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

It is a difficult idea to fathom. But the science is clear: Unless we change the way we live, the Earth’s coral reefs will be utterly destroyed within our children’s lifetimes.

Over the past decades, there have dozens of articles in the media describing dire futures for coral reefs. In the 1960s and ‘70s, we were informed that many reefs were being consumed by a voracious coral predator, the crown-of-thorns starfish. In the 1980s and ‘90s, although these starfish still reared their thorny heads from time to time, the principal threats had moved on — to sediment runoff, nutrients, overfishing, and general habitat destruction.

For me, an Australian marine scientist who has spent the past 40 years working on reefs the world over, these threats were of real concern, but their implications were limited in time or in space or both. Although crown-of-thorns starfish can certainly devastate reefs, the impacts of sediments, nutrients and habitat loss have usually been of greater concern, and I have been repeatedly shocked by the destruction I have witnessed. However, nothing comes close to the devastation waiting in the wings at the moment.

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.

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 fossilize. 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 mollusks 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.

J.E.N. Veron, YALE environment 360, 6 DEcember 2010. Full article.

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