We already have tremendous political instability in the US and Europe right now caused in part by a not-so-great-but-not-devastating economy and the Syrian refugee crisis. Imagine a worldwide crisis with tens or hundreds of millions of people losing their homes and livelihood, how is that going to play out?
40% of global population lives in coastal areas of lesser elevation than 50m.[1] That is more like 3 billion people. Syrian refugee crisis seems quite insignificant in comparison. Of course, only one of those is currently underway, and we don't really know how fast sea levels will rise.
>We already have tremendous political instability in the US and Europe right now
This is definitely not true for the US. At least not by any global notion of political instability. The US government isn't near collapse, there is a very limited likelihood of any kind of coup, the economy is doing very well, and riots have been very rare.
There is a lot of political bickering, but it doesn't seem to be any more-so than during the last party-in-the-office changes (Clinton to Bush, Bush to Obama).
