Warming's hidden danger

Acid oceans are the elephant in the room of global change, a phenomenon potentially so enormous and profound in its implications for life on earth that the world media has largely avoided it, governments have shunned it and scientists have addressed it mostly in muted tones behind closed doors.

The acid oceans theory is quite straightforward: the carbon dioxide 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 carbon dioxide 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 about 7.7 or lower is expected by 2100 if carbon dioxide continues to increase in the atmosphere at the predicted rate. The pH is a measure of the acidity or alkalinity of a solution. The lower the pH level, the more acidic the solution. Solutions with a pH of less than seven are considered acidic. Pure water at 25 degrees C has a neutral pH of seven.

The ARC Centre of Excellence in Coral Reef Studies' Malcolm McCulloch says it is 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 says.

McCulloch and his team have 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 Ove Hoegh-Guldberg suggest it could be enough to shut down coral growth and kill the calcareous algae that holds together the fronts of reefs. He has warned that Australia risks losing the Great Barrier Reef if atmospheric carbon dioxide levels rise above 500 parts per million from their present level of 385ppm, as they are expected to do by mid-century.

If that were not serious enough, an even more profound effect 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 one-third of all life in the oceans.

The likeliest consequence of their loss or decline would be an enormous realignment of marine food chains, reaching right up to land animals and humans.

While other algae would probably replace those that died, this would take time and would not necessarily preserve present 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 that extract carbon dioxide from the atmosphere. This would leave more carbon dioxide in the atmosphere to insulate the earth and warm it more quickly.

In the short run, too, higher levels of dissolved carbon dioxide 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 run-off from the land could lead to stagnant, oxygen-less seas where nothing but bacteria exist. These, ironically, are the conditions thought to have given rise to today's mighty oil fields, such as those of the Persian Gulf, which are one of the greatest sources of the carbon dioxide we are pumping into the atmosphere.