A multi-centennial drought reconstruction from tree-rings reveals a growing threat to Christmas Island's water resources

Small islands that depend on limited freshwater resources are at significant risk from seasonal drought, which poses a major threat to both their ecosystems and communities. Christmas Island, located in the eastern Indian Ocean, presents an example for which severe drought conditions during the wet season not only affects its freshwater resources but also biodiversity on the island, including the migration pattern of the iconic red crab species. However, short-term instrumental climate records on this island make it hard to quantify drought variability and assess its associated risks. Tree growth is affected by drought via reduced soil moisture, and hydroclimate reconstruction from tree-ring chronologies can therefore provide longer-term information on historical variability of dry and wet periods. Here, we reconstructed the wet season (December-May) self-calibrating Palmer Drought Severity Index (scPDSI) for Christmas Island using 64 remote tree-ring chronologies from Asia, Australia, and New Zealand. scPDSI was reconstructed using the Point-to-Point Regression (PPR) method and compared with regional marine coral proxies for independent verification. The remote tree-ring chronologies explained more than 66 percent of scPDSI variance (R-squared) over the calibration period. The trees identified as significant predictors in the regression model were primarily located in areas affected by the Indo-Pacific climate drivers including the Indian Ocean Dipole (IOD). The reconstructions span 1540 CE to 2000. During the first four centuries of this period, the frequency of extreme (5th percentile) droughts and pluvial events rarely exceeded one event per 13 years. In contrast, the frequency of both extremes experienced an unprecedented increase during the 20th century, and with a notable shift towards dry conditions. These findings highlight a significant shift towards more frequent and severe dry conditions during the wet season on Christmas Island, posing a challenge to water resource management and potentially threatening the island's ecosystem and services to the community.