True Left

The latter group (numbering in the hundreds of millions, and who are the ones who caused global warming in the first place) must be exterminated, or else the climate refugees (numbering in the billions, and who are not the ones who caused global warming) will be the ones to die.

This would be fair and just. However, finding enough fair and just people to make this happen may prove difficult, as already seems to be the case. Fair people often (falsely) have empathy for even the unfair it seems, and I think you and I could find thousands of examples to prove that point. Perhaps the better argument would be that those who caused climate change should be deported to the lands most affected by climate change, while those who are not largely responsible for the climate crisis should be allowed to emigrate into the lands that are least affected by climate change? At the least, by preparing the argument for fairness in the climate crisis this way, people with empathy for others outside of their own group (non-tribalists) won't have a panic attack when they hear the word "exterminated"?