Archaeologists have long puzzled over the long pause — a 1,700-year halt in Polynesian eastward expansion after Lapita peoples settled Samoa and Tonga around 3,000 years ago. Between 900 and 1100 AD, voyagers in double-hulled sailing canoes suddenly reached Hawai'i, Aotearoa, and Rapa Nui. New research published in the Journal of Pacific Archaeology identifies a sustained drought as the catalyst.

Researchers reconstructed rainfall history by analyzing hydrogen isotopes locked in algae and plant molecules preserved in swamp sediment cores from Tonga and Samoa. The isotopic signature of tropical rainwater shifts with precipitation volume, creating a chemical archive. The data reveal the driest period in 2,000 years between 850 and 1200 AD, coinciding with peak island populations and the migration pulse.

The drought traces to a westward shift of the South Pacific Convergence Zone, a rain belt driven by sea-surface temperature patterns that operates on decadal timescales beyond El Niño cycles. Genetic evidence shows Samoa's population surged around 1000 AD, amplifying resource pressure. Climate stress, demographic growth, and improved canoe technology aligned to force exploration.

This work demonstrates how paleoclimate proxies can resolve long-standing archaeological debates. The isotope technique applies broadly to tropical regions lacking instrumental records, offering a template for linking environmental change to human migration patterns worldwide.