Ocean acidification will have a much more serious effect on the ability of coral reefs to rebuild themselves than previously thought, research suggests.
In a study, reported today in the journal Nature, scientists pumped CO2 onto a section of the Great Barrier Reef at One Tree Island, to simulate levels of ocean acidification expected to occur by 2050.
They then measured the effect this had on a process known as calcification, which is used by coral and other reef organisms as part of their structure.
The study, led by Dr Rebecca Albright, assistant curator at the California Academy of Science, was the first to investigate the impacts of ocean acidification on an entire natural reef system.
She said this was a much greater effect than found in previous studies, some of which saw a decrease of 15 per cent.
But Dr Albright said most of the previous studies had been done in the lab on individual coral species, and this did not reflect the complex reef ecosystem.
"Coral reefs are so much more than just corals," she said.
Some algae, sponges and other reef inhabitants are also involved in the calcification process.
Interactions between reef organisms could be important
The scientists don't know exactly what on the reef is driving the discrepancy between their field results and those from the lab.
"We're really looking at the cumulative response of the entire system," said Dr Albright, "not just coral".
It is possible that the dramatic response to increased acidity on the reef is due to interactions between different reef organisms, which are not detected in lab experiments on individual species.
Previous lab studies have often used much higher levels of acidity than those used in the current study — generally levels predicted to occur beyond the end of the century.
Dr Albright and team used acidity levels that the Intergovernmental Panel on Climate Change expects will occur under a business as usual emissions scenario by 2050.
"We wanted to do something that's relevant," Dr Albright said, adding that the large impact seen resulted from a very tiny change in acidity — a reduction of pH by 0.1.
The team hopes their findings trigger researchers to use more realistic carbon dioxide levels in future experiments.
"If we wind up seeing a 30 per cent reduction in the ability of coral reefs to grow over the next 30 to 40 years, that is a substantial threat to reefs," Dr Albright said.
Mounting evidence a cause for concern
These new findings build on previous research from the same team. In 2016, they exposed a reef to lower levels of acidification to reflect pre-industrial conditions. Calcification was shown to increase, suggesting ocean acidification was already slowing coral reef growth.
Dr Sophie Dove, Associate Professor at the University of Queensland who was not involved in the study, welcomed the findings.
"They're showing that this coral community potentially starts to dissolve more than it's calcifying when you add carbon dioxide to the water column, and I think that's very important," she said.
Dr Dove said the study helps emphasise the risk posed to reefs from ocean acidification.
While Dr Albright and colleagues had used good experimental methods, Dr Dove said it would be good to find out how much of the decrease in calcification came from corals.
Temperature and acidification acting in tandem
Apart from ocean acidification, coral reefs globally are also threatened by increasing ocean temperature.
Dr Albright said that a coral reef community that has just gone through a mass bleaching event relies on reproduction and growth to be able to recover, but the added impact of ocean acidification would undermine the reef's ability to do that.
"Temperature and ocean acidification are acting in tandem at different scales and on different processes, but they're both eroding away the resilience of these systems," she said.
To learn more about the ocean and climate change tune into The Science Show this week to hear about an idea of burying carbon dioxide in the ocean at a rate 500 times faster than we once thought possible — without dissolving the Great Barrier Reef.