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The team reconstructed past rainfall using hydrogen-isotope measurements preserved in plant leaf waxes collected from sediment cores in Rano Aroi and Rano Kao, two of the island’s primary freshwater reservoirs. These isotope records provide the first continuous hydroclimate reconstruction covering the full period of Rapa Nui’s pre-contact human history. The analysis indicates a sustained decline of 600–800 millimeters of rainfall per year, far exceeding the magnitude of recent droughts that have already strained the island’s water resources.

The onset of this prolonged drought corresponds with well-documented cultural transitions, including a reduction in large-scale megalithic construction (moai i ahu), shifts in the island’s ritual landscape, and changes in settlement patterns. The findings suggest that persistent freshwater scarcity influenced how communities organized their agricultural systems, accessed coastal groundwater, and adapted ceremonial activities.

The study adds new evidence to a growing body of research indicating that Rapa Nui’s population remained stable prior to European arrival (1722 CE), with cultural change driven by environmental pressures rather than societal collapse. By providing a direct record of past precipitation trends, the work helps clarify how long-term climate variability shaped the island’s historical development and offers context for understanding future water challenges on Rapa Nui.