Saving the Mothership

Healthy coral reef, left, and decimated Australian reef, right, after a severe bleaching event. Photographs by Dr. Ove Hoegh-Guldberg.

The coral reefs of the world could be gone in our own lifetime, or that of our children. The good news is that we can still save them.

By Ove Hoegh-Guldberg, Ph.D.

Coral reefs are drop-dead gorgeous. Magical landscapes filled with thousands of unique species, coral reefs rival rainforests in terms of the number of species that can be found in a single ecosystem. Scientists estimate that we only know 30 percent of more than a million species that live in and around coral reefs. 
Coral reefs have been a lifelong fascination for many of us, including me. As a teenager, I almost drove my poor parents crazy with my many aquariums and my desire to spend all my time exploring beneath the waves.

In addition to being storehouses of biodiversity, coral reefs play a critical role for an estimated 500 million people who depend on them for food and income. Besides providing food, building materials, and the basis for fisheries and tourist industries, coral reefs protect coastal areas from the full force of ocean waves, which would otherwise seriously damage land, buildings, and infrastructure.

Given that coral reefs are important to so many people, you might think that humans would be looking after them with great care. Unfortunately, we are doing almost the exact opposite. Over the past 50 years, we have seen between 30 and 50 percent of all the coral reefs on earth deteriorate so dramatically that scientists predict they may be completely lost in the next few decades.

A recent study by Drs. John Bruno and Elizabeth Selig of the University of North Carolina revealed that the rate at which coral reefs are disappearing has
recently accelerated, and we may be losing as much as 2 percent per year. Numerous other scientists have backed up Bruno and Selig’s research.


Problems facing coral reefs
The reason that coral reefs are deteriorating at such a rapid rate is simple. Human populations are growing exponentially in the coastal regions of the world, and this is intensifying the stress that human activity causes for coral reefs. While some of our activities have a direct impact on the reefs, many of the things we do have indirect and insidious consequences.

The rapid deforestation of tropical coastlines by companies greedy for profit, and the use of unsustainable agricultural practices, have led to an increase in the amount of sediment and nutrients flowing into coastal waters. The coral reefs in these areas are being smothered by sediment and algal blooms.

People have fished coral reefs with such intensity that many of them no longer contain some species that perform important ecological roles. For example, many societies have decimated the populations of herbivorous parrotfish, which normally keep the reefs clean—so the reefs have become overgrown by seaweeds. 
Fishermen in some areas have developed destructive techniques to maximize their catch, such as using dynamite or fertilizer to stun the fish. This practice has reduced many coral reefs to piles of rubble. Reefs in this condition take decades to recover.

Climate change
For many years, scientists felt that climate change caused by rising levels of greenhouse gases, such as carbon dioxide, would only be a problem in the distant future. Over the past 10 years, this perspective has changed dramatically.

At the heart of a coral reef is the intimate symbiosis between corals and tiny plant-like organisms known as dinoflagellates or zooxanthellae. Zooxanthellae live inside the gastric tissues of corals, where they photosynthesize like any other plant. However, instead of retaining the products of photosynthesis, zooxanthellae pass the majority of the organic energy trapped to the coral. The result is literally a “super organism,” which acts like an animal but is able to derive copious energy from the sun as plants do.

The efficiency of the symbiosis between zooxanthellae and corals results in an abundance of energy, much of which the coral uses to grow and calcify. By forming skeletons from a limestone-like substance called calcium carbonate, corals build the three-dimensional structures of coral reefs, which provide habitat for thousands of different species.

Starting in 1979, however, coral reefs began to exhibit a phenomenon called mass coral bleaching. Coral bleaching is a sign of stress and results from the breakdown of the symbiosis between corals and zooxanthellae. When the brown zooxanthellae move out of the tissue of the coral, they leave it a brilliant white (hence the term bleaching). Without the zooxanthellae, the coral is deprived of its principal source of energy. The bleached corals begin to starve and become more susceptible to disease.

The impact can be huge. For example, in 1998, 16 percent of the world’s coral reefs were killed by mass coral bleaching due to warmer-than-normal sea temperatures. This global average hides the fact that some regions fare much worse: the Western Indian Ocean, for instance, lost 46 percent of its corals in that one year.

Over the past several decades, the incidence of mass coral bleaching has grown in frequency and intensity. Our understanding of why it occurs has also increased. Both laboratory and field studies have tracked small (1–2°C) increases in sea temperature above the normal maximal temperature. The relationship between warmer-than-normal temperatures and bleaching is so pronounced that satellite oceanographers can predict with great accuracy when and where bleaching will occur. This link is now the basis of the NOAA satellite detection program (http://www.nhc.noaa.gov/satellite.shtml).

Projections into the future suggest that sea temperatures will soon be so warm that corals will bleach on a yearly basis. If this occurs, their populations will plummet. Research done almost 10 years ago showed that even with mild changes in carbon dioxide levels in the atmosphere, sea temperatures would rise above safe levels by the end of this century. The most worrying news is that the rate of climate change is increasing, so it is now estimated that this will occur within the next 30 to 50 years.

