Changing climate pushed islanders to 'chase the rain' across the Pacific 1,000 years ago

Research by the University of Southampton and University of East Anglia (UEA) shows a major shift in South Pacific climate conditions—beginning around 1,000 years ago—that may have pushed people to settle further east and move away from increasingly drier conditions in the west.

Settled islands in Western Polynesia, such as Samoa and Tonga, became drier, while more remote ones in Eastern Polynesia, for example French Polynesia (Tahiti), gradually became wetter and more attractive for colonisation.

This latest study, part of a wider project between Southampton and UEA called PROMS (Pacific Rainfall over Millennial Timescales), examines this shift and its likely impact on migration.

Findings are published in the journal Nature: Communications Earth and Environment.

Principal Investigator for PROMS, Professor David Sear comments: “The Pacific Islands today are under threat from changing climate, but we can see from our research that this is not the first time the inhabitants of the region have had to adapt to a changing climate.

“Our research suggests that beginning around 1,000 years ago, people in the region were effectively chasing the rain eastwards as part of adapting to the stress placed on growing populations by a period of drier conditions developing in the western South Pacific.”

The research team collected sediment cores on the islands of Tahiti and Nuku Hiva in Eastern Polynesia to analyse plant waxes – fatty layers left on leaves. World-leading laboratory analysis of these plant waxes reveals how wet or dry the climate was when the leaves grew. The team combined these new records with other records from across Polynesia, in the Pacific.

From this state-of-the-art data, the research team estimated how rainfall had changed across the Pacific during the last 1,500 years. Together with new climate model simulations, the team were able to uncover when and where this climatic shift in rainfall occurred, and what probably caused it.

The most likely cause is that a natural change in the pattern of sea surface temperatures across the Pacific drove an eastward shift of the South Pacific Convergence Zone (SPCZ) between approximately 1,100 and 400 years ago. The SPCZ is one of the biggest features in the global climate system, a region of high rainfall stretching over 7,000 km from Papua New Guinea to beyond the Cook Islands. The climate shift identified in this new study saw the western part of this rain band become progressively drier, and its eastern part wetter.

The researchers believe this long-term drying of western areas could have acted as a ‘push’ for migration, while the increase in rainfall and freshwater availability in the east may have served as a ‘pull’ to settle new islands. It’s possible the climate shift acted as a driver for people, encouraging them to sail progressively east to islands such as the Cooks and Tahiti.

Co-lead author on the paper, Dr Mark Peaple of the University of Southampton says: “The timing and nature of the hydroclimatic change align with the final wave of human settlement into Eastern Polynesia, which began around 1000 years ago.

“Water is essential for people’s survival, for drinking and successful agriculture. If this vital natural resource was running low, it’s logical that over time the population would follow it and colonise in areas with more reliable water security – even if this meant adventurous journeys across the ocean.”

Co-lead author at UEA, Dr Daniel Skinner adds: “Bringing together knowledge from palaeoclimate archives and climate models has given us key insights into how and why a critically understudied region of the world changed over the last 1,500 years.”

Co-Principal Investigator Professor Manoj Joshi, also from UEA, says: “By better understanding how the climate of the South Pacific has been affected by larger-scale climate changes over past millennia, we can build better predictions for how future climate change will affect the region.”

The scientists hope more research and archaeological analysis can further refine the timing and scale of both environmental and societal changes in the South Pacific.

Fieldwork was supported by Explorer grants from National Geographic Society.

Ends

Notes to Editors

1. The paper ‘Ocean variability drives a millennial-scale shift in South Pacific hydroclimate’ is published in the journal Communications Earth and Environment, DOI: https://doi.org/10.1038/s43247-025-02676-5
    
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3. This research was funded by the UK’s Natural Environment Research Council (NERC).
    
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6. More about the University of East Anglia can be found at: https://www.uea.ac.uk/
    
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