Globally, the frequency of severe coral bleaching events has increased nearly fivefold in the past four decades, from once every 25 to 30 years in the early 1980s to once every 5.9 years in 2016, a new study reports. Coral bleaching occurs when sudden spikes in temperature have a negative effect on the algae residing within corals, which provide nutrients for the bony organisms as part of a symbiotic relationship. Prolonged bleaching over several months leads to high levels of coral mortality. To understand how bleaching events have changed over time, Terry Hughes et al. analyzed numerous historical datasets capturing bleaching events at 100 locations around the world that occurred between 1980 and 2016. The most recent data from 2015 and 2016 reveal that more than 30% of bleaching episodes can be described as "severe," extending at scales of tens to hundreds of kilometers. When the researchers look at how the frequency of bleaching events has changed over recent decades, they found that the risk of such events, and particularly severe cases, has increased by approximately 4% every year since 1980. While the Western Atlantic began to warm and experience bleaching more frequently than other regions at first, the risk of bleaching events has increased most strongly over time in Australasia and the Middle East. The authors also note that, while bleaching events tended to take place more often when the global cycle was naturally warmer (El Niño), general warming trends mean that bleaching events are increasingly taking place regardless of natural warmer/cooler cycles.
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