The biggest and most robust corals on the Great Barrier Reef (GBR) have slowed their growth by more than 14 per cent since 1990, according to an AIMS study. Evidence is strong that the decline has been caused by a synergistic combination of rising sea surface temperatures and ocean acidification.
The study was based on statistical analyses of annual growth bands from 328 Porites corals from 69 reefs across the length and breadth of the GBR, extending back in time up to 400 years. The data are from AIMS' Coral Core Archive (ACCA), the most extensive such collection in the world.
Reef corals create their hard skeletons from materials dissolved in seawater. When large amounts of atmospheric carbon dioxide enter seawater, the resulting chemical changes effectively reduce the ability of marine organisms to form skeletons.
Up to the tipping point in 1990, there were modest fluctuations in calcification, with an annual decline rate recorded that year of 0.3 per cent. However, by 2005 growth was declining by 1.5 per cent per year. On current trends, the corals would stop growing altogether by 2050.
The data suggest that this severe and sudden decline in Porites growth is unprecedented in at least 400 years. The causes remain unknown, but the study suggests that the combination of increasing temperature stress and ocean acidification may be diminishing the ability of GBR corals to deposit calcium carbonate.
Previous laboratory experiments and models had predicted that coral growth would decline in response to acidification, but this study has shown for the first time that corals are already affected in their natural environment throughout the GBR.
More carbon dioxide in the oceans causes the oceans' alkaline/acid balance (their "pH") to shift towards acidic. Oceanic pH has already dropped by 0.1 and could decrease by 0.4 by the end of this century. This is due to the oceans absorbing about a third of the extra carbon dioxide (the main greenhouse gas) that humans have put into the atmosphere.
If projections of a 0.4 decline in pH are correct, this would be well outside the realms of anything organisms have experienced over hundreds of thousands of years.