While ancient Polynesians were highly adaptable and accustomed to seasonal droughts, prolonged and severe droughts during times of high population density could mean that an island could no longer supports its human population. Ultimately, the survival of the islands depends on a single critical resource: precipitation.
Discovering the climate record
Until recently, scientists lacked evidence from the Tonga and Samoa region about what the climate was like in this critical era of migration. But we were able to reconstruct these past changes by analyzing hydrogen isotopes (slightly different forms of the same element) preserved in ancient mud from swamps and lakes.
In the tropics, the isotopic composition of rainwater reflects the amount of rainfall. As algae and plants grow and absorb this water, they lock this chemical signature into molecules that can survive in sediments for thousands of years, providing a natural archive of past precipitation.
Using this technique, we find evidence for a severe and sustained dry period in the southwestern tropical Pacific between 850 and 1200 AD Our results, recently published in the Journal of Pacific Archeologyindicate that this was the driest period the region had experienced in the last 2,000 years. Crucially, this drought coincided with a time when island populations were larger.
The great migration to the eastern Pacific coincided with a dry climate in the western Pacific:
Most humans arrived in the eastern Pacific shortly after a dry period (marked in orange) of long-term climatic conditions further west (top graph) and a series of sudden “dry shocks” (marked in orange, middle graph).
Why would some islands experience a drought that would last decades or centuries? Precipitation in the tropical South Pacific depends on the position of the South Pacific Convergence Zone, or SPCZ, a major belt of clouds and rain that moves east and west over time, driven by sea surface temperature patterns. Short shifts are related to The Child and La Niña, but the SPCZ can also move on much longer time scalesbringing decades of unusually dry or wet conditions to different parts of the Pacific.
All of this is consistent with genetic data indicating that Samoa’s population increased rapidly around 1000 AD, perhaps thanks to arrival of new people. This suggests that several factors are aligned (severe climate stress, expanding populations, better canoe technology) to drive bold eastward exploration.
The story of Polynesian expansion is notable in its own right. As Moana introduces new audiences to the traditions of Pacific voyages, scientists continue to deepen our understanding of the environmental challenges these extraordinary sailors faced and how they responded with ingenuity, resilience, and ocean-scale exploration.
David SearProfessor of Physical Geography, University of Southampton; Manoj JoshiProfessor of Climate Dynamics, University of East Angliaand brand peopleResearcher, Paleoclimate, University of Southampton. This article is republished from The conversation under a Creative Commons license. Read the original article.
