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

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.

You may well feel that dire predictions about anything almost always turn out to be exaggerations. You may think there may be something in it to worry about, but it won’t be as bad as doomsayers like me are predicting. This view is understandable given that only a few decades ago I, myself, would have thought it ridiculous to imagine that reefs might have a limited lifespan on Earth as a consequence of human actions. It would have seemed preposterous that, for example, the Great Barrier Reef - the biggest structure ever made by life on Earth - could be mortally threatened by any present or foreseeable environmental change.

Yet here I am today, humbled to have spent the most productive scientific years of my life around the rich wonders of the underwater world, and utterly convinced that they will not be there for our children’s children to enjoy unless we drastically change our priorities and the way we live.

A decade ago, my increasing concern for the plight of reefs in the face of global temperature changes led me to start researching the effects of climate change on reefs, drawing on my experience in reef science, evolution, biodiversity, genetics, and conservation, as well as my profound interests in geology, palaeontology, and oceanography, not to mention the challenging task of understanding the climate science, geochemical processes, and ocean chemistry.

When I started researching my book, A Reef in Time: The Great Barrier Reef from Beginning to End (Harvard, 2008), I knew that climate change was likely to have serious consequences for coral reefs. But the big picture that gradually emerged from my integration of these disparate disciplines left me shocked to the core.

In a long period of deep personal anguish, I turned to specialists in many different fields of science to find anything that might suggest a fault in my own conclusions. But in this quest I was depressingly unsuccessful. The bottom line remains: Science argues that coral reefs can indeed be utterly trashed in the lifetime of today’s children. That certainty is what motivates me to spread this message as clearly, and accurately, as I can.

So what are the issues? Most readers will know that there have been several major episodes of mass bleaching on major reef areas worldwide over the past 20 years. In the late-1980s when the first mass bleaching occurred, there was a great deal of concern among reef scientists and conservation organisations, but the phenomenon had no clear explanation. Since then, the number and frequency of mass bleachings have increased and sparked widespread research efforts.

Corals have an intimate symbiotic relationship with single-celled algae, zooxanthellae, which live in their cells and provide the photosynthetic fuel for them to grow and reefs to form. The research showed that this relationship can be surprisingly fragile if corals are exposed to high light conditions at the same time as above-normal water temperatures, because the algae produce toxic levels of oxygen, and excessive levels of oxygen are toxic to most animal life. Under these conditions, corals must expel the zooxanthellae, bleach, and probably die or succumb to the toxin and definitely die. A tough choice, one they have not had to make at any time in their long genetic history.

We tend to think of temperature in terms of our day-to-day comfort level. We don’t have to be told that atmospheric temperature shows huge swings and variations from day to night, among seasons, and cyclically on other scales. Early critics of global warming used this variability to argue that there was no evidence for overall thermal increases. This missed the point and delayed our recognition of the true problem because atmospheric temperature is only a minor part of the Earth’s thermal picture.

By far the most important mobile heat sinks on the planet are the oceans. As the greenhouse effect from elevated CO2 has increased, the oceans have absorbed more heat. The surface layers are affected most as mixing to the depths can take hundreds of years. Large ocean masses such as the Indo-Pacific Warm Pool do not continue to warm further, but rather they broaden and deepen. Now they commonly become so large that their outer edges are pulsed onto the continental margins, where waters are warmed further. This creates the mortal dilemma for corals - to expel or not to expel their oxygen-producing zooxanthellae.

Ecosystems can recover from all sorts of abuse, and coral reefs are no exception. Good recoveries from bleaching have been observed, provided that further events do not occur while the ecosystem is re-establishing. Unfortunately, there are no signs that greenhouse gas increases are moderating, and so we can assume that the frequency and severity of bleaching events will continue to increase - on our present course, the worst bleaching year we have had to date will be an average year by 2030, and a good year by 2050. Ocean and atmospheric rises in temperature are also predicted to increase the severity of cyclones, which will add an extra burden on the recovery process.