formed 600 million years ago, there was no life on land.
But our atmosphere also does a great job in enabling temperature on earth that a beneficial for life to thrive. We refer to this as the natural greenhouse effect. It is caused by the circumstance that certain (greenhouse) gases are selective in the energy frequencies they absorb. When energy from the sun is entering the atmosphere, it has a different frequency/wavelength compared to when it leaves. This allows the greenhouse gases to let the energy in, but “trap” it on its way out. More energy in the system then leads to higher temperatures.
Thankfully so! Without the greenhouse effect, the temperature on earth would be around -18°C (now it is on average around +14°C). So, in general, we want the greenhouse effect; we just do not want too much of it.
What is global warming potential (GWP)
There are many gases (and other things) in our atmosphere which contribute to the greenhouse gas effect, and some of them are more effective in absorbing and emitting energy than others. In more professional terms: the “radiative efficiency” differs. Additionally, they have a different “lifetime,” i.e. how long they stay in the atmosphere. Carbon does not break down in the atmosphere and therefore has a constant GWP (But it can be taken out of the atmosphere by plants, for example). All other gases are therefore expressed as a multiple of the GWP of carbon.
If we combine these two properties of a certain gas (lifetime and radiative efficiency), we can come up with its global warming potential (GWP). Since we take their lifetime into account, the GWP is dependent on the timeframe. As you can see on the right side of the table.
This table is part of a bigger table from the IPCC on the GWP of certain “species”.
And herein lies part of the answer to our methane question. Methane does break down in the atmosphere. So its effect on global warming vanishes over time. The convention is to use the GWP over a hundred years. So if someone says methane is 80 times worse than CO2, he or she is technically right, but not mentioning that this ratio only holds true for a specific timeframe. In the case of 80 times that had to be a timeframe of shorter than 20 years. But indeed, even if we look at the 100-year timeframe, methane is 21 more effective than CO2.
So whats the missing part?
So GWP compares gases one by one. But in our atmosphere, the number of gases are very different. As you see in this table below, CO2 makes up more of the atmospheric composition. (the table is outdated. Currently, Carbon Dioxide makes up 0.0415%)
And that is why talk so much about CO2. There is just so much more CO2 in the atmosphere, that even though it has a lower GWP, the overall effect of CO2 is higher than the effect of methane or Nitrogen Oxide. To be able to compare the different gases easily, we can convert them to CO2-equivalents (often written as CO2e or CO2eq). As visualised in this nice infographic by Our World in Data:
Those equivalents help to quickly and easily communicate and compare certain emissions.
Those equivalents are calculated by: (amount of gas) x (GWP).
Wrapping up:
So wrapping up, the impact of a gas on the global temperature comes down to:
- How effective it is at absorbing and emitting energy.
- How long it stays in the atmosphere
- How much of it there is in the atmosphere.
So, I hope this was insightful and de-confusing and helps you to get a better handle of your next literature on sustainability.
As we just saw, there are some good reasons to talk mainly about CO2. But the importance of methane and nitrous oxide should not be underestimated. Especially when looking at sectors like agriculture.
Technically it would be more precise to talk about greenhouse gases, expressed as CO2 equivalents or CO2e.
Sources:
Ozone Layer:
- http://www.eartharchives.org/articles/life-on-land-made-possible-by-ozone-layer/
- https://royalsocietypublishing.org/doi/abs/10.1098/rsta.2007.2049
Atmosphere without GHG-effect:
GWP of greenhouse gases:
