As global warming wreaks havoc on coral reefs, evidence is mounting that another problem caused by carbon dioxide is an even bigger threat. But is it too late to fix?

Corbis

It's six o'clock on a Sunday morning and I'm sitting on Queensland's Four Mile Beach. There's still a night chill to the air. Though the light is dim, a red glow is building on the horizon as the Sun is about to emerge from beyond the Pacific Ocean.

I'm playing with the sand between my toes and fiddling with a small piece of coral rubbed smooth by the tide.

I've spent the preceding few days out on an Australian government marine survey vessel snaking its way along the Great Barrier Reef. The trip has given me a lot to think about, both good and bad, and this morning I'm mulling over everything I've experienced.

In late July, the CSIRO invited me to join a team of 14 scientists, led by oceanographer Bronte Tilbrook and climate modeller Richard Matear, as they collected data to predict the future health of the reef.

The issue on their agenda is ocean acidification, commonly referred to by those in the know as "the other CO₂ problem" - separate, but linked to climate change. Though acidification has had a lot less press, there is mounting evidence to suggest that it will be a bigger problem for marine life than the warming of the oceans themselves.

Our waste carbon dioxide (CO₂) is mostly maligned for causing climate change as it builds up in the atmosphere, trapping heat, but for the past 200 years it's also been quietly dissolving into the oceans, slowly making them more acidic.

In fact, the oceans are in equilibrium with the atmosphere and have been credited with absorbing something like 40 per cent of all the CO₂ we've pumped out in the last 200 years. In this way, they have acted as a useful brake on global warming, but experts have slowly come to realise that this service has come at a terrible price.

"The oceans have this huge buffering potential for CO₂, and until around a decade ago we thought there was plenty of capacity left and [CO₂ dissolving] wouldn't have a big effect," says Tilbrook, a tall man with white hair, pale blue eyes and a gentle disposition. But research in the 1990s on corals and early maps of oceanic CO₂ concentrations painted a very different picture.

THE PROBLEM WAS NEATLY ILLUSTRATED by an accidental discovery made more than a decade ago by Victoria Fabry, a biologist at California State University in San Marcos, USA. While out on another research cruise, Fabry found that something strange was taking place in sealed jars of seawater populated with shelled planktonic organisms called pteropods. After a few days the shells of the pteropods started to look thinner, and eventually dissolved.

The reason, Fabry realised, was that CO₂ from respiration was building up in the jars, dissolving in the seawater as a weak solution of carbonic acid, and drastically reducing the availability of calcium carbonate (the major constituent of limestone or chalk), which many marine animals need to build their skeleton or shells.

"As CO₂ levels rise in the ocean it makes the actual chemical formation of calcium carbonate difficult for organisms," says Matear, a Canadian lured over to Australia by the CSIRO in the 1990s. "The important thing here is that ... the calcium carbonate becomes a little less stable and corals seem to calcify less."

Joanie Kleypas, based at the U.S. National Centre for Atmospheric Research (NCAR) in Boulder, Colorado, was one of a handful of scientists to realise there was a problem. In the late '90s, she battled tirelessly to have the research community accept their conclusions.

She explains to me via email that, while warming of the oceans will swiftly kill coral outright through bleaching, ocean acidification will work by hindering recovery. "Increasing atmospheric CO₂ is like an underlying disease that causes two different kinds of symptoms," she says. "One is acute, like a heart attack, and the other is chronic, like osteoporosis."

The difference in pH we're talking about seems trifling, but it is enough to have a profound effect on life. Pure water is neutral with a pH of around 7. Seawater is slightly alkaline, and for millions of years has been a fairly constant pH 8.2. But already dissolved CO₂ has reduced that to 8.05, and as carbon continues to build up in the atmosphere, it's going to drop much further.

If projections are correct, an additional drop in pH of 0.4 by the end of this century will be "well outside the realms of anything organisms have experienced in over hundreds of thousands of years," says Janice Lough, a climate scientist with the Australian Institute of Marine Science, in Townsville, Queensland.

Tilbrook, based with the CSIRO in Hobart, Tasmania, believes that though the Earth's temperature has fluctuated throughout history, the CO₂ concentration of the atmosphere - and oceans - hasn't been as high as it's likely to be by 2100 at any time in the last 23 million years.

