Global warming causing war between nuclear armed opponents. Migration flows and competition over ever shrinking resources.

It's silly to treat these things as separate categories as if only one will happen at a time. These issues are happening, and are going to happen all at once. That is the fundamental challenge. AI plays into that because it can make wars and nuclear exchanges so much more effective.

If an AI tells the President the USA can survive China's nuclear barrage mostly unschathed, perhaps he'll press the button...

Runaway warming leading to a hot "supergreenhouse" climate was theorized. Alterbatively a snowpiercer scenario creating a snowball earth.

Or, once billions of people die from climate change, planetary wars will start on the last hospitable areas, leading to the end of humanity.

In contrast, AI ending "humanity" is limited by our own material power, would likely be stoppable, and would not easily reach areas isolated from technology

There's a lot to unpack with this, and I am not an expert, but: crop failure across industrial monocrops leads to food shortages, wild fires destroy significant areas which support pollinator ecologies, fresh water become contaminated by rising sea water and blighted by drought, and perhaps most significantly, as resources become strained, humans become desperate and armed conflict proliferates. This is just top of mind.

Worst case is warming oceans create a hypoxic environment where anaerobic bacteria thrive en masse and generate H2S over country-scale areas undersea. The chemocline breaches the surface and the ocean and atmosphere become poisonous to most complex life and agriculture, also stripping the ozone layer in the process, irradiating the surface. This is one mechanism posited for the end-permian mass extinction that eradicated most ocean and surface life, including the trilobites.

Not sure how long we'd survive such a scenario, even sheltering underground. But surely it couldn't happen to us.

(However, it now seems like the AI might get us first.)

People already entrenched near the poles will fight to keep others out. Right-wing anti-immigrant parties are on the rise right now already.

They also tend to not want to recognise climate change -- because that would imply that people could migrate because of it.

Citation needed for evidence of human cooperation at this scale. As you say, climate change is slow moving, and there's no real evidence across the last 60 years of knowing about it suggesting humans are willing to do what it takes to survive it.

Climate change isn't real because we can just move under the oceans hahahaha

Because nuclear war balances itself: more nuclear bombings = less nuclear weapons, less people to send them. It will naturally stop at some fraction of humanity left which is incapable of making and launching more nukes

In the exact same way major sudden changes in the climate have lead to the extinction of the majority of plants an animals over this planets history. We are not special.

Humans need food. Food does not just appear in supermarkets.

Hard disagree here, population growth is still happening and will keep happening for some time, maybe step out of your bubble if you don't see that.

Africa is still growing massively for example, world is not just western civilization. Sure at that rate and incerase of living conditions for everybody maybe in 1000 years population will be smaller, but its not that hard to fix if wanted - people used to have 10-15 kids as default.