China, as noted by others, stands to lose a lot of arable land and makes plenty of weaponry. India and Pakistan are both likely to become more or less uninhabitable, and have nukes and (for the former) no shortage of people to draft. The United States of America, under a 4C rise, would also see large regions become uninhabitable, and has a northerly neighbour that could not repel an invasion. Russia has already shown a fondness for acquiring territory (see Crimea) when the opportunity presents itself, and Europe is weakly defended.

Not to mention that "set up turrets on the southern border and mow them down" is not the most humane way to handle this, especially since the nations likely to fare OK(ish) are generally the ones that caused this trouble in the first place.