The consequences for the Great Barrier Reef in particular will be grave, he says. By way of example of what we might expect as ocean pH continues to drop, a study in the journal Science in March 2007 showed that skeletons of Scleractinian corals not only stopped growing, but completely dissolved when kept in a tank of water at a pH of 7.4 for 12 months (about the same pH as the human body).

Lough's own study, published earlier this year, points to a 21 per cent decline in the rate at which a type of Porites coral has been able to grow its calcium carbonate skeleton since 1980. The researchers - who monitored 38 colonies of the common reef builder in two parts of the Great Barrier Reef - speculate that this is evidence of ocean acidification already doing its dirty work.

But without baseline data on the carbonate chemistry of the reef waters, it's impossible to place the finger of blame, and that's what the researchers I've joined on the Southern Surveyor are planning to collect. "We need more information about the chemistry of the GBR and how this has changed, is changing and will change," says Lough, who was not a member of the research team on the ship.

The 66 metre Southern Surveyor is the only large, government-funded marine survey vessel for Australia’s own waters. The Aurora Australis gathers data in Antarctica and the Southern Ocean – John Pickrell

"Although the potential to disrupt ecosystems is high, there is virtually no information on the carbon chemistry of the GBR region needed to evaluate the risk of acidification," agrees Tilbrook. He hopes the data they collect will be a benchmark for tracking the progression of acidification. "It's a major step in assessing how vulnerable the reef is to increasing CO₂ emissions in coming decades," he says.

IN THE DAYS PRECEEDING the scheduled start of the 17-day research cruise, the weather has been bad, so as I board my flight from Sydney to Cairns, I'm unsure whether I'll even be able to go out on the 66-metre science vessel. To add further suspense, the Great Barrier Reef Marine Park Authority, concerned about the disturbance to parts of the reserve that are usually off-limits, is yet to issue the necessary permits.

The next morning is the start of a clear day, however, and the park authority grants the permits. On board are Tilbrook and Matear, their team of scientists and a reporter and cameraman for the ABC's The 7.30 Report. Another researcher on board, James Cook University's Robin Beaman, is using a high-tech sonar method to map out a massive and newly discovered fossil reef that runs along the edge of the continental shelf (see "The drowned reef", Cosmos 23, p52).

During the few days aboard the ship, the researchers explain what they're doing and I watch them at work. Day and night, every couple of hours, the engine comes to a full stop and the researchers get to work in a bay that has a crane and opens onto the side of the vessel.

Decked out in lifejackets and hard hats, they dance around each other attaching cables, emptying and filling water bottles, and checking apparatus. The main purpose of this voyage is to collect thousands of samples of water from a series of depths down to the seafloor, in hundreds of different spots along the entire Great Barrier Reef.

Starting in Cairns (and then dropping me and the ABC guys off in Port Douglas) they will continue as far north as Cape Direction, before doubling back and weaving through 2,500 nautical miles all the way down to Gladstone.

In the cool night air, under the harsh glare of floodlights, they attach 24 half-metre-long grey bottles to a device called a CTD (conductivity, temperature and depth) probe. This is lowered as far as two kilometres below the surface of the ocean on a cable. The scientists then slowly winch it back up, triggering each of the open bottles to close at different depths to collect a sample of water.

Once the CTD is back on deck they cluster around it like bees to a flowering shrub, and dribble the water into hundreds of smaller bottles. These are ferried to a lab below deck, where marine chemists work through the night analysing the samples to collect data on the oxygen and CO₂ content, pH and other parameters.

The next day, sitting in the ship's common room, Matear explains to me that similar work has already been completed in other parts of the world's open oceans. Research on the acidification of the vast Southern Ocean has produced some of the most alarming results so far.

In most of the world's tropical regions, experts are concerned that acidification will be an additional stress factor on corals already struggling with warming waters, he says, but "the next extreme is that water becomes so corrosive that calcium carbonate becomes unstable and dissolves".

The worrying thing is that Matear's calculations predict that large portions of the Southern Ocean will experience this state when CO₂ in the atmosphere hits 600 ppm (parts per million). Prior to the Industrial Revolution 280 ppm had been the norm, today it's 380 ppm, and it is expected to quickly rise to 600 ppm. "That's not far off," says Matear, "we're talking 2050 or 2060 based on current emissions predictions."

"What then happens is, if you have an organism with an aragonite [a form of calcium carbonate] shell, and you put it in that water...it would lose its shell," he says. The pteropods that dissolved in Victoria Fabry's jars are just one such species, and they are one of the most plentiful plankton species in parts of the Southern Ocean, such as the Ross Sea.

