By Julian Cribb*

Five times in the history of life on Earth, the corals have perished. Each time they have taken tens of millions of years to return or evolve anew. Eminent Australian marine scientist Dr J.E.N. “Charlie” Veron argues we are at the brink of a sixth mass extinction – and that the killers of the largest living organism on the planet, the Great Barrier Reef, will be none other than ourselves.

In A Reef In Time, published by Harvard University Press, Dr Veron traces the story of the GBR from beginning to what he sees as its likely destruction towards the end of the present century.

Charlie Veron is no ecological Hanrahan, crying “We'll all be rooned". As former Chief Scientist at the Australian Institute of Marine Science and author of one of the world's leading reference books on coral species, his is a voice that speaks with calm authority about a subject he has known intimately as a diver and professionally as a scientist for all his working life.

But it is a voice tinged with despair, questioning whether humanity has the resolve and sense of urgency to draw back from the planetary chaos we are unleashing: “It cannot rationally be doubted that we are now at the start of an event that has the potential to become the Earth's sixth mass extinction. This time there are no bolides (asteroids), no supervolcanoes, and no significant sea level changes … it is a case of humans changing the environment,” he says

As diver and researcher Dr Veron has visited and studied all of the world's great coral ecosystems. He regards the Great Barrier Reef as Nature's pinnacle of achievement in the ocean realm, a place of endless beauty and richness that has endured when other places on Earth have changed beyond recognition. It is the only living organism large enough to be viewed from outer space. It is also the work of a wondrous collaboration, between the corals themselves and their partner algae (zooxanthellae) which turn sunlight into food for the corals and other algae which cement their homes together. A coral reef, he points out, is not a battleground of competing organisms but rather a marvel of co-operation and mutual support in which each creature is a member of a “guild” that plays its part in supporting the whole. It is a system humans can learn from.

What a tragedy, then, if the GBR – along with all the world's other coral reefs – were reduced to a crumbling, weed-infested heap of limestone rubble within the lifespan of our children, never to return while humans still exist.

The processes that may bring this about are already at work beneath the ocean's impassive countenance. Invisible eddies of over-heated water bring a sudden white death to vast tracts of corals when they linger over them for a few days. There are already signs sea levels will rise far faster than corals can grow. Diseases never before seen are on the rampage. And, molecule by molecule, the CO2 we produce each time we start our cars, heat or cool our homes or do our work dissolves into the upper oceans, turning them ever so slightly acidic. Even mild acidity spells death to corals and the calcareous algae that 'glue' the reef together, as well as many of the ocean's planktonic organisms, the most numerous creatures on Earth and the main producers of the air we breathe.

Attempting to reconstruct what scientists now fear will be the likely fate of the GBR and, indeed, all the earth's corals (not to mention the 500 million people they support), Veron reaches back in time to try to decipher the processes that obliterated corals in the ancient past and to see if those conditions are being replicated in the present.

Previous coral extinction episodes. Source: Veron JEN

Five times (above) before the present, some cataclysm either totally or largely obliterated all the corals on Earth, along with a great many other species in the sea and on land. For ten million years or more following each event the fossil record is devoid of corals – and of the vast limestone formations, entire mountain ranges, which they produced. After events like the “Great Dying” at the end of the Permian era (251my ago), it appears corals, along with 96 per cent of all marine life, were completely wiped out and had to begin evolving again from scratch.

The causes of these mass extinctions are unknown, though asteroid impacts, supervolcanoes, tectonic upheaval and climate change figure large among possibilities.

A likely consequence of all such events would have been the mobilisation of vast amounts of the Earth's stored carbon into the atmosphere. This CO2 would dissolve back into the oceans, turning them acidic and shutting down all forms of life which depend on alkaline water to form their chalky shells and skeletons. It is probable that the initial great die-off was followed by vast blooms of fungi and bacteria, feasting on the carcases, which in turn stripped all the oxygen from the water (as they do today in highly polluted waters), killing fish, molluscs and other creatures which had survived the initial acidity. Between them, acidity and anoxia would explain the extinction of most sea life, revealed by the fossil record.

"The prospect of ocean acidification is frightening,” Veron states. “It is serious because of commitment – a word that will soon be used with increasing frequency in the scientific literature.” Commitment here does not mean something as frangible as a human vow. In science it means a process that is literally unstoppable. If the oceans turn acidic – as they are already doing – the only known way of reversing this is the slow weathering and dissolution of limestone mountain ranges into the sea, a process which takes millions and millions of years to gradually buffer the water back to its normal alkalinity. The public, keenly aware today of the role of CO2 in heating the planet, is far less conscious of its second activity in acidifying the oceans – or of the vast spans of time necessary for the oceans to re-balance themselves, which are vividly suggested by the immense gaps in the fossil coral record.

Unlike coral bleaching, which is visible within days, acidification is a creeping death. “The long-term outlook is that reefs will be committed to a path of destruction long before any effects are visible,” Veron says. If global atmospheric CO2 levels attain the 650-700 parts per million they are forecast to reach by the latter part of this century even if we adopt all the CO2 reducing measures now proposed, some of this human-generated CO2 will still be around in 30,000 years time, helping to acidify the oceans. This is not a process that can easily be 'switched off' and highlights the immense lags in the Earth's systems, even were we to cease burning fossil fuels tomorrow.

"Ultimately, and here we are looking at centuries rather than millennia, the ocean's pH will drop to a point where a host of other chemical changes, including a lack of oxygen, may kick in. We have set the stage for the sixth great mass extinction, and another few decades like our last century will see the Earth committed to a trajectory from which there will be no escape,” Veron observes. “A continued business-as-usual scenario of CO2 production will ultimately result in destruction of marine life on a colossal scale.” The need for action to quell CO2 is urgent but Veron – along with other scientists – is critical that the media, in its quest for “balance” by including both scientific and non-scientific claims in its reporting of greenhouse as if they were of equal weight and validity, helps to keep the public and governments in a perpetual ferment of indecision. “Such public uncertainty, in combination with pressure from groups with vested interests, has prolonged government inaction in democratic countries (notably the USA and Australia) and this delay is already having far-reaching consequences. The GBR will be among the first in a long line of dominoes to fall … .” he warns.

"A Reef in Time” is an alarming, but not alarmist, book. Passionate yet objective, Veron presents the science behind his argument lucidly and accessibly: much will be new to lay audiences unacquainted with the latest thinking about climate science and unaware how very much graver than the present official climate outlook many eminent scientists privately consider the situation to be. Veron has blazed a vital trail in this regard, choosing an example so graphic and an icon so huge in the human consciousness that it is hard to ignore – indeed, the largest living organism on the planet. If we kill it, the book insinuates, what does it say of us?

"This account … may seem like a science fiction horror story, but there is little evidence of fiction either in the science or in the simplified interpretation I have given here,” he states, acknowledging that his is not an account burdened with optimism. Even so, he continues, there is room for hope. Reductions in greenhouse emissions will soon be forced on humanity in any case, so we may as well get on with it – and at a far more drastic rate.

So far as the Great Barrier Reef is concerned the first and ultimately only step we can take to preserve it is to cease polluting the Earth with CO2, Veron argues. At the same time we must minimise all the other stresses to which we subject it. This is the decade in which we must decide, he says. Then, though scarred, the corals will probably pull through.

(Julian Cribb is a science writer and Adjunct Professor
of Science Communication at the University of Technology Sydney.)