I think here's where you're going wrong. There's a difference between "is caused by" and "contributes to".
It would be internally consistent to form an argument that supposes an alternate root cause, e.g., solar activity cycles and cycles of the Earth's magnetic field. One can state that these natural problems are (hypothetically) the root cause, while still acknowledging that CO2 is a greenhouse gas and thus decreasing our CO2 emissions - even if we weren't doing anything naughty on our own - might serve to mitigate the damage expected to be caused by those long-term natural cycles.
The common jargon in Liberalese is "anthropogenic climate change", i.e., climate change created by humans. My argument is that they'd have an easier time getting people on board if they went about this without assigning blame.
Saying "there's a problem we need to solve" is going to get a more productive reception than saying "you created this problem and now we need to solve it".
How would someone go about it, and would you trust them?