Mass coral reef bleaching: A recent outcome of increased El Nino activity?

Coral reefs are generally considered to be the most biologically productive of all marine ecosystems, but in recent times these vulnerable aquatic res ources have been subject to unusual degradation. The general decline in ree fs has been greatly accelerated by mass bleaching in which corals whiten en masse and often fail to recover. Empirical evidence indicates a coral reef bleaching cycle in which major bleaching episodes are synchronized with El Nino events that occur every 3-4 years on average. By heating vast areas o f the Pacific Ocean, and affecting the Indian and Atlantic Oceans as well, El Nino causes widespread damage to reefs largely because corals are very s ensitive to temperature changes. However, mass bleaching events were rarely observed before the 1970s and their abrupt appearance two decades ago rema ins an enigma. Here we propose a new explanation for the sudden occurrence of mass bleaching and show that it may be a response to the relative increa se in El Nino experienced over the last two decades.