Mr. deBrum’s focus is squarely on the West’s wallets — recouping “loss and damage,” in negotiators’ parlance, for the destruction wrought by the rich nations’ industrial might on the global environment. Many other low-lying nations are just as threatened by rising seas. InBangladesh, some 17 percent of the land could be inundated by 2050, displacing about 18 million people. But the Marshall Islands holds an important card: Under a 1986 compact, the roughly 70,000 residents of the Marshalls, because of their long military ties to Washington, are free to emigrate to the United States, a pass that will become more enticing as the water rises on the islands’ shores.