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Paired measurements of¹⁴C/¹²C and²³⁰Th ages from two Hulu Cave stalagmites complete a precise record of atmospheric¹⁴C covering the full range of the¹⁴C dating method (~54,000 years). Over the last glacial period, atmospheric¹⁴C/¹²C ranges from values similar to modern values to values 1.70 times higher (42,000 to 39,000 years ago). The latter correspond to¹⁴C ages 5200 years less than calibrated ages and correlate with the Laschamp geomagnetic excursion followed by Heinrich Stadial 4. Millennial-scale variations are largely attributable to Earth’s magnetic field changes and in part to climate-related changes in the oceanic carbon cycle. A progressive shift to lower¹⁴C/¹²C values between 25,000 and 11,000 years ago is likely related, in part, to progressively increasing ocean ventilation rates. 

Abstract A full‐spectrum characterization of past interglacial climate is a necessary prerequisite for the detection and attribution of climate changes during the current interglacial. Here we present a speleothem record of Asian summer monsoon (ASM) during Marine Isotope Stage (MIS) 11 interglacial (MIS 11c), from Yongxing cave, China. The record's unprecedented chronologic constraints and decadal‐scale temporal resolution allow a precise and direct comparison of ASM between the MIS 11c and the Holocene. Our data suggest that orbital‐centennial patterns of ASM were remarkably similar during both interglacial, including their pacing and structure. Notably, a multi‐millennial stronger monsoon late in MIS 11c, the “Late‐MIS 11c shift,” is similar to the Late Holocene strengthening of the ASM, the “2‐Kyr shift.” Thus, the multicentennial ASM weakening at the end of the Late‐MIS 11c shift could imply that the current century‐long ASM waning trend may persist into the future, if only natural forcings are considered.