Most of the research looking at the effect of acidification on plankton has so far been on pteropods and coccolithophores (a microscopic plant covered in calcium carbonate platelets), but a recent pilot study from the Australian Antarctic Division and the University of Tasmania is the first to show that acidification negatively impacts the development of Antarctic krill, too.

The study found physical abnormalities and decreased activity in the tiny crustaceans, when grown in tanks at the pH expected in the Southern Ocean by the end of this century. It's unlikely such krill would survive to adulthood, says Lilli Hale, an honours student involved in the research.

"Antarctic krill play a key role in the structure and function of the Southern Ocean ecosystem, serving as an important grazer and critical prey item for reproductive successes of whales, seals and seabirds," she adds. It's hard to imagine how species such as the blue whale will survive without the krill on which they rely.

"The potential impact on the dynamics of ecosystems will be very large," says Matear, also based in Hobart with the CSIRO. "But our understanding of these ecosystems at the moment is so poor that it's difficult to make a projection of the scale of the effects."

NCAR's Kleypas likens it to the unfolding global economic crisis: "When one bank fails, then it affects the ability of another financial institution to remain solvent. Because of the interconnected nature of marine ecosystems, a similar domino effect will occur."

The outlook for the Southern Ocean is grim. I begin to wonder what will become of the Great Barrier Reef itself - described by Tilbrook as "the largest living [calcium] carbonate platform on Earth".

Ken Anthony, a marine ecologist at the University of Queensland in Brisbane, says that most existing predictions of the effect of ocean acidification on coral reefs are limited by the fact that they are based on what we know about processes in the open ocean.

The work Tilbrook and Matear are doing will fill an important gap in the data. "This cruise is the first to provide us with a good background understanding of how the chemistry of ocean water changes when it interacts with the reef, and will help us better predict the threat to the Great Barrier Reef," he says.

To really appreciate what we're in danger of losing, I decide I'd better have a look for myself.

TWENTY-FIVE KILOMETRES OUT TO SEA from Port Douglas and Four Mile Beach are the Low Isles, and it's here, in the last days of my trip, that I spend some time snorkelling, seeing the reef up close. These islands are coral cays vegetated with mangroves and palm trees, and the reefs offshore are famous as the location of one of the first detailed ecological studies of coral reefs ever attempted - an expedition mounted by Britain's Royal Geographical Society between 1928 and 1929.

As I paddle around, I am dumbfounded by the variety of species: an array of brightly coloured fish, hard and soft corals, giant clams, anemones and even a turtle. I see as much life here, just below the surface, as I had scuba diving in deeper waters elsewhere in the tropics, which is testament of the incredible diversity of the Great Barrier Reef.

Even here though, in a seemingly pristine spot, damage to the reef is evident. While I'd been out on the Southern Surveyor, Prime Minister Kevin Rudd and the Minister for Climate Change, Penny Wong, had been taken out to the Low Isles by researchers to see examples of healthy reefs and those that have been heavily damaged by bleaching.

Reefs worldwide have had a tough time in recent years: overfishing, pollution and the warming effect of climate change have all taken a heavy toll. Bleaching, a stress response to warm water (whereby a coral ejects the symbiotic algae that it needs to survive, and often dies) is one of the most immediate problems.

An Australian study published in the journal Ecology Letters in 2006 provided compelling evidence that coral reefs have suffered more damage between the 1970s and today than at any other time in the last 220,000 years. If the reefs are already struggling so much, what hope do they have in coming years as acidification accelerates?

Chris Langdon runs the Corals and Climate Change Laboratory at the University of Miami in Florida, and worked alongside Kleypas in the early days to impress the severity of the problem on other experts. He predicts that - though they can live for 300 years - as coral colonies die, they will not be replaced by new recruits at the rate needed to sustain them.

"The latest results are showing that the growth rate of baby corals in the first days of their lives are strongly impacted by ocean acidification, such that they develop more slowly, prolonging their time at a highly vulnerable stage," he says.

Unfortunately, because ocean acidification is such a new problem, much of the fieldwork that will help us predict the effects has yet to be completed. But the research that does exist is not encouraging.

A study published in the Proceedings of the National Academy of Sciences in July 2008 looked at an area of the Pacific Ocean off Central America where there is a natural upwelling of CO₂-rich waters from the sea bed. It found that there is little of the calcium carbonate cement that holds reefs together, and that reefs here grow slowly and erode rapidly.

