If methane decays to $\small\sf{CO_2}$ in the atmosphere, shouldn't there be much more $\small\sf{CO_2}$?

Question

I am trying to understand the atmospheric ppm data, specifically around how CH₄ decays to CO₂.

This data says 2017 CH₄ is about 1850ppm, and 2017 CO₂ is 402ppm increasing about 3ppm/year. (https://www.eea.europa.eu/data-and-maps/daviz/atmospheric-concentration-of-carbon-dioxide-5)

I've also read that CH₄ has a half-life in the atmosphere of about 9 years, decaying with the chemical equation CH₄ + O₂ → CO₂ + H₄

So if you'd expect methane's 1850ppm to decay to 925ppm over 9 years, that means you'd also expect to see an increase of 925ppm of CO₂, or about 100ppm/yr just from methane decay. That'd be insanely high. But the data I linked earlier shows an increase of 3ppm.

So what am I missing in this analysis?

Answer 1

So what am I missing in this analysis?

The linked data says 2017 CH₄ was about 1850 ppb, not 1850 ppm. Dividing parts per billion (ppb) by 1000 results in parts per million (ppm). In other words, the atmospheric methane concentration was about 1.850 ppm in 2017. Assuming an exponential decay with a half life of 9 years means that about 0.14 ppm $\left(1850*(1-2^{-1/9}\,)\right)$ of the 2.4 ppm increase in the 2017 to 2018 CO₂ concentration was due to methane decomposition. That's a small but not insignificant portion of the increase.