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High-precision uranium-thorium (U-Th) dating of dead branching corals from Luhuitou reef, Sanya, northern South China Sea indicates that the reef framework grew episodically over the past 7,000 years. Episodes of coral reef growth ("switch-on" phases) occurred in response to regional warming during the mid-Holocene Climate Optimum, Medieval Warm Period, and Current Warm Period, when the East Asian summer monsoon was strong and the East Asian winter monsoon was weak. In contrast, episodes when reef growth dramatically slowed or ceased ("switch-off" phases) occurred during comparatively cold periods (e.g., Dark Age Cold Period and Little Ice Age) and are linked to abrupt weakening of the East Asian summer monsoon and concurrent strengthening of the East Asian winter monsoon. In the context of global warming, the northern South China Sea may become an important refugium for coral reef growth of up to 2 degrees C above present, but only if local anthropogenic pressures are reduced.

Plain Language Summary Understanding the response of coral reef growth to past warm periods allows better predictions of the response of coral reefs to future climate change and evaluations of the potential for the locality to act as climate change refugia for coral reefs in the 21st century. We identify abrupt coral reef recovery in the northern South China Sea (SCS) since the last century (especially post-1960 CE), indicating that the Current Warm Period is an optimal episode for reef growth in the northern SCS. As our results suggest that expansive coral growth occurred in the mid-Holocene when sea surface temperatures were up to 2 degrees C higher than present, the northern SCS may represent critical refugia for future reefs in the 21st century.