The atolls and coral banks of the Chagos Archipelago (British Indian Ocean Territory) in the central Indian Ocean were badly affected by the warm water event that started in 2015 and lasted for nearly two years. On these reefs, coral mortality was very severe, reducing coral cover to <10% cover and usually about 5%, almost eliminating soft corals and reducing the 3-Dimensional structure of the reefs. Most atolls are not inhabited, so any changes are driven by climate changes rather than by any direct, local anthropogenic effect. Coral cover has been measured for 20 years using the same methods, while temperature loggers have recorded water temperature at various depths for over 10 years. Water temperatures have risen by one third of a degree on ocean reefs and over one half of a degree in lagoons over this period, causing the recent severe mortality. Juvenile corals have also been recorded at intervals during the last few years, and numbers severely declined following the mortality of the adults. Estimates of calcification suggest a marked reduction, from a state of vigorous reef growth that had not long recovered from the previous severe warming event of 1998, to one of net erosion. Predictions suggest that recurrences of mass mortalities will take place too frequently for any significant recovery of reef health in these atolls by the late 2020s.