Reef in American Soma. Top: Dec 2014. Bottom: Same location 3 months later.
Credit: The Ocean Agency / XL Catlin Seaview Survey.

We are about to lose all our shallow water tropical reefs. From Australia’s Great Barrier to the shifting shoals of the Bahamas, the world’s warm-water ecosystems are facing total destruction—possibly within ten to twenty years. The only way to avoid this catastrophe is to keep global temperature rise below 1.5 centigrade.

There are many reasons why climate scientists are coming to the view that even a 2 °C rise is too much. From increasing incidence of extreme weather events—such as the still unfolding disaster in southern Africa—to the likely impact on world crops and the complete disappearance of island nations, there is much that could be said on this topic. And the change, from believing that 2 degrees is “safe” to now looking for no more than a 1.5 centigrade increase, is the main reason why we are suddenly seeing headlines such as “only 12 years to save the planet”.

In a short column I can’t properly discuss all the potential problems. So, I’m going to concentrate on coral reef destruction. There are several reasons for this. For a start, it’s a subject I can claim expertise in as, 30 years ago, I was one of the first people to build computer models of how coral reefs grow. Furthermore, large-scale extinctions are going to be the longest lasting effect of global-warming. The fossil record shows us that, following big losses of biodiversity, it take 5-10 million years for Earth to recover. The final reason for looking at tropical reefs is that they are a “climate canary”. Canaries were taken down mines to give early-warning of dangerous gas build-up and, if increasing loss of our reefs does not provide a wake-up call to the world that greenhouse gas build-up is serious, I suspect nothing will.

Total destruction of all tropical coral reefs sounds like an exaggeration but there are good reasons for making such a dramatic statement. The issue is coral-bleaching—the fact that excessively warm water turns corals white. This happens because corals are not one organism but two. The coral polyp itself is an animal but it hosts a plant in what’s called a symbiotic relationship—a partnership that provides benefits to both participants. In this case, the plant feeds the coral and the coral shelters the plant. Bleaching happens if water becomes a few degrees too warm, for more than a few weeks, since the warmth kills the symbiotic plant and the coral then loses its colour and eventually dies.

Bleaching is a natural process that happens now and again and has done so for millions of years. Normally it’s not a problem because new corals grow back after about ten years and the frequency with which coral reefs find themselves in hot water is much less than once a decade. The problem is that global warming is increasing this frequency. Today, with one degree of warming, bleaching occurs about once a decade and our reefs are struggling. By fifteen years’ time or so—when warming will probably get to 1.5 degrees if we don’t do anything—the frequency will have risen to about once every two and half years and most reefs will be dying. If we get to a two centigrade rise, reefs will bleach every year and they are doomed.

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