What specifically is going to cause mass migrations in your mind, and on what time scale? Sea level rise on the order of 5 feet in 80 years is hardly going to make most countries uninhabitable. Over longer time scales will see much larger increases, but the yearly change is never projected to be that rapid.
Water availability is a concern for farming and industry, but not really people. Desalination is cheap enough for personal use even in India. Every 10 gallons a day is an extra ~4$ per year. Projecting US use age on global poor is unaffordable, but the global poor don’t use nearly that much water.
As armchair academic I kind of understand you. People live in all sort of horrible conditions today - famine, malaria, diseases. As living person I ask myself what if my home was destroyed by flood? What if there is no water to grow food? What if person from the country which caused most of CO2 emissions [1] expects me to be contempt? Shouldn't I blow them up? Maybe all I need to commit is a good preacher.
Last mass migration caused by ISIS. COVID caused reduced oil consumption is enough to make a trouble for oil exporters. Global warming is worse - drought, flood, fires. World is extremely fragile, resources supplied from another part of the world, even today cobalt mining is horrible, what would be then? I've heard of another civilization and broken supply chains [2].
Supply disruptions occur with every war or crisis yet modern economies are extremely resilient. The trap is thinking the economy needs exactly what it’s getting rather than it being the result of a giant optimization problem on available resources. Just look at the US GDP drop from Covid vs the percentage of people staying home.
I also think that you are assigning agency to the general public in ways that are unlikely to hold up. Clearly groups could whip up hatred over global warming, but they can do that over just about anything. The root cause is almost never as important as the people guiding things and the goals being pushed.
As to [1] what’s interesting to me is the US emissions are currently about 1/2 of China’s and dropping fairly quickly while China’s are still expanding rapidly. Dropping US emissions will continue to help, but it’s currently less important than slowing how fast China’s increase. And thus someone aims the mob at a slightly different target.
Water availability is a concern for farming and industry, but not really people. Desalination is cheap enough for personal use even in India. Every 10 gallons a day is an extra ~4$ per year. Projecting US use age on global poor is unaffordable, but the global poor don’t use nearly that much water.