When the Rains Moved East

For generations, the story of Polynesian expansion has been told as a triumph of navigation. Double-hulled canoes, star paths, memorized swells, and birds on the wing have rightly taken center stage. But ships do not sail without reasons. A new climate reconstruction¹ suggests that, around a thousand years ago, one of the most basic forces of all began to tilt the balance of island life across the Pacific: freshwater.

a Overview map of site TAH-A in Tahiti and b site NHS-2 in Nuku Hiva. The digital elevation model is the 30 m Copernicus DEM. b, c Corresponding high-resolution satellite imagery for each site, obtained from Google Earth Pro.

A millennial-scale shift in rainfall patterns quietly redrew the hydroclimatic map of the South Pacific. Islands in the west, long settled and densely occupied, grew drier. Farther east, rainfall increased. The result was not a sudden collapse or a single exodus, but a slow change in incentives, pressures, and possibilities. People, quite literally, followed the water.