Acidifying seas
As if global warming wasn’t enough, another problem has reared its ugly head: ocean acidification. This phenomenon occurs when an overabundance of carbon dioxide dissolves in the ocean, where it reacts with water to produce a dilute acid known as carbonic acid. One of the major impacts of this acid is to reduce the concentration of carbonate ions in the water. The doubling of carbon dioxide in the atmosphere will decrease the concentration of carbonate ions in sea water by 30 percent.

Scientists have now shown that the decreasing concentration of carbonate ions in seawater has a direct effect on the rate of calcification by corals and other marine calcifiers. Because much of the calcium carbonate that is laid down is removed by physical and biological erosion, a small drop in calcification can mean that corals are no longer able to build and maintain the calcium carbonate structures that are so important to them and to the species that live in and around them.

Earlier this year, scientists reported that calcification of corals (in this case members of the genus Porites) had decreased by 15 percent on the Great Barrier Reef over the past 20 years, a rate that has not been seen at any other time during the last 400 years of record. Researchers in Thailand have found similar evidence of a slowdown in calcification. While scientists are still exploring the underlying reasons for these changes, it appears that these changes might be the “smoking gun” of ocean acidification.

One of the lessons that scientists have learned is how little we know about the synergy of climate change. In research done in my laboratory at the University of Queensland, we have shown that increasing the acidity around corals makes them more susceptible to coral bleaching and mortality. Climate change is also likely to affect a large number of other factors, such as storm intensity and sea level. Scientists have yet to define precisely what these changes are likely to mean for coral reefs in the future.

Saving the mothership
Coral reefs have an enormous amount to teach us about the connection between the well-being of natural ecosystems and that of human beings. Even though they only occupy 0.1 percent of the earth’s surface, coral reefs support 10 percent of the world’s population and provide the basis for multibillion-dollar tourist and fishing industries. Losing corals puts all of this in jeopardy.

Essentially, we are starting to destroy the earth, our mothership, which has supported us for millions of years. What can we do about this dire situation?
There’s no doubt that the most urgent action we must take is to reduce our emissions of carbon dioxide and other greenhouse gases. The best science available demonstrates that if we continue with our current emission levels, we will increase global temperatures by 4–8°C by 2050.

Given the other changes that are occurring—such as the catastrophic breakdown of polar summer ice and the landlocked sheets of Antarctica and Greenland—it is very clear that we must take immediate action on this critical issue. If we don’t, the ramifications go far beyond coral reefs, and may threaten the very existence of humanity.

According to the experts (the Intergovernmental Panel on Climate Change, or IPCC), changing course with respect to the carbon intensity of our economy and lifestyle will cost less than 2 percent of global Gross Domestic Product (GDP) growth. This is akin to getting as rich in 26 years as we would have in 25 years—not much to pay to avoid planetary catastrophe!

As well as urging our leaders to take on this challenge and steer our societies back to a safe climate, it is important that we make changes as individuals. There are many things we can do in our daily lives that will have a huge collective impact on the problem. For example, insulating our homes and investing in solar hot water will reduce the average carbon emissions by 20 to 30 percent per family. Simple things such as carpooling, walking or riding a bike to work, and using public transportation can also reduce our carbon footprint.

There are other things we can do for coral reefs to buy time while we strive to reduce carbon dioxide and other greenhouse gas emissions. Coral reefs will have a better chance of beating the climate-change challenge if other stresses on them are reduced. They will have little chance of recovering from climate-change impacts, such as coral bleaching, if they are polluted and physically destroyed at the same time. For this reason, we must increase our efforts to improve the water quality along coral coastlines, reduce the pressure on reefs caused by fishing, and help countries educate their citizens about the folly of using destructive fishing techniques.

Is there hope for coral reefs?
There is no doubt that the current situation for coral reefs is extremely grim. Local pressures on coral reefs are expanding rapidly as humans rely more and more on coastal resources for their daily food and livelihood. At the same time, our current emission trajectory rises above the worst-case scenario spelled out by the IPCC.
The good news is that we do have the ability to turn the situation around. The cost of transforming our societies from ones that depend on carbon dioxide–emitting energy sources to those reliant on renewable energy sources, such as solar, wind, and tidal, is well within our means, and is tiny compared to the cost of repairing the catastrophic damage we will incur if we don’t.
 

Ove Hoegh-Guldberg is professor and director of the Centre for Marine Studies at the University of Queensland in Brisbane, Australia.

He has worked in polar, temperate, and tropical regions, focusing on the impacts of ocean warming and acidification on coral reefs. He has produced over 160 peer-reviewed publications and has mentored more than 30 postgraduate students. Hoegh-Guldberg is currently a Queensland Smart State Premier’s fellow. He works closely with several industry and NGO groups on climate-change issues and is a reviewing editor at Science Magazine. In 2009, Hoegh-Guldberg was made director of the Global Change Institute at the University of Queensland.
Details of his laboratory group can be found at www.coralreefecosystems.org. His blog is at http://www.climateshifts.org

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