Repeat bouts of warming ocean waters could lead to a 'knock-out punch' that kills tropical corals.

A new study has found high ocean temperatures are harming tropical coral reefs almost five times more often than in the 1980s.

The average time between severe 'bleachings' shortened to six years in 2016 from 25-30 years in the early 1980s, research suggests.

Corals die if bleachings are long-lasting, wrecking reefs that are nurseries for fish, a source of food to millions of people and a destination for scuba-diving tourists.

Heat makes the stony-bodied creatures that make up coral reefs expel colourful algae which causes the bleaching.

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Severe bleaching outbreaks are hitting coral reefs four times more often they were a few decades earlier. They are now hit by episodes of bleaching once every six years compared to once every 25 - 30 years

PARIS AGREEMENT The Paris Agreement, which was first signed in 2015, it is an international agreement to control climate change. It hopes to hold the increase in the global average temperature to well below 2°C (3.6ºF) 'and to pursue efforts to limit the temperature increase to 1.5°C (2.7°F)'. If the Paris agreement is successful there will still be a Great Barrier Reef, but it will look different. In June, President Trump announced his intention for the US, the second largest producer of greenhouse gases in the world, to withdraw from the agreement. Advertisement

A team of experts from the ARC Centre of Excellence for Coral Reef Studies in Australia looked at 100 coral reefs around the globe and how often they have suffered severe bleaching since 1980.

Bleaching is caused purely by warmer waters, when it's nearly 1°C (2°F) above the normal highest temperatures for an area.

'Tropical reef systems are transitioning to a new era in which the interval between recurrent bouts of coral bleaching is too short for a full recovery' of mature corals that can require 10-15 years, the researchers wrote in the journal Science.

Climate change will 'inevitably' make underwater heat waves and bleachings more frequent, they wrote.

Bleaching isn't quite killing the delicate corals, but making them extremely sick by breaking down the crucial microscopic algae living inside the coral.

Bleaching is like 'ripping out your guts' for coral, said study co-author Mark Eakin, coordinator of the Coral Reef Watch program for the U.S. National Oceanic and Atmospheric Administration.

Warming waters are having a devastating impact on coral reefs. The Florida Keys, Puerto Rico and Cuba have been hit seven times by bleaching and the study found that it has wide reaching implications for the ecosystems

It takes time to recover from bleaching, and the increased frequency means coral doesn't get the chance to recover before the next outbreak, Dr Eakin said.

'If you go into the ring with a heavyweight boxer, you could probably stand up for one round, but once that second round comes - you're going down,' said Dr Mark Eakin told the BBC.

Only six of the 100 coral reefs weren't hit by severe bleaching: four around Australia, one in the Indian Ocean and another off South Africa.

Georgia Tech climate scientist Kim Cobb, who studies reefs but wasn't part of this international team, applauded the research and said that as the world warms more there will be 'profound and lasting damage on global reefs.'

Guam has been one of the hardest places hit with eight severe bleaching outbreaks since 1994, four of them in the last five years, Dr Eakin said.

In the 1980s, bleachings happened during local heat waves, and then started to occur in the 1980s and 1990s during natural El Nino weather events that release heat from the Pacific Ocean

The Florida Keys, Puerto Rico and Cuba have been hit seven times.

U.S. President Donald Trump, who doubts climate change is man-made, plans to pull out of the Paris accord and instead promote U.S. fossil fuels. Australia's goals under the Paris accord are less ambitious than those of some other rich nations.

The Paris pact seeks to limit a rise in average global surface temperatures to 'well below' two degrees Celsius (3.6°F) above pre-industrial times, ideally 1.5°C (2.7°F) by shifting from fossil fuels.

Corals are already under threat with warming of 1°C (1.8°F) so far, a U.N. panel of climate scientists says.

In the 1980s, bleachings happened during local heat waves, and then started to occur in the 1980s and 1990s during natural El Nino weather events that release heat from the Pacific Ocean.

100 coral reefs were looked at and only six had avoided severe bleaching. Guam has been one of the hardest places hit with eight severe bleaching outbreaks since 1994, four of them in the last five years

'Now we're seeing the emergence of bleaching in every hot summer,' said DR Mark Eakin, an author of the study at the U.S. National Oceanic & Atmospheric Administration, said in a statement.

Lead author Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies in Australia, urged more action to limit greenhouse gas emissions under the global 2015 Paris climate agreement.

Professor Hughes said there was evidence that some corals could adapt to rising temperatures.

The Great Barrier Reef, the world's biggest, was not doomed if governments stepped up action.

'If the ... Paris Agreement is successful we will still have a Great Barrier Reef. It will have a different mix of coral species, but it will still function and look like a reef,' he said.