We explore the hypothesis that a shift in global climate 4200 years ago (the 4.2 ka event) was related to the El Niño–Southern Oscillation (ENSO). We summarize records of coral reef development in the tropical eastern Pacific, where intensification of ENSO stalled reef growth for 2500 years starting around 4.2 ka. Because corals are highly sensitive to climatic changes, like ENSO, we suggest that records from coral reefs may provide important clues about the role of ENSO in the 4.2 ka event.