(Rob Griffith | AP file photo) An excavator moves rocks and sand in 2015 to aid in the construction of sea walls around the airport on Majuro Atoll in the Marshall Islands. Climate change poses an existential threat to places like the Marshall Islands, which protrude only 6 feet (2 meters) above sea level in most places. King tides, when the alignment of the Earth, moon and sun combine to produce the most extreme tidal effects, and storm surges cause floods that contaminate fresh water, kill crops, and erode land. As a result, some Marshallese think an exodus as inevitable, while others are planning to stay and fight.