Of all the threats facing coral reefs, the most worrying is global warming. Few dispute that this phenomenon has been exacerbated by greenhouse gases pumped into the atmosphere as a result of man’s industrial activities. Increasing sea levels and warming seas are among the effects resulting from this problem. Both these changes are bad news for coral reefs, but it is increasing sea temperature that is the main threat.
Coral reefs are sensitive to temperature changes and have a very small range of tolerance which allows for an increase of only one to two degrees centigrade. When the sea temperature rises above this level the corals react in a very dramatic way and coral bleaching occurs. In 1998, the highest sea temperatures on modern record were noted. In that year, most of the coral reefs around the world experienced a major coral bleaching event. It was the sixth bleaching event since 1979 and the worst on record.
Sea temperatures in the tropics have increased by almost one degree centigrade over the past century and look set to increase at nearer double this rate in the next. From the recent bleaching events we know that reef building corals are getting close to their upper thermal limit. Corals tend to die off in vast numbers following the most extreme bleaching events because they no longer have their algal partners to provide them with necessary nutrients. This post bleaching die off can stretch across thousands of square kilometres of ocean.
In 1998, 80-90 per cent of all the corals in the central Indian Ocean (Maldives, Chags and Seychelles) – represented about 5 per cent of the world’s coral reefs – died in this way. Recovery is now underway on many of these reefs, with new corals appearing in many places. Scientists still have little idea whether the new corals will be any better adapted than those which died.
Complex computer models of global climate have all projected that sea temperatures will increase past the level of tolerance of reef building corals within the next few decades. If this happens, bleaching events will become more intense and more frequent. The 1998 event which caused the worst bleaching to date could be the first of many similar events over the next few decades. If the predictions of the climate models are correct, bleaching events could occur annually in 30-50 years time. Some people argue that this time scale will give corals the time to adapt to the changes, but the fact that corals are already dying off in huge numbers proves fairly conclusively that this is not the case.
There seems little doubt that if global warming continues at the present rate, global losses of coral reefs will be catastrophic. Most evidence shows that corals will not be able to recover from more frequent bleaching events and will not be able to adapt to increased temperatures. This will eventually lead to a shift in the make up of the reef to less diverse coral communities made of the hardiest coral types or even communities dominated by algae.
