Corals reveal ENSO-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo.

Extreme climate events, such as the El Niños in 1997/1998 and 2015/16, have led to considerable forest loss in the Southeast Asian region following unprecedented drought and wildfires. In Borneo, the effects of extreme climate events have been exacerbated by rapid urbanization, accelerated deforestation and soil erosion since the 1980s. However, studies quantifying the impact of interannual and long-term (>3 decades) climatic and anthropogenic change affecting Borneo's coastal and coral reef environments are lacking. Here, we used coral cores collected in Miri-Sibuti Coral Reefs National Park, Sarawak (Malaysia) to reconstruct the spatio-temporal dynamics of sea surface temperature and oxygen isotopic composition of seawater from 1982 to 2016, based on paired oxygen isotope and Sr/Ca measurements. The results revealed rising sea surface temperatures of 0.26 ± 0.04 °C per decade since 1982. Reconstructed δ18Osw displayed positive excursion during major El Niño events of 1983, 1997/98 and 2015/16, indicating drought conditions with less river runoff, rainfall and higher ocean salinities. La Niñas were generally associated with lower δ18Osw. We observed a long-term shift from more saline conditions between 1982 and 1995 towards less saline conditions after 1995, which are in agreement with the regional freshening trend, punctuated by saline excursion during El Niños. The decadal shifts were found to be driven by the Pacific Decadal Oscillation (PDO). This study provides the first long-term data on El Niño Southern Oscillation (ENSO)-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo. Our results suggest that coral records from northern Borneo are invaluable archives to detect regional ENSO and PDO impacts, and their interaction with the Asian-Australian monsoon, on the hydrological balance in the southern South China Sea beyond the past three decades.