I am not certain about volcanoes, although I tend to lean on Beskar's side (volcanoes have always been the decisive factor in climate shifts), but I will have to point out that CH4 is precisely twenty times more potent grenhouse gas than CO2. In addition to CO2, volcanoes likewise produce a plethora of other gases. Which are generally more pivotal than the CO2, as carbon dioxide is a relatively light compound, quite weak as a "greenhouse gas".
EDIT:
Plant as many as you wish, but you are wasting your time, effort, and funds, climatologically speaking. I repeat: while they take in some CO2, counting on them to lower the planet's temperature is akin to trusting a single maggot to digest a bull, isntead of letting a tiger loose. Dump ground iron in the oceans, and that is a quite sure bet to cool down the Earth as well as increase its oxygen supply. But that coupled with global dimming and the possibility of an impending natural Ice Age will turn our planet into a snowball one. I would rather let global warming do its job, if it is still in force.
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