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Is the end in sight for the world’s coral reefs?

By Charlie Veron - posted Friday, 10 December 2010


Scientists don’t need a pocket calculator to conclude that compressing the time periods between events in this way will prevent recovery: If we do not take action, the only corals not affected by mass bleaching by 2050 will be those hiding in refuges away from strong sunlight.

But there is more bad news. A decade or so ago, we thought that mass bleaching was the most serious threat to coral reefs. How wrong we were. It is clear now that there is a much more serious crisis on the horizon - that of ocean acidification. This will not only affect coral reefs (although reefs will be hit particularly hard), but will impact all marine ecosystems. The ultimate culprit is still CO2 but the mechanism is very different.

Normally there is a balance between CO2 in the atmosphere and its derivatives in surface waters of the ocean. As with temperature, the oceans act as a huge repository, absorbing and buffering any excess CO2 in the atmosphere. For this process to be efficient the oceans must have time for mixing to occur between its different layers, renewing the surface buffers from below. When CO2 increases too rapidly, these chemical reactions can falter, altering the balance of the buffers and gradually allowing the oceans to become less alkaline.

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All organisms that produce calcium carbonate skeletons (including shells, crabs, sea urchins, corals, coralline algae, calcareous phytoplankton, and many others) depend on their ability to deposit calcium carbonate, and this process is largely controlled by the prevailing water chemistry. As alkalinity decreases, precipitation of calcium carbonate becomes more and more difficult until eventually it is inhibited altogether. The potential consequences of such acidification are nothing less than catastrophic.

In my book, I examine the events that led up to each of the five mass extinctions in Earth’s history. Corals offer a unique insight into the past, both because they have been around for most of the history of life on Earth and also because they readily fossilise. I examine the theories offered to explain these global extinctions and find that ocean acidification is the only explanation which fits the evidence well. Ocean acidification has played a major part in the marine devastations which took place in those ancient times.

A particularly galling aspect of the past four mass extinction events (very little is known about the first) is that, following them, reefs disappeared - not just for a few tens of thousands of years, but for millions of years - long after adverse climatic conditions may have returned to benign levels. One of the characteristics of acidification is that while it can be initiated by high CO2 levels over relatively short periods, there are no short-term geochemical fixes to reverse the process. Reversal can take place only through the immensely slow weathering and dissolution processes of geological time, processes that take hundreds of thousands to millions of years.

Ocean physics dictates that we will observe the effects of acidification in colder and deeper waters before it spreads to shallower tropical climes. The early stages of acidification have now been detected in the Southern Ocean and, surprisingly perhaps, in tropical corals. On our current trajectory of increasing atmospheric CO2, we can expect that by 2030 to 2050 the acidification process will be affecting all the oceans of the world to some degree. At that point, the relatively cool, deep-water tropical regions that have offered refuges to corals from temperature stress will be those most affected by acidification.

No doubt different species of coral, coralline algae, plankton, and molluscs will show different tolerances, and their capacity to calcify will decline at different rates. But as acidification progresses, they will all suffer from some form of coralline osteoporosis. The result will be that corals will no longer be able to build reefs or maintain them against the forces of erosion. What were once thriving coral gardens that supported the greatest biodiversity of the marine realm will become red-black bacterial slime, and they will stay that way.

Another concept of great importance is that of commitment - a word climatologists use only too often. Many of the consequences of our current actions cannot yet be seen, and yet the Earth is already committed to their path. This delayed reaction is due to the inertia of the oceans, both thermal and chemical. The greenhouse gases we produce today will take a number of decades (and sometimes more) to unleash their full fury, but their effects are unavoidable and unstoppable. We cannot afford to wait until the predictions of science can be totally verified, because by that time it will be too late. How many of us wish to explain to our children and children’s children that the predictions were there but we wanted confirmation?

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Coral reefs speak unambiguously about climate change. They survived Ice Age sea-level changes of 120 metres or more with impunity. They once survived in a world where CO2 from volcanoes and methane was much higher than anything predicted today. But that was over 40 million years ago, and the increase took place over millions of years, not just a few decades, time enough for ocean equilibration to take place and marine life to adapt.

This is not what is happening today. Ponder these facts: The atmospheric levels of CO2 we are already committed to reach, no matter what mitigation is now implemented, have no equal over the entire longevity of the Great Barrier Reef, perhaps 25 million years. And most significantly, the rate of CO2 increase we are now experiencing has no precedent in all known geological history.

Reefs are the ocean’s canaries and we must hear their call. This call is not just for themselves, for the other great ecosystems of the ocean stand behind reefs like a row of dominoes. If coral reefs fail, the rest will follow in rapid succession, and the Sixth Mass Extinction will be upon us - and will be of our making.

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First published by Yale Environment 360 on December 6, 2010.



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About the Author

Dr J.E.N. (Charlie) Veron is Former Chief Scientist at the Australian Institute of Marine Science and an author based in Townsville Queensland. His website is here: www.coralreefresearch.org. He is the author of numerous books, including the three-volume Corals of the World. His research has taken him to all the major coral reef regions of the world during 66 expeditions. His latest book is A Reef in Time: The Great Barrier Reef from Beginning to End.

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