AN underwater picture of a reef turning into a bacteria-dominated ecosystem was shown in a courtroom to prove just how an increase in carbon dioxide emissions will affect the Great Barrier Reef.
A marine science professor gave evidence at a court hearing on Tuesday and described what would happen to the reef if there was an increase in carbon emissions.
Global Change Institute director Ove Hoegh-Guldberg's evidence formed part of an argument from environmental group Coast and Country, which is fighting mining company Adani in the Queensland Land Court against the Carmichael Mine proposed for the Galilee Basin.
During the hearing, which will continue for several more weeks, Prof Hoegh-Guldberg said if carbon dioxide emissions continued "along our current track, within 20 years we won't have a Great Barrier Reef with corals".
During his evidence he was asked to describe a graphic from his research that depicted images of what a reef would look like under different circumstances.
One image featured murky water with no coral, which Prof Hoegh-Guldberg said was a reef which bacteria dominated because of the effects of an increase in carbon emissions.
"You get to a point where corals ... only the toughest survive," he said. "Other less appealing organisms like seaweeds and sponges tend to take over reef systems."
But he said it did not have to be that way. He said if steps were taken to deal with the issues, the reef could stabilise and potentially go back to its best state.
Under cross examination, Adani's lawyer Peter Ambrose questioned Prof Hoegh-Guldberg whether mining itself - not the burning of coal - would directly impact the Great Barrier Reef.
The witness replied that it would be extracting a substance that people knew would increase carbon into the atmosphere when burnt. But he said the actual extracting of the coal itself would not directly impact the reef.
During the hearing Prof Hoegh-Guldberg also said impacts on coral reefs had only been known for about 15 to 20 years and that people should be cautious.
He also mentioned how ocean acidification overseas was causing hundreds of millions of dollars' worth of damage to aquaculture industries, including oysters.
"The same effect is happening on the Great Barrier Reef but we probably haven't actually uncovered them or described them," he said. "I think we have to be very cautious in what we're doing with the ocean."
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