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I have a theoretical exercise where I calculated the amount of CO2 that is released to the atmosphere per year (4.6 ppm). We assume that the relative increase of CO2 is 2 ppm, hence the percentage of CO2 that stays in the atmosphere is around 2/4.6 ~ 40 %.

What is the technical term for this percentage?

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  • $\begingroup$ Possibly residual or accumulated CO2. $\endgroup$ – Fred Oct 21 '18 at 7:52
  • $\begingroup$ Atmospherically persistent CO2? $\endgroup$ – Trevor J. Smith Oct 23 '18 at 14:37
  • $\begingroup$ Mauna Loa CO2 data shows the actual amount in the atmosphere and the continuing increase ; one does not need to assume anything. Unless you are separating "good" from "bad" CO2. $\endgroup$ – blacksmith37 Oct 27 '18 at 16:39
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We call this the airborne fraction, although as the name suggests, it's normally expressed as a fraction rather than a percentage.

Raupach et al (2014) is an (open access) example of it being used in the literature, and in that paper they quote a long-term 1959 to 2012 value of 0.44, so in the same ball-park as your estimate. Similarly, here's an example of it's use in Chapter 7 of the IPCC AR4 with a range of modelled values from 0.4 to 0.5. It's also often calculated over single years, as in your example, to look at the trends and variability.

As an aside, the 2 ppm in your calculation is called the atmospheric CO2 growth rate and the total mass of CO2 in the atmosphere would be the atmospheric CO2 burden.

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