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I suppose I should have emphasized "measured carbon dioxide". That word is crucial.

What you measure is independent of how many plants there are. There can be many plants, or there could be none; either way, the ratio of CO2 in the atmosphere is higher. As a result, temperatures go up.

As an aside, this also means that plants and ocean systems are unable to keep up with our current rates of CO2 production. If they were, homeostatis wouldn't have been disturbed, and the ratio of gasses in the atmosphere wouldn't have changed.