Comment: Corals pale with global warming

Posted on 23 February 1999
Gland, Switzerland: You hear people saying it all the time  the world is getting smaller. It has become almost a cliche of our age, born of the modern ease of travel and the increasing speed and range of communications technology. Yet even as the sentence passes our lips, I wonder how many of us realise just how small and limited is the space on earth shared by us and millions of other creatures.

The fact is that the biosphere supporting life on earth reaches only about 12 kilometres upwards and 12 kilometres down. In planetary terms, that represents something like the thickness of the skin on a tomato, and it is within this `skin' that evolution has taken place over millions of years.

Now, though, the tomato skin is suffering damage on a scale it has never before experienced, largely as a result of the behaviour of the six billion people  well, that will be the number by June  who crowd this small world. At their most damaging, our activities are provoking changes in the climate of the biosphere that are beginning to affect the very basis of the life within it.

We have seen the results in the melting of glaciers, in the spread of drought across Africa, in the growing incidence of tropical forest fires and, most recently, in the climatic ravages of the most powerful El Nino effect ever. These are very obvious consequences on the visible side of the skin, but now we are beginning to see that serious damage is being caused on the underside, too.

Reports are coming in that rising sea temperatures and increasingly strong El Ninos are now affecting coral reefs round the world. They have been called the rainforests of the sea, because of the rich diversity of species they support, and the evidence is gathering that coral reefs are suffering in a similar way to their land-based equivalents.

During the past two years, in parallel with the El Nino effect, the bleaching of coral reefs has been noted in all of the world's main tropical oceans  the most widespread incidence ever recorded. Bleaching is the name given to the expulsion of the coral's symbiotic algae, which results not only in loss of colour but also in the cessation of photosynthesis. It is the natural reaction of corals to stress, but it takes place most commonly when sea temperature rises by one or two degrees Celsius for several weeks. In some places, coral bleaching is a seasonal event. It happens in the Pacific, the Indian Ocean and the Caribbean, and normally the coral is recolonised by the all-important algae within a matter of weeks. But the latest surveys are showing that in some places the stress on the coral is such that it is turning completely white and dying.

This has been catastrophic in Sri Lanka, the Maldives, Tanzania and the Arabian Gulf, with mortality reaching 95 per cent in shallow corals. Mortality of between 50 and 70 per cent has been noted in Kenya, the Seychelles, Belize and the Far East, while in the northern Caribbean up to 50 per cent of coral was found to have died.

Equally worrying are reports that bleaching is occurring On Florida's coral reefs, which form one of the largest tracts in the world after the Belize reef and the Great Barrier Reef of Australia. The Florida tract has been protected since 1990 as part of the Florida Keys National Marine Sanctuary. But it is still under intense pressure from human activity  fishing, boating, diving and so on. Now it appears that a rise in sea temperature has caused this important reef to be included in the latest worldwide bleaching phenomenon.

As in many other places where bleaching has occurred, in Florida it took place against a background of sea temperature more than three degrees Fahrenheit higher than the top of the seasonal range at which `normal' bleaching would occur.

Recently, a new global warming related problem has been identified which could have catastrophic effects on coral reefs. Increased carbon dioxide in the atmosphere directly affects the amount of calcium carbonate in sea water, and this is an important factor in the coral's ability to make its skeleton. Scientists already think that coral `calcification' rates have been reduced by 6-11% because of increases in atmospheric carbon dioxide over the last century. Weakened coral skeletons and slower growth rates will result from this, making reefs more vulnerable to damage and less able to keep up with sea level rise. Experts at a workshop in Boston, USA, commented that in the light of this new evidence, conservation or management strategies aimed at removing or mitigating only human-derived stresses on coral reefs were likely to be inadequate in the face of the climatic threat.

It is now considered highly likely that the general level of sea temperatures will rise as a result of greenhouse gas emissions, so it is probable that the incidence of coral bleaching - already at record levels worldwide  will increase. If coral dies, it cannot be replaced. Part of the complex web of life is gone forever and the world is more fragile because of it. But while we watch this happening, along with the droughts and the forest fires, the catastrophic storms and the melting glaciers, there are many among us who still wallow in complacency and allow ourselves the luxury of `debate' over the need to reduce carbon dioxide emissions.

When will we finally wake up to the fact that, just like the tomato, we need to keep our 'skin' intact in order to survive?

(1,029 words)