While ancestral Polynesians were highly adaptable and accustomed to seasonal droughts, prolonged and severe droughts during times of high population density might mean an island could no longer support its human population. Ultimately, island survival hinges on a single critical resource: rainfall.
Unlocking the climate record
Until recently, scientists lacked evidence from the Tonga and Samoa region of what the climate was like in this critical migration era. 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 sediment for thousands of years, providing a natural archive of past rainfall.
Using this technique, we found evidence of a sustained, severe dry period in the southwest tropical Pacific between 850 and 1200 AD. Our results, recently published in the Journal of Pacific Archaeology, indicate this was the driest period the region had experienced in the past 2,000 years. Crucially, this drought coincided with a time when island populations were larger.
The great migration into the eastern Pacific coincided with a dry climate in the western Pacific:
Why would some islands experience a decades or centuries-long drought? Rainfall 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 shifts east and west over time, driven by patterns of sea surface temperature. Short-term shifts are linked to El Niño and La Niña, but the SPCZ can also move over much longer timescales, bringing decades of unusually dry or wet conditions to different parts of the Pacific.
All this matches up with genetic data that indicates Samoa’s population rapidly increased around 1000 AD, perhaps thanks to the arrival of new people. This suggests several factors aligned—severe climate stress, expanding populations, better canoe technology—to prompt daring exploration eastward.
The story of Polynesian expansion is remarkable in its own right. As Moana introduces new audiences to Pacific voyaging traditions, scientists are continuing to deepen our understanding of the environmental challenges these extraordinary navigators faced—and how they responded with ingenuity, resilience and exploration on an oceanic scale.
David Sear, Professor in Physical Geography, University of Southampton; Manoj Joshi, Professor of Climate Dynamics, University of East Anglia, and Mark Peaple, Research Fellow, Palaeoclimate, University of Southampton. This article is republished from The Conversation under a Creative Commons license. Read the original article.
