ON AN idyllic coral atoll just a two-hour boat ride from Queensland's Gladstone Harbour, out past the endless line of tankers queued to load coal for export, a half-dozen scientists work frantically against the tide.
Their objective? To explore the consequences of rising atmospheric carbon - which evidence overwhelmingly attributes to the burning of coal and other fossil fuels - on the delicate chemistry of the reef and the creatures living there.
The project team, led by David Kline, a young scientist from the University of Queensland's Global Change Institute, is completing tests on a new underwater laboratory that will expose living corals on the Great Barrier Reef to the more acidic conditions forecast for oceans by the end of the century.
The team has spent weeks working around the ebb and flow of tides, connecting four narrow, two-metre transparent chambers peggedover the reef shelf to the complex technology required to manage and monitor them. Small fish and currents move naturally through the porous structures, two of which will be constantly dosed with seawater flushed with carbon dioxide to lower the pH.
''This system here is the heart of the experiment,'' Dr Kline explains to a film crew from the BBC natural history unit as he stands in the shallows, patting his hand on a floating platform loaded with pumps, cables and 50 instruments, all in constant conversation with ''the brains'' - a computer program running in a laboratory a few metres away on shore.
International interest is high because this is the first in situ investigation of its type. Findings from the Free Ocean Carbon Enrichment (FOCE) project will be keenly studied by scientists around the world.
Fathoming the effects of ocean acidification - the ''other'' carbon problem, one that emerged in scientific literature only a decade ago - has become one of the most urgent issues on the science agenda. The potentially diabolical consequences were highlighted in major briefing papers presented last week by the United States National Research Council to the US Congress and by the European Science Foundation to national leaders. The papers appealed to governments to give the issue priority for investigation and action.
''The chemistry of the ocean is changing at an unprecedented rate and magnitude due to anthropogenic carbon dioxide emissions,'' the NRC has said. ''The rate of change exceeds any known to have occurred for at least the past hundreds of thousands of years.''
Ocean acidification occurs when carbon dioxide (CO2) dissolves in naturally alkaline seawater, forming weak carbonic acid. Studies show the world's oceans have a huge appetite for carbon, and have insulated humanity from greenhouse warming by gulping in about one-third of the emissions pumped into the atmosphere since the Industrial Revolution.
But the process lowers the overall pH of seawater - by about 30 per cent over the past 200 years. It also soaks up carbonate ions, which are crucial to marine organisms making their calcium carbonate shells and skeletons.
The Heron Island experiment assumes a future with seawater twice as acidic as today, a more conservative take than published business-as-usual scenarios, which put the increase at 150 per cent by 2100. The question scientists are racing to answer is what a more acidic environment will mean for the tiny shelled zooplankton on which the marine food chain depends, and for the skeletons corals build into reefs.
The fear, explains the director of the Global Change Institute and head of the Australian Research Council-funded research team, Professor Ove Hoegh-Guldberg, is that the change hits these creatures on two fronts - creating a more corrosive environment, and depleting stocks of building materials. ''If these organisms can't compensate for that … reef growth will slow until the reef superstructure begins to crumble. If coral populations disappear you put at risk about a million or so species, and all of the beautiful benefits to humans such as fisheries, coastal protection, tourist industries and so on.''
Meanwhile, he says, reefs are struggling with the effects of rising temperatures, which can trigger bleaching - when the stressed coral hosts expel the microscopic algae on which they rely for survival. He likens simultaneous bleaching and acidification to ''having two rhinos run at you from different directions''. Maybe by some miracle you will escape, but the odds are not good.
''Ocean acidification is already occurring and will get worse,'' said Professor Jelle Bijma, lead author of the European Science Foundation document, when it was presented last week. Combined with warming, ''we are in double trouble. The combination of the two may be the most critical environmental and economic challenge of the century''.
Dr Kline says some of his corals will be airbrushed to mimic bleaching, to see how damaged structures respond to the more acid environment. This will provide clues on whether reef atolls will continue to provide a platform for new communities to grow - ''or is the balance going to shift … are these massive reef structures going to end up dissolving?''
To date, exploration of these questions has been limited to laboratory aquaria. ''But seeing how they behave in the natural world is vital to gaining a reliable sense of where the future lies,'' says Dr Kline. Ecosystems may turn out to be more resilient - or less - than the models show. ''Here in the world, the corals are surrounded by their natural community - you have natural water, natural light,'' he explains, making final adjustments to the chambers.
They rest on layers of sands that have their own complex chemistry of carbonates and biota, which may help corals cope with a more acidic future.
''I'm hoping that these environments have some ability to buffer the impact of ocean acidification, and thereby part of the biodiversity of islands and coral reefs would be preserved,'' says Dr Hoegh-Guldberg. ''But its 50:50. I actually think that what we are seeing in the laboratory is repeated in nature.''
The Queensland coalmines may be just over the horizon, but ''this is a big fat canary'', he says of the reef.
''Something as complex and broad a feature as coral reefs is now sickening and dying … This is really giving us a warning sign that maybe the whole basis of our dependence on this planet, the biological and ecological services, will change.''