BANTANG KILLING, Gambia—In this little village in West Africa, Ebrima Nyan is watching his farmland slowly wither away. When Nyan, 47, was a teenager, the village grew all the rice it consumed, in a field alongside the Gambia River. Now that field lies dry and empty, after the river’s brackish water intruded, rainfall flagged and the soil became too salty for crops. By the early 2020s, the salt pr...