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The trade winds cause strong upwelling in the eastern equatorial Pacific, and create the eastern Pacific Cold Tongue (EPCT) that has far‐reaching impacts on global climate. However, large discrepancies persist in quantifying 20th‐century EPCT sea surface temperature (SST) changes across different instrumental data sets. Here we synthesize four coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a Cold Tongue Index (CTI) reconstruction for 1887–1997. The coral CTI record shows a rapid 20th century warming of the EPCT, suggesting an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in reconstructed EPCT SST show an association with the Walker Circulation. Our reconstruction indicates that recent EPCT cooling during the global warming hiatus is not unusual in the context of the 20th century. Our results provide new evidence for 20th century EPCT SST changes and an observational constraint for predicting future tropical climate changes. Plain Language Summary: The tropical Pacific exhibits an asymmetric pattern of sea surface temperatures (SST) along the equator, with a warm pool in the west and a cold tongue in the east. The Eastern Pacific Cold Tongue (EPCT) plays a crucial role in influencing global climate. To date, however, considerable disagreement persists about changes and causes of the EPCT SST over the 20th century due to the sparse instrumental observations before ∼1960. Here we synthesize published coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a coral Cold Tongue Index (CTIcoral) for the period 1887–1997. The CTIcoral record correlates significantly with instrumental data since 1960, and thus allows reliable assessment of 20th century EPCT SST changes. Our record exhibits a rapid 20th century warming trend of the EPCT, indicating an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in CTIcoral also show an association with the Walker Circulation. Comparing the magnitude of the 1992–2011 trend in instrumental CTI with our reconstruction reveals that recent EPCT cooling during the global warming hiatus is not anomalous in the context of 20th century. Our findings have implications for predicting future EPCT SST changes by offering a constraint for model simulations. Key Points: Synthesizing coral Sr/Ca‐SST records yields robust estimates of 20th century warming trend in the eastern Pacific Cold Tongue (EPCT)A coral‐based reconstruction indicates an underestimation of 20th century EPCT warming trends in instrumental SST recordsRecent EPCT cooling during the global warming hiatus is not anomalous in the context of the 20th century [ABSTRACT FROM AUTHOR]

Widespread coral bleaching and mortality associated with global warming have occurred frequently since the phenomenon was first documented in the early 1980s. One such episode that occurred in 1991 over the tropical Pacific‐Indian Ocean region has been attributed to high summer sea surface temperatures. However, we found that sea surface temperature values from the South China Sea region do not provide a comprehensive explanation for the event. Our results, based on time series records of rare earth elements, trace element Al/Ca ratios, and microdomain images from corals in the South China Sea, suggest that this coral mortality event was exacerbated by heavy ash fallout from the cataclysmic 1991 volcanic eruption of Mount Pinatubo. Our findings highlight the profound impact of a volcanic eruption on the modern vulnerable coral reef ecosystems, already under the stress of global warming. Plain Language Summary: Intensive and frequent coral bleaching events have occurred under global warming conditions in recent decades. However, abrupt and extensive mortality episodes, such as which occurred in the South China Sea in mid‐1991, cannot be explained solely on the basis of high summer sea surface temperatures. Here we examine this event in the light of time series of rare earth elements, trace element Al/Ca ratios, and microdomain images for coral samples collected in the South China Sea region. Our results suggest that the second‐largest volcanic eruption of the 20th century, the 1991 Mount Pinatubo volcanic eruption, exacerbated this coral mortality event. The sensitive impacts of such a volcanic eruption on modern vulnerable coral reefs, associated with the stress of thermal conditions, are demonstrated. Key Points: This study reports that the mortality of modern coral reefs could be exacerbated by volcanic eruptionsThe anomalies of rare earth contents and Al/Ca ratios associated with microdomain images indicate an external impact from the 1991 PinatuboOur results highlight amplified responses of vulnerable coral reefs to environmental changes under global warming [ABSTRACT FROM AUTHOR]