Let's say temps rise by 10 degrees and sea levels rise a lot. I would think there will still be a lot of areas that will be inhabitable. I don't think that the atmosphere will become unbreathable. I think you can have a pretty sophisticated civilization with 100 million or even 10 million people.
Again, I am not saying these things are OK. We should worry about displacing millions of people and probably wars but civilization dying seems overblown and damaging to the argument that we should be doing something.
Of course the planet will be inhabitable. Will there be any of us to inhabit it? Or will war break out because of population migration due to the effects of climate change? I don't know the answer to the first question, but the second one is easy -- it's already happening.
These are important issues but I still doubt civilization will get destroyed. Let's warn of likely scenarios. These are bad enough. No need for exaggeration that then can easily be refuted.
Africa is undergoing a mass migration leading to local wars. The Middle East is subject to the same pressure. There’s plenty of evidence that this is directly leading to European immigration and the rise of nationalist movements. None of this is exaggerated — it is current reality.
Again, I am not saying these things are OK. We should worry about displacing millions of people and probably wars but civilization dying seems overblown and damaging to the argument that we should be doing something.