Climate change poses a dual threat for sea levels. For one, when land-based polar ice melts, it finds its way to the sea. Second, when water warms, it expands to take up more space — a major yet unheralded cause of sea-level rise. With sea-level rise accelerating at a rate of about one-eighth of an inch per year, the effects on humanity are plain. Though only 2 percent of the world’s land lies at or below 10 meters above sea level, these areas contain 10% of the world’s human population, all directly threatened by sea-level rise. Small island nations such as those in the Pacific Ocean stand to be wiped off the map. The people of Kiribati, for example, are among the world’s first refugees of sea-level rise, and two of the nation’s islands have all but disappeared into the ocean.