Ocean acidification - The other CO2 problem

Coral reef

Coral reefs are among the most vulnerable ecosystems to ocean acidification

5 January 2009 by Tom Marshall

Climate change has many dangerous consequences, but few of them have risen to prominence as quickly as the threat of ocean acidification. Tom Marshall explores the problem.

In the last half-decade the issue has climbed high up the environmental research agenda from almost-total obscurity. But its consequences still aren't well understood, and more research is urgently needed.

Our economy is so dependent on burning fossil fuels that almost everything people do emits some carbon dioxide (CO2) into the atmosphere. The oceans absorb a large portion of this CO2.

That's a good thing in as far as the seas are soaking up carbon that would otherwise remain in the atmosphere and worsen climate change. Less welcome is the fact that dissolved carbon dioxide is slowly making the oceans more acidic by forming a weak carbonic acid.

So far, this has pushed the oceans from a pH of around 8·2 to 8·1 - on this scale, lower numbers mean more acidity. That might not sound like much, but in the context of such a vast and intricate ecosystem as the world's oceans, it's a big deal indeed. And the number looks set to keep dropping as atmospheric CO2 levels continue to rise.

Dr Ken Caldeira and Dr Michael Wickett from Lawrence Livermore National Laboratory in the US were among the first to point out the problem in a Nature paper in 2003. This was followed up by a Royal Society report two years later. Before these studies, researchers had tended to think ocean acidity remained fairly constant, and that the ocean could absorb large amounts of carbon without coming to harm.

The United Nations Intergovernmental Panel on Climate Change (IPCC) predicted in its 2007 fourth Assessment Report that ocean pH will fall by a further 0·14 to 0·35 units over the 21st century. "While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (eg corals) and their dependent species," it added.

"By the end of this century, the oceans will be more acidic that they have been for more than 20 million years," says Dr Ian Joint, a marine microbiologist at Plymouth Marine Laboratory.

More worrying still is that much of the predicted change is already underway because of the carbon we've already emitted; even if we stopped burning fossil fuels today, the oceans would keep getting more acidic for years to come.

Soured seas

If they come true, the IPPC's predictions would still leave the seas a little less acidic than pure water. But the effects would be far-reaching. Among the biggest risks is that rising acidity will harm plankton and other microbes in the water.

These creatures make chalky shells and exoskeletons for themselves out of calcium carbonate. Researchers fear these could dissolve in more acid oceans. Larger creatures like lobsters, mussels, sea urchins and oysters could also be at risk.

Microbes with carboniferous shells sit near the bottom of the marine food chain, and any significant drop in their numbers could end up starving fish and mammals higher up. Corals may be even more vulnerable. The stony skeletons they build to live in are also made of carbonates; these too could dissolve in CO2-soured waters.

Dead coral

Coral killed by the changing environment

This would have catastrophic knock-on effects. Many of the world's poorest people rely on the rich ecosystems around coral reefs for the fish they eat, and these ecosystems are already under severe pressure from environmental changes like stronger sunlight and warmer water as well as from human activities like fishing with explosives or poison. Many reefs are dying, while others are stretched to the limit.

Since the Royal Society report in 2005, evidence that there's a problem has continued to mount. NERC-funded research by a group including Plymouth Marine Laboratory's Dr Steve Widdicombe showed that burrowing heart sea urchins are extremely vulnerable to acidity. As well as the expected damage to their carbonate exoskeletons, the urchins suffered damage to their digestive systems that would ultimately harm their ability to feed.

This isn't just a problem for sea urchins, because these digging creatures play a vital role in stirring up the seabed and releasing nutrients for other organisms to use, similar to the job done by earthworms on dry land. If acid oceans disrupted these activities, the knock-on effects on communities of plankton could be far-reaching.

Acid interactions

The picture isn't simply one of impending doom, though. Recent NERC-funded experiments surprised the scientific community by suggesting some of the sea's calcifying inhabitants may not be quite the sitting ducks some had feared.

Research published in Science in April 2008 suggested that coccolithophores - single-celled underwater organisms covered in calcium carbonate armoured plates, whose compressed remains now form the world's chalk deposits - respond to more acid conditions by growing thicker shells.

This matters because coccolithophores form large-scale 'blooms' that can be detected from satellite covering hundreds of thousands of square kilometres. In such numbers, they control important ecological and carbon cycle processes, and make a major contribution to the Earth's capacity to sink carbon.

"Field evidence from the deep ocean is consistent with these laboratory conclusions, indicating that over the past 220 years there has been a 40% increase in average coccolith mass," the report's authors said, adding that these creatures "are already responding and will probably continue to respond to rising atmospheric CO2 partial pressures."

"Our findings went against the established view that more acidic oceans would simply be harmful to marine calcifying microorganisms" explains Dr Debora Iglesias-Rodriguez of the National Oceanography Centre, Southampton (NOCS), who was one of the paper's lead authors. "But, from a physiological point of view, they were not all that surprising," she adds.

