Yuli Seperi/ReutersEarth’s political boundaries will always shift as conflict rises and regimes fall. We are used to this sort of change and the out-of-date maps it produces. But nothing in the history of human diplomacy has prepared us for what’s coming next — countries that physically cease to exist as the land they are made of is swallowed up by rising seas. For low-lying nation states like the Maldives, Tuvalu, the Marshall Islands, and Kiribati, it’s not a question of if, but when.