Australia reef shallows may cross "damage threshold" in 30 years

Corals are seen at the Great Barrier Reef in this January 2002 handout photo.

CANBERRA (Reuters) - Coral reefs in shallow ocean waters are far more vulnerable to sea temperature rises blamed on global warming than previously thought, with some areas of the Great Barrier Reef likely to pass critical damage thresholds in roughly 30 years.

Australian researchers looked at shallow corals, found in tropical waters under 70 metres (230 ft) in depth, along Australia's iconic Great Barrier Reef, and found that even tiny increases in overall ocean acidity could lead to extreme changes.

"Honestly, we thought damaging thresholds would not be crossed in coral reefs this century. Now we know that for parts of the day, at least on our reef, it will be passed by 2040," said the study's lead author, Emily Shaw.

"We know that if we continue on our current CO2 emissions trajectory that the ocean will take thousands of years to return to chemical conditions resembling those of today," said Shaw, from the Climate Change Research Centre in Sydney.

Heralded as one of the seven natural wonders of the world, the 2,000 km (1,200 mile) Great Barrier Reef is home to 400 types of coral, 240 species of birds and 1,500 species of fish. It is worth A$6 billion a year in tourism to the economy.

Coral reefs are particularly vulnerable to acidification and rising sea surface temperatures, as increased CO2 levels in the oceans make it harder for tiny reef-building coral organisms to secrete their slow-growing calcium carbonate skeletons.

Scientific studies last year found that coral reef growth in areas of the Caribbean had dropped by as much as 70 percent cent, particularly in shallow water areas around the state of Florida in the United States.

In the worst cases, warming temperatures cause tropical corals to bleach and die off, as oceans absorb more CO2 and undergo a chemical process causing increased levels of acidity.

Current scientific predictions are for most coral reefs to become "functionally extinct" if atmospheric carbon dioxide levels continue to rise toward forecast levels causing climate shift by mid-century.

Shaw's team, based at the University of New South Wales, said organisms living in shallow coral reefs and marine ecosystems would be exposed to extreme acidity more than those living in deeper ocean environments.

On shallow reefs, natural acidity levels varied more during tidal changes and as chemical activity among symbiotic algae living in the corals shifted between daytime and night. Seasonal shifts also played a smaller role.

The Australian researchers looked at waters around Lady Elliott Island, on the Great Barrier Reef, but said the results would be replicated globally in coral reefs concentrated around the tropics.

"Rising carbon dioxide levels lower the ocean's ability to buffer these changes. This amplification in shallow areas and the more extreme conditions will actually happen everywhere," Shaw said.

(Reporting by Rob Taylor; Editing by Elaine Lies)