She notes that the geological record from the last period of major ocean acidification around 55 million years ago - known as the Paleocene-Eocene Thermal Maximum - suggest it had little impact on the abundance of calcifying microorganisms, even though this potentially involved levels of CO2 much higher than those expected even under worst-case scenario predictions of what will happen this time around.

The research drew on both experimental work and on analysis of the geological record to understand how coccolithophores have reacted to past upswings in acidity, and how they're likely to behave this time around.

It's still not clear how such an increase in calcification would affect coccolithophores' net absorption of carbon. The paper's authors suggest it could stay roughly stable; more research is needed to be certain, particularly in areas of the ocean that are especially vulnerable to acidification, such as the waters around the Antarctic.

Iglesias-Rodriguez and colleagues are also quick to point out that even if more acid doesn't harm coccolithophores, this doesn't mean it won't harm other marine organisms. Coral in particular still looks vulnerable - unlike coccolithophores, corals do their calcification outside their bodies, so their homes have less protection from the environment as they form.

The research on coccolithophores is still the subject of heated debate, and other scientists note that several experiments on these organisms have suggested the opposite and more intuitive conclusion - that coccolithophores are harmed by more acid conditions.

In a comment published in Science in December 2008, the other scientists criticised the methodology of the experiments published in the same journal in April the same year. They note that these studies were of short duration, and argue it's possible that coccolithophores are able to adapt for a while but won't stay healthy in the long term under more acid conditions.

Iglesias-Rodriguez and colleagues in turn responded to the criticism of their paper in a rejoinder in Science, arguing that their methodology more accurately simulates ocean conditions than previous lab work. The debate continues; if nothing else, it's clear that scientists have a long way to go before they fully understand the effects of ocean acidification on marine microorganisms.

Another piece of NERC-funded research, published in Proceedings of the Royal Society in February 2008, confirmed that more acid conditions could boost calcification rates, but found that this came at a cost to the creatures in question.

This time, the authors - again including Steve Widdicombe at PML - worked with Amphiura filiformis, a species of brittle star, finding that it can increase its metabolism and rate of calcification in response to greater acidity, but at the cost of muscle wastage. They concluded this means such adaptations aren't likely to be sustainable in the long run.

Decoding the data

Other researchers recently tested the effects of changing the environmental conditions of copepods - tiny, water flea-like crustaceans - from pH 8·2 to pH 7. This is an extreme shift representing the level of acidity expected if we extracted and burnt all known reserves of fossil fuel. Even at this level, adult copepods showed few signs of harm - they seemed to live and reproduce just as effectively over the period of the research.

Acidity did cause far fewer eggs to hatch, though, with only 4% surviving, in comparison to around 40% in normal sea water. And the researchers found clear evidence that these levels of acidity were damaging the shells and internal organs of other animals like sea urchins.

A team led by Ian Joint at PML in 2006 used mesocosms - large bags filled with seawater that let researchers test the effects of subtle changes in environment on microbial communities - in a fjord near Bergen in Norway. The data these produced is still awaiting publication, but it isn't expected to support any simple take on the problem. A technique known as metagenomic analysis, which identifies all the DNA present in a sample of water, allowed the researchers to analyse the whole spectrum of organisms living in the mesocosms.

In a sign of the subject's complexity, these experiments suggested very different conclusions about coccolithophores and how they'd adapt to more acid conditions than the NOCS research from 2008. In the more acidic environments in the mesocosms, coccolithophores were less able to form their armoured shells than in normal conditions.

"There are huge discrepancies between the results of different experiments," comments Iglesias-Rodriguez at NOCS, arguing that most of these are probably to do with different experimental design. Clearly more work is needed.

Neutralising the problem?

Levels of CO2 in the atmosphere naturally fluctuate, but the changes seen since the industrial revolution are likely to have a far more dramatic impact on the oceans than previous shifts. That's probably because previous changes in CO2 concentrations happened much more slowly, giving marine ecosystems time to adapt.

The Royal Society's report on ocean acidification in 2005 said that current ocean acidity is already at the highest levels seen for hundreds of millennia - and, crucially, that the rate of change has been "100 times greater than at any time over this period".

We still don't know the final costs of ocean acidification. If the effects on already-stressed ecosystems are as bad as they could be, the ultimate consequences will be profound.

Workable ideas to solve the problem are thin on the ground. One possibility would be a programme of dumping crushed limestone or something similar in the sea to neutralise the acid.

But the scale on which this would have to be done makes it seem impractical. And at the moment digging up this much limestone and towing it out to sea would use up so much fossil fuel, and emit so much carbon, that it's not even certain that there would be any net benefit.

Cutting CO2 emissions could be the only practicable approach. But several of the most promising potential methods for doing this themselves risk making the oceans still more acidic.

Carbon capture and storage, or CCS, involves trying to squirrel away the CO2 emitted by burning fossil fuels, rather than letting it escape into the atmosphere. One idea is to inject it into the underground voids and caverns left after an oil or gas field has been sucked dry.

This could even let the energy industry extract more fossil fuels, by flushing out residual deposits of oil and gas. An international team of scientists and engineers worked on a test project off the Norwegian coast between 1996 and 2004, eventually managing to inject nearly eight million tonnes of liquefied CO2 into the seabed. So far it seems to have stayed put. But there's always the risk of a leak, which would send a plume of CO2 rising into nearby waters.

