Robyn Williams: This could be one of the most important talks we've broadcast. Ockham's Razor has been going for nearly 25 years and we've had many a dire warning, many a cause for concern, as well as plenty of real or apparent breakthroughs.

But today is a cry of alarm from one of Australia's leading scientists, someone known all over the world where marine biologists gather, someone who's done the definitive recording of what corals we have and how they live.

Charlie Veron is a former Chief Scientist with AIMS in Townsville, the Australian Institute of Marine Science. He got the Darwin Medal in 2004 and now he's trying to get us to realise that the Great Barrier Reef and corals everywhere could be threatened with extinction. Here's why.

Charlie Veron: Over the past decades there have been many stories in the media about the plight of the Great Barrier Reef.

In the '60s and '70s we all heard that the Great Barrier Reef was about to be consumed by that voracious predator, the Crown of Thorns Starfish.

In the '80s and '90s, the principal threats turned out to be sediment runoff, to nutrients, overfishing and general habitat destruction.

For me, an ancient marine scientist who has spent thousands of hours diving on the Great Barrier Reef these past 40 years, each of these threats has been of concern. But nothing comes close to the devastation waiting in the wings at the moment.

Very likely you have a feeling that dire predictions about anything almost always turn out to be exaggerations. This view is understandable. Once I also would have thought it ridiculous to imagine that the Great Barrier Reef might have a limited future as a consequence of human activity. It would have seemed preposterous that the greatest coral reef on Earth, the biggest structure made by life on Earth, could be mortally threatened by any present or foreseeable change.

I was wrong. Yet here I am today, utterly convinced that The Great Barrier Reef will not be there for our children's children to enjoy. Unless we dramatically and immediately change our priorities, and the way we live.

I have been immensely fortunate in my career to have worked on coral reefs around the globe. And to have worked in many different disciplines of science. I have had the opportunity to make a significant contribution to reef science, and to reef conservation. Now comes time for payback, for responsibility. Responsibility to speak out, on behalf of that life which cannot plead its own cause.

When I started writing my book, I knew that climate change was likely to have serious consequences for coral reefs. But the big picture which emerged, quite frankly, left me shocked to the core.

This really led to a period of personal anguish. I turned to specialists in many different fields of science to find anything that might suggest a fault in that big picture. I was depressingly unsuccessful. The bottom line remains: the combination of the best science today argues that the Great Barrier Reef can indeed be utterly trashed in the lifetime of today's children. That is what motivates me to broadcast this message as clearly, as accurately, and as far as possible.

So, what are the issues? You probably know that there have been several major episodes of mass bleaching on the Great Barrier Reef since this began in the late 1980s. Since then the frequency of bleaching events has increased, and this has sparked an intense research effort.

Corals have an intimate give-and-take symbiotic relationship with single-celled algae called zooxanthellae which live in their cells and provide most of the food they need. A lot of research has shown that this symbiosis can be surprisingly fragile. If corals are exposed to high light at the same time as high temperature, the algae produce toxic levels of oxygen. Corals must expel this zooxanthellae, bleach and probably die. Or succumb to the toxin and definitely die. A tough choice. It is one they are not designed to make.

As the greenhouse effect from elevated carbondioxide has increased, the oceans have absorbed more and more greenhouse heat. The surface layers are being affected most, but large ocean areas have a temperature limit, about 31 degrees. Once this limit is reached, the surface does not warm further, but it broadens and it deepens. This creates the largest mobile heat mass on earth.

We are seeing this effect now. We are seeing abnormally heated water pulsed onto the Great Barrier Reef during El Nino cycles. When this happens, the ocean is further heated, to levels that corals have not experienced for millions of years. This leads to their mortal dilemma - to expel or not to expel their zooxanthellae - that becomes the question.

I've seen spectacular recoveries from mass bleaching on as little as a decade, provided that further El Nino cycles do not occur while the ecosystem is re-establishing. Unfortunately, El Nino cycles appear to be becoming more frequent. This is because the oceans are reaching their upper temperature limit more and more frequently. In a couple of decades, every year will appear to be an El Nino year. The frequency and severity of bleaching events will continue to increase. That is certain.

On present forecasts, the worst bleaching year we have had to date will be an average year by 2030. And it will be a good year by 2050.

If we keep increasing greenhouse carbondioxide, by 2050 at the very latest, the only corals left alive will be those hiding in refuges such as deep outer reef slopes. The rest of the Great Barrier Reef will be unrecognisable. Bacterial slime, largely devoid of life will be everywhere.

