Corals and Climate Change

By Christine Evans 

This is part one of a two-part series.

Corals are among the oldest living organisms on the planet and have existed for some 500 million years. Coral reefs are one of the most biodiverse ecosystems on Earth with higher genetic diversity than tropical rain forests.

Corals play an integral and irreplaceable role in the world’s oceans, providing a habitat for between one and nine million marine species. Even though they cover less than 1 percent of the sea floor—just 0.2 percent—they support an estimated 25 percent of all life in the oceans, including 25 percent of known fish species. They provide critical habitat during the life cycles of many marine species, including shrimp, crabs, starfish, sea cucumbers, sea urchins and sea turtles. They protect coastlines against storms, improve water quality, are an important source of new medicines, and support the fisheries upon which hundreds of millions of people around the world depend, not just for their livelihoods, but for their sustenance and way of life.

Different studies have tried to put a dollar value on the ecosystem services that coral reefs provide, from direct use values, like fisheries and tourism, to indirect use values like coastal protection and water filtration.

One widely cited estimate is based on an analysis of over 80 coral reef valuation studies. It places the average annual economic value of a hectare of coral reef at $130,000 a year, with an upper limit of $1.2 million a year. In total, the services that reefs provide are worth approximately $172 billion annually, according to the analysis. Other estimates put this number closer to $375 billion.

Despite their enormous importance to humans and to life in the oceans, the future for corals is not bright. Faced with many stressors already—pollution, coastal development, destructive fishing practices, overfishing, sedimentation and more—many scientists believe climate change will prove to be too much for coral reefs. According to J.E.N. Veron, former chief scientist at the Australian Institute of Marine Science, “Unless we change the way we live, the Earth’s coral reefs will be utterly destroyed within our children’s lifetimes.”

Since the start of the industrial revolution, humans have been adding large amounts of carbon dioxide (CO2) to the atmosphere, much of which is now stored in the oceans. In the coming decades and centuries, climate change may have many impacts on marine ecosystems and coral reefs. Food webs, oxygen content of the water, disease outbreaks, ocean currents, and changes in the frequency and strength of storms are some of the ocean processes affected by climate that could impact reefs. Furthermore, climate change will exacerbate many existing stressors that coral reefs face, like fishing, pollution, and habitat destruction.

But there are two impacts of climate change that are particularly important for corals: warmer temperatures and increased ocean acidity.

Since 1955, around 93 percent of planetary warming has occurred in the world’s oceans. Corals are highly sensitive to changes in their environment, including temperature, and such stress can cause a type of algae, known as zooxanthellae, to leave the corals’ tissue. Zooxanthellae and coral have a symbiotic relationship, with the tiny photosynthetic algae supplying corals with energy and food and the corals providing nutrients and a home for the algae. Zooxanthellae are what give corals their bright colors, and when zooxanthellae leave, corals become “bleached,” making them more vulnerable to things like disease. Bleaching events may be taking place more frequently now due to the many stressors that corals are facing.

Two wide-spread bleaching events have been recorded previously—one in 1998 and another in 2010—and a third is currently underway. According to the U.S. National Oceanic and Atmospheric Administration (NOAA), roughly 12 percent of the world’s coral have bleached in the last year, and over 9,000 square miles could be lost before this most recent event is over, which could be well into 2016. “It probably won’t be as big as 1998, so we’re probably talking hopefully no more than 10 percent. Even if we’re talking one to 10 percent of the coral reefs around the world that’s a huge amount of coral reef area,” says Mark Eakin of NOAA’s Coral Reef Watch Program.

And according to Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, coral bleaching of the magnitude of these events has not taken place in “hundreds, if not thousands, of years.”

Around the turn of the century, coral bleaching was thought to be the most serious danger facing corals, but since then some scientists have suggested a potentially far more sinister threat—ocean acidification. Ocean water has become approximately 26 percent more acidic since the Industrial Revolution, and will become much more so unless we curb CO2 emissions.

As more CO2 finds its way into the oceans, it reacts with seawater to form carbonic acid. This reduces the water’s pH, leading to acidification. Higher levels of acidity reduce the availability of carbonate ion, which is a necessary component of the calcium carbonate shells and skeletons that are formed by organisms like plankton, shellfish and coral. As a result, these organisms grow more slowly and their shells and skeletons become less dense.

This will have long-term consequences for coral complexity, growth and recovery. “The decline of the structurally complex corals means the reef will be much simpler and there will be less habitat for the hundreds of thousands of species we associate with today’s coral reefs,” says Katharina Fabricius of the Australian Institute of Marine Science. “There are also fewer juvenile corals in areas with high CO2 levels, therefore coral reefs in those environments face greater challenges recovering from disturbances like tropical storms.”

Corals have faced several extinction events in the past—as far back as 435 million years ago and as recently as 24 million years ago—and it has taken millions of years for them to come back after such events. According to Veron, “ocean acidification has played a major part in the marine devastations which took place in those ancient times.” And scientists have found nowhere in the geologic record where these changes have happened as quickly as they are taking place today. In fact, the rate of change may be entirely unprecedented. Dramatic changes in climate and ocean chemistry usually take place on timescales of thousands of years, rather than a few hundred.

“Coral reefs speak unambiguously about climate change,” says Vernon. “They survived Ice Age sea-level changes of 120 meters or more with impunity. They once survived in a world where CO2 from volcanoes and methane was much higher than anything predicted today. But that was over 40 million years ago, and the increase took place over millions of years, not just a few decades, time enough for ocean equilibration to take place and marine life to adapt.”

Says Fabricius, “It’s really important for me that the next generation can grow up diving in the ocean, enjoying it and getting pleasure from it. I’d like my girls to see it (as it is now). I’m seriously concerned how the ocean will look like in 2100. Because with the current predictions we will not have coral reefs, with the current predictions they will not enjoy eating mussels and oysters. They will not have a lot of things we take for granted.”

Although the outlook for coral reefs, not to mention the oceans more generally, may seem bleak, there is still much that can be done. Part two of this post will dive into more hopeful developments in coral reef research and conservation.

Christine Evans is a graduate student in Columbia’s Sustainability Management program.