Oceans face the acid test

Acid oceans are the elephant in the room of global change – an event potentially so massive and profound in its implications for life on Earth that the world media has largely avoided it, governments shunned it and scientists discussed it mostly in muted tones, usually behind closed doors.

The acid oceans theory is quite straightforward: the CO2 emitted by human activity dissolves out of the atmosphere into the seas, gradually turning them more acidic.  This is largely independent of global warming or other effects. It is a straight equation – about half of all the CO2 produced since the start of the industrial revolution has ended up in the sea, reducing the surface pH by 0.1 (some experiments indicate as much as 0.3 pH).


A further reduction of 0.5 pH to around 7.7 or lower is expected by 2100 if carbon dioxide continues to increase in the atmosphere at the predicted rate. The ARC Centre of Excellence in Coral Reef Studies’ Professor Malcolm McCulloch says it is now beyond question the oceans are becoming more acidic, and this is happening 100 times faster than at any time in recent geological history. “The reason this effect has snuck up on us is that we don’t have long term records for ocean acidity,” he explains.  McCulloch and his team have recently developed a world-first way to measure past acidity in sea water.

The change in acidity may seem minor but experiments by the Centre’s Professor Ove Hoegh-Guldberg suggest it could be enough to shut down coral growth and kill the calcareous algae that hold together the fronts of reefs. He has warned that Australia risks losing the Great Barrier Reef if atmospheric CO2 levels rise above 500 parts per million from their current level of 385ppm, as they are currently expected to do by mid-century.

If that were not serious enough, an even more profound impact is likely to occur among calcareous plankton which will be unable to form their chalky skeletons as the water acidifies. The vulnerable corals and plankton make up about a third of all life in the oceans.
The most likely consequence of their loss or decline would be a massive realignment of marine food chains, reaching right up through fish and birds to land animals and humans. While other algae would probably replace those that perished, this would take time and would not necessarily preserve current fish and marine animal populations.

A second likely consequence of acid oceans is accelerated global warming, caused by the partial or complete shutdown of the oceans’ biological and physical carbon pumps, which extract CO2 from the atmosphere. This would leave more CO2 in the atmosphere to insulate the Earth and warm it more quickly.

In the short run, too, higher levels of dissolved CO2 in the sea also make it harder for fish and squid to extract oxygen from the water, and may kill those creatures with a high oxygen demand. 

The worst-case scenario is that a mass death of sea life combined with heavy nutrient runoff from the land could lead to stagnant, oxygen-less seas where nothing but bacteria exist. These, ironically, are the very conditions thought to have given rise to todays’ mighty oil fields, such as those of the Arabian Gulf, which are one of the greatest sources of the CO2 we are pumping into the atmosphere.

Finally, many of the vulnerable marine creatures play a significant role as oxygen producers for the planetary atmosphere.  The effect of their removal or reduction on our ability to breathe is not yet understood, but some researchers fear it could reduce the habitable biosphere.

About 251 million years ago the Great Dying at the end of the Permian took out 90 per cent of all sea life and 75 per cent of all land species. The causes are still debatable, but high CO2 levels released in a massive episode of volcanism in Siberia are one explanation.  This may have produced acidic, oxygen-less oceans. Not long after, fungi were the dominant lifeform on the planet, dining on the carcases.

Today the scientific evidence indicates clearly that the surface oceans are already acidifiying, and the effects of this can be seen in corals and other organisms.

Australia, having the world’s largest coral reef – with the $5 billion industry and 60,000 jobs that hang off it – and being next to the Southern Ocean whose chilly waters provide the Earth’s main mechanism for pumping CO2 out of the atmosphere, has a larger stake in this phenomenon than other countries and should be putting maximum scientific effort into understanding it.  It is not a trivial issue for any government or nation.

People did not believe the melting icecaps idea a few years ago but, if television has performed one good deed, it has been to bring the reality of polar dissolution into a billion living rooms. Finding out whether the burning of fossil fuels is killing sea life and how this will affect all life on Earth is even more important and urgent.

Julian Cribb is adjunct professor of science communication at the University of Technology, Sydney, and edits R&D Review and ScienceAlert.

Editor's Note: First published in The Australian on 7 November 2007.