The plight of the Great Barrier Reef

We are seeing this effect now. We are seeing abnormally heated water pulsed onto the Great Barrier Reef during El Niño 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 Niño cycles do not occur while the ecosystem is re-establishing. Unfortunately, El Niño 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 Niño 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 carbon dioxide, 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 carbon dioxide.

Normally there is a balance between carbon dioxide 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 carbon dioxide. 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 the full impacts. But the effects will be unavoidable, because commitment is unstoppable.