Tropical Indo-Pacific intermediate waters remain in a Little Ice Age cold state

Model simulations predict that the deep Pacific Ocean still harbors cooling from the Little Ice Age (LIA; ca. 1450 to 1850 CE), implying vast unrealized heat uptake. Yet proxy records capable of testing this prediction at intermediate depths over the past millennium remain scarce across the Pacific. We address this gap using Li/Mg thermometry on a living cold-water coral from 1345 m in the South China Sea, with U-Th dating confirming continuous growth from the Medieval Climate Anomaly (MCA; ca. 850 to 1250 CE) to present. Li/Mg data reveal a cooling of 1.0 ± 0.2 °C from the MCA to the LIA, with the trend persisting at a reduced rate into the early Current Warm Period (ca. 1850 CE to present) and no recovery to MCA levels, a pattern also captured by Community Earth System Model (CESM) Last Millennium Ensemble simulations. In contrast, sea-surface temperatures have recovered from LIA levels, revealing a pronounced vertical thermal decoupling. These results demonstrate that deeper intermediate waters beneath the Indo-Pacific Warm Pool remain in a LIA cold state, implying that the ocean interior’s heat uptake capacity is far from exhausted and that recent increases in intermediate-depth heat content likely mark the onset of a protracted equilibration process rather than the approach to a new steady state.