“We’ll be among the first 15 countries in line,” Mr. deBrum said.

He envisions elevating Marshallese cities as much as six feet and building resilient new drainage systems. “That could buy us at least 20 years,” he said.

For now, on Majuro, the Marshall Islands’ capital, the adaptation to sea-level rise is lower tech. In the neighborhood of Jenrok, a seaside cemetery has been eroded by the rising waves — about 10 rows of coffins and headstones have washed out to sea. To adapt, the Marshallese encase their dead in aboveground concrete tombs, but the rising waves have started to lap at those, too.