The non-hypothetical is that climate change is going to massively worsen outcomes of poor people, especially in tropical and subtropical regions. The rate of migration from Africa to Europe is only going to increase longer term, no matter how many dictatorships EU and UK are making deals with. Unless we act to limit climate change and help people in these regions to adapt. But in any case, we need to learn to better deal with migrants rather than try to keep them at bay at all cost.