Let me preface this by saying I believe in AGW, and that it's an existential risk.
> I don't see why this needs to be politicized this heavily
One huge reason is that the coincident ideologies of the left can't be taken seriously by those on the right. Or, put another way, they don't believe them.
Specifically, if one was truly serious about the urgency of solving climate change, they would not simultaneously advocate for raising the standard of living of non-Western populations to Western levels, before the carbon footprint required of said standard of living was dramatically reduced first.
Carbon emission reduction right now, with the urgency required to prevent collapse, is fundamentally incompatible with open borders. So when someone on the right sees an open-borders advocate clamoring for immediate carbon emissions reductions, the conclusion is not necessarily that they don't believe the science, it's that they don't believe you do. And the only valid conclusion is that you're motivated by politics, not by legitimate concern for the species.