I find the fear of mass migration a funny one to be championed by the left(who do the heavy lifting for climate change response, at least in the US), considering their, at best, "meh" reaction to the mass migration that is already happening across the US southern border.
I don't believe the fear/apprehension is specifically about immigration of peoples, but the likelihood that we'll see intergroup conflicts (fuelled by fear mongering rhetoric that usually follows), as well as the fact that these peoples –who are least poised to handle the challenges of global warming– are being displaced in the first place.
The left prefer people don’t have to migrate, but that they can seek asylum when they need to
There is no contradiction between wanting migrants treated humanely and wanting fewer people forced to become migrants. The more migrants there are, the worse they are treated.