Studies of the communities around shallow volcanic vents in the Mediterranean Sea provide more evidence. The water here is cool, rich in CO₂ and has a pH of about 7.5, so it provides some clue as to what life may be like in a more acidic ocean. Around these vents, "you don't get many organisms with calcium carbonate skeletons; you get a lot of algae and seagrass growing," says Tilbrook. "This is probably an indication of what might happen in the extreme."

Towards the end of the century, Tilbrook envisages a reef structure that is severely weakened, easily damaged by storms and with few new corals growing on it. "It might be dominated by algae, we really don't know, but there are likely to be some pretty significant changes," he says. "The picture is: no reef as we know it, a completely changed habitat."

"In 50 years time, the Great Barrier Reef will probably be severely affected by coral bleaching and the coral diseases that seem to follow," Kleypas tells me. "At these levels, ocean acidification won't cause their death, but it will already be affecting the ability of corals and coralline algae to grow and compete for space."

She points to research showing that some algae and seagrasses are more successful in high CO₂ conditions, meaning that ocean acidification will give them an advantage over the corals and coralline algae that cement the reef together.

Other studies are starting to hint that the problem is going to hit a wider sweep of species than just those that have calcium carbonate shells or skeletons.

Research from the Monterey Bay Aquarium Research Institute in California, U.S., recently found that lowering the pH of the oceans allows sound to travel farther - potentially 70 per cent farther by 2050 - with knock-on effects for whales, which rely on sonar, and are already struggling with a cacophony of man-made noise pollution.

Another issue that could affect a range of marine invertebrates and fish is increased acidity of their body fluids, a condition called acidosis. A study published in the journal Current Biology in July 2008 reported that the fertilisation of an Australian sea urchin (Heliocidaris erythrogramma) fell by 25 per cent in water at a pH of 7.7, which some models suggest will be common by 2100.

"A 25 per cent drop in fertility is the equivalent of a 25 per cent drop in the reproductive population," says Jon Havenhand, lead author behind the study, and a marine ecologist at the University of Gothenburg in Sweden. "It remains to be seen whether other species exhibit the same effect, but, translated to commercially and ecologically important species, such as lobsters, crabs, mussels and fish, acidification would have far-reaching consequences."

Though it's yet to hit the mainstream agenda in a similar way to climate change, ocean acidification could be damaging for our economies as well as our ecosystems.

Because of the size of the Australian coastline, 60 per cent of its territory is ocean. The CSIRO estimates that 10 per cent of the Australian economy is tied to the seas and in 2002/03 the maritime industry contributed about A$70 billion to the national economy. Oysters, mussels and abalone represent important fisheries to Australia (one quarter of the world's wild abalone is fished from Tasmanian waters) and all are shelled species that grow more slowly as pH drops.

Kleypas points to a 2006 report which predicts that a 10 to 25 per cent decline in the harvest of calcifying shellfish could create economic losses for the U.S. economy totalling up to U.S.$1.5 billion a year.

It's not just the impact on fisheries either. One study estimates that A$800 million out of A$900 million total tourism revenue in coastal regions of Far North Queensland in 1999 was related to the Great Barrier Reef - and tourism related to the reef is likely to take a nosedive if the reef deteriorates. "People probably will not pay to come and see weed-covered, rocky areas," says Tilbrook.

The sun rises over Four Mile Beach – John Pickrell

AS I SIT BAREFOOT, WATCHING THE SUN rise over Four Mile Beach on the last day of my trip, a couple with a child appear, along with other vacationers taking an early morning run along the tide. It's pretty evident how the entire local economy is tied up in tourism.

The quandary is, even with international agreement to drastically cut CO₂ emissions - something that now, more than ever, I understand is desperately important - atmospheric levels of the greenhouse gas will still swell before they subside again, and there's not a lot we can do to stop it getting into the oceans.

Part of the reason ocean acidification threatens to be a bigger long-term problem than global warming is that marine life has so far shown little capacity to adapt to falling pH levels. "We know that, given enough time, many organisms can eventually adapt to climate change," laments Kleypas, "[but] it does not appear that many organisms can adapt to ocean acidification."

With action now, we can stop the worst-case scenarios from playing out. But I'm left wondering if we're already too late to do much to stop this disaster from happening. Looking out towards the Low Isles, I'm glad I came to see the reef myself before it was too late.