Because if large-scale nuclear or biological war is reasonable likely then climate change really doesn't matter anyway. End of the world scenarios are always an easy "sell" but they are usually not helpful for constructive progress.
Before nuclear war, lots of constructive things like geoengineering could happen.
You're looking at it from the wrong end. Nuclear war seems unlikely now, but it might become reasonable once half of the world's population is banging on the borders of the other half.
Do you think if e.g. India or Pakistan start experiencing lethal wet-bulb temperatures and people there decide to mass-migrate to survivable lands, only to be stopped at the border and denied entry, will those people and these countries just throw their hands up in the air, seeing there's nothing they could do, and accept imminent death? Or maybe they'll try to threaten or force to be let in, using any means available, up to and including nuclear weapons?
(This applies to all nuclear-armed countries - I mentioned these two, because AFAIK they'll be the first nuclear powers likely to experience lethal wet-bulb temperatures - but that's not the only way climate change could make a country mass-evacuate, and such other factors could hit other nuclear powers first.)
So they commit suicide to escape deadly temperatures? If they nuke the countries they want to move to - how does that play out? The choices of providing local cooling, putting more sulfur dioxide into the atmosphere, shades in space etc. will all look more attractive than extended suicide. That also includes negotiating solutions.
See my comment on brinkmanship elsewhere in the thread.
Nuking your desired safe haven is obviously a stupid idea. But threatening them with nukes so they accept you in, that's a reasonable negotiation tactic when you're otherwise up against the wall. If they refuse, making a stronger threat is an obvious next move. There is, however, no obvious stopping point before tensions are so strong that a small mistake - or someone at high enough level getting irrational - will cause the nukes to be launched.
As I pointed out, it's not a reasonable negotiation tactic as following through on the threat would accomplish nothing - and both sides know that. Even if you succeed with your threat and some of your people are allowed in and then the border is closed again - what then? Nuke your own people?
All cooperative moves are much better than extended suicide - for both sides.
> it's not a reasonable negotiation tactic as following through on the threat would accomplish nothing - and both sides know that.
It's effective, because the other side is forced to bet their own lives on you eventually accepting defeat and agreeing to die, so that they may live. It's not an easy bet to make and stick to.
All cooperative moves are indeed better than suicide, but when we're at the point everyone wants to get to NZ, we're way past the time of cooperative solutions.
There is no indication that it will be even an effective tactic in the situation described. Even in simple cases, the other side could assume it might intercept some of nukes, some might not work, it might get some with conventional weapons on the ground - it will never be so clearcut (even ignoring any preemptive moves).
Also, assuming that there are only zero or negative sum moves left is a really big assumption.
