Climate Change

The End of Coral Reefs? Not Yet.

November 12, 2015

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Limestone islands surround a sheltered lagoon where hard corals grow within centimeters of the low tide line. Wayag, Raja Ampat, Papua, Indonesia, Pacific Ocean. Photo © Ethan Daniels
Limestone islands surround a sheltered lagoon where hard corals grow within centimeters of the low tide line. Wayag, Raja Ampat, Papua, Indonesia, Pacific Ocean. Photo © Ethan Daniels

When I think of my life on coral reefs I feel old.

I’ve spent much of my life exploring reefs around the world — working on a Great Barrier Reef research station, surveying reefs in the Sinai Peninsula before the adjacent deserts were paved with hotels, and diving among the world’s least-disturbed coral reefs in the British Indian Ocean Territory.

Thirty-five years after my first-ever snorkel, we now know that corals are in trouble. Concerns first raised by scientists are now being echoed by island nations threatened by sea level rise, fishers who depend on their fecundity, by the multi-billion dollar dive tourism sector, and by nations and international entities from the United Nations to the Papacy.

It would be naive to think that we can restore the world’s reefs, or any other ecosystem, to a pre-Anthropocene state. But despite the apparent gloom it’s also too early to predict the end of reefs altogether.

Corals in West Papua Province, Indonesia. Photo © Jeff Yonover
Corals in West Papua Province, Indonesia. Photo © Jeff Yonover

Climate and the Problem of Compounding Stress

Coral reefs are naturally dynamic. They have always been hit by storms, plagued by starfish and hammered by disease. They have been knocked down countless times, and yet they bounce back with a surprising degree of resiliency.

What’s different now is that humans are adding enormously to the burden of stress. An estimated 75 percent of the world’s warm-water reefs are threatened by human activities, and degradation is almost ubiquitous.

This too isn’t all that new­­­­­­­­ — overfishing in many places goes back a century or more. Pollution is also not a new problem. The addition of human stressors to the natural background had already affected some reefs even when, as a ten-year-old, I first enjoyed their frenzied kaleidoscope of life. Not long after that, the worst affected reefs started showing what we scientists called “phase-shifts,” the coral being replaced by thick, cloaking seaweed. They were no longer reefs, and they were not nearly so rich and productive.

Climate change is the new threat on the block and it worries me deeply. When stressed by warm waters, corals show a stress response known as coral bleaching. If it stays even just a couple of degrees too warm for too long, the corals which build the reef will die.

In 1998 I led an expedition to survey the coral reefs across the vast archipelago of the Seychelles. That year was an El Niño year, and the world’s oceans had never been hotter. Our fantastic exploration was changed, not ended, as we witnessed the demise of 90 percent of all of the living corals in our 1500-kilometer transit of the Indian Ocean.

What I witnessed in the Seychelles was mirrored worldwide, although it was probably at its very worst in the Indian Ocean. The only upside on this is that it wasn’t the end. Ten years later, most of those reefs were partly or fully recovered. They were more resilient than we first thought.

Now, a similar, wide-scale bleaching event looks set to sweep the world’s oceans. Fueled again by El Niño, the National Oceanic and Atmospheric Administration recently declared it the third global coral bleaching event ever recorded. We can only hope that the world’s reefs will eventually recover, as they did in 1998.

The other piece of climate change that is just beginning to affect coral reefs is ocean acidification. A large part the great surplus of carbon dioxide we are creating through the burning of fossil fuels is dissolving in the ocean. From a temperature perspective we might be grateful — the air would be a lot hotter if it weren’t for this dissolution. But as it dissolves the carbon dioxide makes the oceans gradually more acidic. Corals build their structures from limestone, or calcium carbonate, but the minerals they take from the waters to do this are dissolved in an increasingly acidic ocean. The fear is that as ocean acidification accelerates, corals will build weaker skeletons, grow more slowly, or even that their very skeletons may start to dissolve.

Bleached coral © Mark Spalding
Bleached coral © Mark Spalding

Adaptation and Resilience in the Anthropocene

Now, coral reefs are expected to survive in the face of a cocktail of pressures. Warming and acidification are worldwide challenges, but they simply add to existing burdens; there are few reefs anywhere that are not already threatened or degraded by overfishing or pollution. It’s hard to keep optimistic in the face of all these challenges, but it’s not the end for reefs. In a recent perspective for Science, I pointed out why we still have reasons to hope, if we act now.

We now know that some corals and reefs are showing some adaptive capacity to both warming and to acidification. Even better, some appear to be adapting faster than we expected to high temperature exposure, and some appear to have some smart physiological mechanisms that enable them to extract minerals for their skeletons even from very low concentrations in the water.

Some reefs may be more resilient than we expected. And it seems this resilience may be something we can support. The reefs of the British Indian Ocean Territory were among the worst hit by bleaching in the Indian Ocean in 1998, but they also showed dramatic recovery. In the Seychelles, similarly impacted reefs showed a mixed message — some recovering, others phase-shifting into a less productive habitat dominate by algae. Various factors can be linked to this differing response: some of the factors building resilience were physical, chemical, or oceanographic, others such as nutrients and overfishing are elements we can manage.

And we must try.

As long as there are still coral reefs, there will be riches for local people. An estimated 275 million people live near reefs. Reefs generate sand for beaches, break the waves during the wildest of storms, produce tons of fish year after year, and they draw travelers from across the globe, spending critical finances on local communities. To all visitors, from near or far, reefs are rich, magical places, perhaps Earth’s most beautiful and entrancing ecosystems.

The future of coral reefs is in our hands. The urgency is certainly worrying. But I don’t think this urgency unique to reefs. Other habitats may already be in the same situation, or soon will be.

If we can halt or slow the decline with better management then we are buying critical time. We must cut emissions deeply and quickly. It is perhaps all the more urgent for coral reefs than for other ecosystems, but they can survive. And millions of people need them to.

To learn more, listen to this podcast:

The End of Coral Reefs? Not Yet - Podcast with Mark Spalding
Mark Spalding

Mark works for our Global Oceans Team. He is a marine scientist with a passion for the world’s oceans. He has worked for The Nature Conservancy since 2004, and is based out of the Department of Physical, Earth and Environmental Sciences, University of Siena, Italy, while also remaining part of the Conservation Science Laboratory in the University of Cambridge (where he also does a small amount of lecturing). More from Mark

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