World's reefs under siege from global warming: study

The window for saving the world's coral reefs from the effects of global warming is rapidly closing, according to a study.
For the first time, researchers have measured the escalating rate of coral bleaching at locations throughout the tropics over the past four decades.
The study published in the journal Science documents a dramatic shortening of the gap between pairs of bleaching events, threatening the future existence of these iconic ecosystems and the livelihoods of many millions of people.
"The time between bleaching events at each location has diminished five-fold in the past 3-4 decades, from once every 25-30 years in the early 1980s to an average of just once every six years since 2010," said Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) in Australia.

"Before the 1980s, mass bleaching of corals was unheard of, even during strong El Nino conditions, but now repeated bouts of regional-scale bleaching and mass mortality of corals has become the new normal around the world as temperatures continue to rise," said Hughes.
The study establishes a transition from a period before the 1980s when bleaching only occurred locally, to an intermediate stage in the 1980s and 1990s when mass bleaching was first recorded during warmer than average El Nino conditions, and finally to the current era when climate-driven bleaching is now occurring throughout ENSO cycles.

The researchers show that tropical sea temperatures are warmer today during cooler than average La Nina conditions than they were 40 years ago during El Nino periods.

El Nino and La Nina are opposite phases of a naturally occurring global climate cycle.
"Coral bleaching is a stress response caused by exposure of coral reefs to elevated ocean temperatures. When bleaching is severe and prolonged, many of the corals die. It takes at least a decade to replace even the fastest-growing species," said Professor Andrew Baird of Coral CoE.
"Reefs have entered a distinctive human-dominated era - the Anthropocene," said C Mark Eakin of the National Oceanic & Atmospheric Administration in the US.
"The climate has warmed rapidly in the past 50 years, first making El Ninos dangerous for corals, and now we are seeing the emergence of bleaching in every hot summer," said Eakin.

For example, the Great Barrier Reef has now bleached four times since 1998, including for the first time during back- to-back events in 2016 and 2017, causing unprecedented damage, researchers said.