There is worse news. A decade or so ago we thought that mass bleaching was the most serious threat to coral reefs. We were wrong. We now know that there is a much more serious crisis on our horizon: ocean acidification. Acidification will not only affect coral reefs, it will impact all our oceans and all life in them. The culprit is still carbondioxide.

Normally there is a balance between carbondioxide in the atmosphere, and its chemical derivatives in the ocean. As we saw for temperature, the ocean acts as a huge repository, absorbing, then neutralising excess atmospheric carbondioxide. To do this effectively, they must have time for mixing to occur between shallow and deep layers, time for alkaline water to act as a buffer. When carbon dioxide increases too rapidly, the balance of the buffers change. The oceans become less alkaline.

When this happens, marine life will not be able to produce their normal calcium carbonate skeletons. The consequences of that are nothing less than catastrophic.

In my book I examine the events which led up to each of the five mass extinction events in Earth's history. Reefs offer a unique insight into these because they are made of calcium carbonate. That is the connection, and it is an unhappy one. I cannot escape the conclusion that ocean acidification has played a major role in all five mass extinctions of the past.

A particularly disturbing aspect of all this is that, following all mass extinctions, living reefs completely disappeared. Not just for thousands of years, but for millions. One characteristic of acidification is that while it can be initiated quickly, it cannot be easily reversed. That process requires the evolution of new life and the slow weathering of rock. It takes millions of years.

We know that we will observe the effects of acidification in colder and deeper waters first. That is already happening in the Southern Ocean. On our present trajectory, we can expect acidification to start impacting the Great Barrier Reef by around 2030.

At that time, the cool outer reef slopes which provided a safe haven from bleaching will be the very places most affected. Sod's Law. The result will be that corals will no longer build reefs, nor maintain them against the forces of erosion. They will be nothing but mounds of bacterial slime and algae.

There is another aspect of this which is of enormous consequence. That is commitment. Most of the consequences of our current actions cannot yet be seen. However, the Earth is already committed to their path. This delayed reaction is due to the inertia of the oceans, thermal and chemical. The greenhouse gases we produce today will take decades to unleash their full impacts. But their effects will be unavoidable, because commitment is unstoppable.

The longer we delay, the greater the damage.

How many of us would like to explain to our children's children, that the predictions were there, but - sorry - we just didn't take them seriously enough?

Corals speak unambiguously about climate change. They once survived in a world where carbondioxide from volcanoes and methane was much higher than anything predicted today. But that was 50 million years ago. The accumulation of carbondioxide then took millions of years, not just a few decades. Then there was time enough for oceans to equilibrate. And for life to evolve solutions.

This is not what is happening today. Think about it. The levels of carbondioxide we are already committed to reach, has no equal over the entire longevity of the Great Barrier Reef. Perhaps 25 million years, and most significantly, the rate of carbon dioxide increase we are now experiencing has no precedent in all known geological history.

Reefs are the ocean's canaries. We must heed their call. This call is not just for the reefs themselves, but for all the great ecosystems of our oceans. These stand behind reefs like a row of dominoes. If reefs fall, the rest will follow. In quick succession. The Sixth Mass Extinction will be upon us. It will be of our own making, and it will be unstoppable by any means whatsoever.

Climate change is the greatest challenge humanity has ever faced. The longer we delay the costlier the remedy will be, and the more likely we will reach the point of no return.

On our present tack the future looks bleak, but it is still far from hopeless. We still have a window of opportunity, which we must take for the sake of our children and all the fauna and flora that share our planet. We are the custodians.

A brief look back at the staggering and accelerating technological advances of the past century is persuasive evidence that humans can find solutions if the political will is there to focus innovations in the right directions. We must buy ourselves time. Time for the innovators to do their job, to develop solutions and to create a future that is not dependent on fossil fuel. We, the citizens of the wealthy countries, are capable of achieving 50% cuts in greenhouse emissions virtually immediately. At the same time we need to put pressure on our governments, to help our governments, to support far-reaching national and global efforts to provide the permanent solutions.

What is required is willingness and immediate action. It is time for Australia to become a leader in this endeavour. If not, our Great Barrier Reef will be the first of the dominoes to fall. And that fall will be forever as far as we humans are concerned.

Robyn Williams: Forever!

Charlie Veron is a former Chief Scientist at the Australian Institute of Marine Science in Northern Queensland. He has done the definitive work on corals and now he's written a book called A Reef in Time: The Great Barrier Reef, from Beginning to End, published by Harvard University Press.

I'm Robyn Williams.

This transcript was typed from a recording of the program.
Complete accuracy cannot be guaranteed because of the possibility of mishearing and occasional difficulty in identifying speakers.