[gambiting]

>>Other than displacing a large number of people who live on coastlines, what's the big deal?

Those people won't peacefully move somewhere else. Especially in Europe as the middle east becomes literally unliveable and millions of people will start walking north to just survive another day - I can't imagine even the most tolerant european countries willingly accepting this, especially since it won't happen overnight, but over many decades, with more and more people flowing in. That is bound to cause huge amounts of unrest, possibly civil war/ or just war.