The Polynesian expansion is among the most remarkable maritime exploration endeavors in human history. It includes a chronological anomaly that has occupied archaeologists for decades, known as the long pause. Research published in the Journal of Pacific Archaeology provides a new piece of evidence to this case. Around 3,000 years ago, the ancestors of the Polynesians, the Lapita populations, advanced eastward in the Pacific up to the archipelagos of Samoa and Tonga. They brought characteristic ceramic styles and an insular culture. Then the movement stops. For the next 1,700 years, traces of eastward navigation become scarce. The populations of Tonga and Samoa grow in place and develop a post-Lapita culture that is their own. Between 900 and 1100 AD, everything changes. A phase of eastward migration resumes, of considerable magnitude. Within a century, crews aboard large double-hulled canoes reach Hawaii, Aotearoa, New Zealand, and Rapa Nui, Easter Island. The spread of sweet potatoes in the Pacific also suggests contact with the American continent, a hypothesis that recent genetic analyses have supported. When European navigators reach these waters several centuries later, they discover even tiny atolls populated by communities sharing common cultural and linguistic traits. The great migration of Polynesian peoples toward the eastern Pacific seems to have coincided with a climatic change: droughts in the western Pacific, which may have pushed these populations to explore new lands to the east. Three competing explanations have been proposed by specialists. A first hypothesis invokes a technological advance, vessels finally capable of sailing against the easterly trade winds. A second highlights demographic pressure and the social tensions that result from it. A third seeks an environmental trigger. It is this one that these studies explore. What conditions ensure survival on an island? The reasoning starts from a simple material constraint: on a Pacific island, survival depends on fresh water and food, and these two resources ultimately depend on precipitation. Polynesian populations had adapted to seasonal droughts. A prolonged and severe drought occurring when population density is high poses a problem of an entirely different nature: the island may cease to be able to feed its inhabitants. It remained to determine what the actual climate was in the region of Tonga and Samoa at that time, a question on which data were lacking until recently. The method used relies on hydrogen isotopes, these variants of the same chemical element, preserved in ancient sediments collected from marshes and lakes. Under the tropics, the isotopic composition of rainfall reflects the amount of precipitation. Algae and plants that absorb this water incorporate this chemical signature into their molecules, which can remain in sediments for millennia. This results in a natural archive of past rains. The result is clear. The tropical region of the southwest Pacific experienced a severe and prolonged dry period between 850 and 1200 AD, the driest of the last two thousand years. It coincides with a time when insular populations reached their highest numbers. Why such a long drought? The mechanism lies in the position of an atmospheric structure, the South Pacific Convergence Zone, a vast band of clouds and rain that shifts east or west depending on the surface temperatures of the ocean. Its short-term movements are linked to the phenomena El Niño and La Niña. But this zone can also shift on much longer time scales, imposing on certain parts of the Pacific decades of unusually dry or wet conditions. A convergence of factors. The authors do not present the drought as the sole cause of the phenomenon. Genetic data indicate a rapid increase in the population of Samoa around the year 1000, possibly linked to the arrival of newcomers. Several factors would thus have aligned: a severe climatic stress, an expanding population, and more advanced vessels. A caveat should be noted. The existence of the long pause itself is the subject of discussion among archaeologists, some seeing it partly as an artifact of dating methods and uneven excavation coverage. These studies provide a solid climatic context, they do not close the debate on the motivations of these navigators.