Another possible solution is simply to pipe liquefied carbon dioxide down into the deep oceans. Here, the constant pressure of the water above should keep the CO2 in liquid form and prevent it from rising to the surface.

This poses even greater risk of making surrounding waters more acid; very large volumes of water could potentially be polluted. The research on copepods used levels of acidity thought to mirror those around a deep sea CO2 injection plant, and found these conditions could be seriously harmful.

Ironically, it was interest in the possible consequences of such a leak from a CCS facility that first spurred awareness of the acidification that's going on simply because of increasing atmospheric carbon dioxide, and the harm this could cause.

Many now think the risks of CCS are worth taking. Researchers point out that governments need to weigh the risk of high levels of acid pollution in a comparatively small area against the near-certainty of significant acidification all over the world if they don't move aggressively to curb CO2 emissions.

"The impact of ocean acidification caused by increased atmospheric CO2 on marine environments if we don't act could be devastating," says Dr Carol Turley, a microbial ecologist and senior scientist at PML who was among the authors of the 2005 Royal Society paper. "Storing CO2 under the seabed is an important method of reducing atmospheric CO2 emissions from burning fossil fuels - just as long as we can make sure it stays there." She adds, however, that research into CCS should be accompanied by work on low-carbon energy sources as part of a wider energy strategy.

The next steps

Environmental scientists have learned the lessons of the global warming debate; ocean acidification has risen into the awareness of policy-makers almost as fast as it has gained prominence among researchers. Interest in, and funding for, research into the issue is now significant.

Ocean acidification is the subject of one of the first large-scale research programmes to come out of NERC's theme action plans. The £7m initiative will shed light on areas including the effects of more acid oceans on vulnerable ecosystems, and how these effects will interact with other expected global changes, such as higher temperatures. Some of the work carried out under this programme is likely to contribute to EPOCA, the European Project on Ocean Acidification.

This is the biggest international consortium currently investigating the issue. It launched in June 2008 and will last for four years. It breaks down the impact of ocean acidification into 16 manageable areas - for instance, its expected impact on calcification, or on the diversity of marine microbes.

The project is split into four teams; the first focuses on the chemistry of ocean carbon, the second on the effects on marine organisms, the third on the impact on ocean biogeochemistry and the fourth on potential tipping points in the ocean system.

Among EPOCA's initiatives so far was a workshop in November including presentations from scientists including Debora Iglesias-Rodriguez on best practice for the design of experiments, with the aim of making the results of different experiments easier to compare. Iglesias-Rodriguez and colleagues are also preparing follow-up research in Antarctic waters to refine the results of their coccolithophore experiments.

Turley at PML is leading the team investigating tipping points, looking for points at which ocean ecosystems may be particularly vulnerable. Scientists from across Europe are working with representatives of organisations ranging from BP and Rolls Royce to WWF and Greenpeace. Turley is also part of the group that will ultimately synthesise all EPOCA's findings and present them to policy-makers and the public in accessible form.

Elsewhere, the story is similar. Germany is in the process of setting up its own national research programme into ocean acidification. Meanwhile US scientists are awaiting the passage of a Senate bill, which they hope will provide some $30m a year in funding for ocean acidification research.

A range of innovative research is now underway. PML's modelling group is creating computer simulations to tell us how higher levels of CO2 will affect Britain's coastal waters. So far the researchers have modelled the effects of atmospheric CO2 concentrations as high as 1000 parts per million - around two and a half times present levels.

US researchers suggested in PNAS that creatures such as the Humboldt, or Jumbo Squid, which grows up to seven feet long and plays an important role in the Pacific marine ecosystem both as predator and as prey, will struggle to breathe in more acid conditions.

Scientists from the University of Plymouth recently published a report in Nature describing their experiments at a site off the Italian island of Ischia where geologically-formed CO2 naturally seeps through the seabed.

They hope this will complement existing research, which has primarily involved work in the lab. They've found that while just outside this 'natural jacuzzi' the ocean's pH is a normal 8·2, directly above the vent it drops to 7·4.

Moving towards the centre of the zone, the scientists note a sharp change in ecology at around pH 7·8 - at this point, corals disappear and the environment become dominated by lush sea grasses and algae.

Many of these species are invaders from very different ecosystems. These take advantage of this strange habitat when native organisms die out. Many carbonate-shelled animals like limpets still live there, but their shells are paper-thin.

Another programme that's underway in the US, called the Free Ocean CO2 Enrichment experiment, is pumping CO2 into the seabed off California and monitoring how this affects the local environment.

Finally, a recent study by scientists at the University of Chicago and published in Proceedings of the National Academy of Sciences suggested that ocean acidity is rising at least ten times faster than previously thought, and perhaps as much as 20 times. This is having having a marked effect on the health of shellfish.

The researchers took half-hourly samples of the coastal waters of the north-west Pacific over a period of eight years. They also monitored the health of marine organisms in the same area, noting that numbers of species like mussels declined sharply over the period.