Although their ancestral Polynesians were highly adaptable and accustomed to seasonal droughts, prolonged and severe droughts during periods of high population density became common on the island. no longer supports the human population. Ultimately, the island’s survival depends on one critical resource: rain.
Opening the climate record
Until recently, scientists had no evidence of what the climate was like during this critical migration period from Tonga and Samoa. 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 precipitation. As algae and plants grow and absorb this water, they attach this chemical signature to molecules that can survive in the sediment for thousands of years, providing a natural archive of past rainfall.
Using this technique, we found evidence of a continuous, severe tropical dry period in the southwest Pacific between 850 and 1200 AD. Our results were recently published Journal of Pacific Archaeologyindicating that this is the driest period the region has experienced in the last 2000 years. Most importantly, this drought coincided with a time when the island’s population was higher.
The great migration to the eastern Pacific coincided with a drier climate in the western Pacific:
Humans arrived mainly after a dry period in the eastern Pacific (marked in orange), long-term climatic conditions further west (top graph) and a series of sudden “dry shocks” (marked in orange in the middle graph).
Why do some islands experience drought for 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 shifts east and west over time, controlled by sea surface temperature patterns. It is associated with short-term changes Child and La Nina, but so can the SPCZ move over longer periods of timeIt brings decades of unusually dry or wet conditions to various parts of the Pacific Ocean.
All of this is consistent with genetic data showing that Samoa’s population rapidly increased around 1000 BC. arrival of new people. It suggests that several factors combined to spur bold exploration eastward—severe climatic stress, expanding populations, better canoe technology.
The story of Polynesian expansion is remarkable in itself. As Moana introduces new audiences to the traditions of Pacific voyaging, scientists continue to deepen our understanding of the environmental challenges these extraordinary navigators faced and how they responded with ocean-scale ingenuity, resilience, and exploration.
David SearProfessor of Physical Geography, University of Southampton; Manoj JoshiProfessor of Climate Dynamics, University of East Angliaand Mark PeapleResearch Associate, Paleoclimate, University of Southampton. This article is being republished Conversation Under Creative Commons license. read it original article.
