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Acid test for the seas

The basic facts on ocean acidification.

Key text

Box 1: Iron fertilisation of the oceans Activities
Further reading
Useful sites

Key text

Chemists have known for a long time that a beaker of water sitting in a lab will absorb carbon dioxide from the air and turn acidic. Would it happen at a larger scale? If we greatly increased the concentration of carbon dioxide in the world's atmosphere, for example, would the oceans become a vast acid bath? What would be the ecological effects? Over the next century or so, we are going to find out.

Increasing atmospheric carbon dioxide

For more than two hundred years, the human race has been releasing large quantities of carbon dioxide and other greenhouse gases into the atmosphere. It has been doing this in two main ways: by burning fossil fuels, such as coal, oil and natural gas, and by clearing and burning vegetation, such as forests.

Carbon dioxide, methane, water vapour, nitrous oxide and ozone are known as greenhouse gases because they trap heat in the atmosphere, like glass does in a greenhouse; in this way they help warm the Earth. From 1750 to 2007, the concentration of atmospheric carbon dioxide increased from 278 to 383 parts per million, or by about 38 per cent; with 22 per cent occurring in the past 50 years. This might not seem much, but most climate-change scientists agree that it has been enough to heat up the Earth by an average of about 0.7 ºC. This also might not seem like much, but if the concentration of greenhouse gases in the atmosphere continues to increase, average surface temperature of the Earth could rise by as much as 6 ºC by 2100. Life in our already-sunburnt country would become very sticky indeed.

Ocean acidification

Not all the carbon dioxide released into the atmosphere stays there; some of it – about a third of total human-induced emissions – has been absorbed by vegetation during photosynthesis and a similar amount has been soaked up by the ocean. We're lucky that it has, or global warming would be happening much more quickly than it is.

The oceans are naturally alkaline, or basic, with a pH of about 8.2. When carbon dioxide dissolves in sea water it forms carbonic acid (H2CO3), which releases hydrogen ions (H+), lowering the pH and making it more acidic. Scientists estimate that the additional carbon dioxide in the atmosphere and the subsequent absorption of some of this by the oceans has lowered oceanic pH by about 0.1 units since 1750. They also estimate that the oceans will continue to absorb the excess carbon dioxide present in the atmosphere and that oceanic pH will fall by a total of about 0.5 units by the end of this century, bringing it down to about 7.7. This is still slightly basic – so we won't be creating a vast acid bath. But the pH scale is logarithmic, which means that even a decline of half of one unit will mean a several-fold increase in the concentration of hydrogen ions.

Plankton at risk

One of the victims of these extra hydrogen ions could be a type of phytoplankton called coccolithophores, one of the most abundant single-celled algae in the ocean. They are found in the upper, sunlit layers of the sea and play a vital ecological role. Coccolithophores produce a large proportion of the planet's oxygen, sequestering huge quantities of carbon and providing the primary food source for many of the ocean's animals. Coccolithophores use calcite, a form of calcium carbonate, to form tiny plates, or scales, on their exterior. Calcium carbonate starts to dissolve as pH declines: ocean acidification could therefore have a harmful effect on the abundance of coccolithophores and, consequently, on the health of the oceans and the planet.

Further ocean acidification could also be damaging for corals, such as those in the Great Barrier Reef. Corals are constructed with the skeletons of countless generations of small animals called anthozoans, which, in turn, are made largely of calcite. A reduction in calcite in the ocean could limit the formation of new corals, weaken existing corals and could also prompt coral bleaching.

An unsaturated ocean

As well as being highly soluble in even weak acids, calcium carbonate also dissolves at low temperatures and at high pressures. Scientists have discovered what they call a 'saturation horizon' in the ocean, above which the water is 'supersaturated' in carbonate ions (CO32-). This means that there is so much calcium carbonate in the water that organisms that use it (or other forms of carbonate) can flourish. Below this horizon, where the pressure is higher and the temperature lower, calcium carbonate tends to dissolve and is not so readily available to calcifying organisms. With the increasing oceanic absorption of carbon dioxide, the saturation horizon is starting to move upwards, shrinking the habitat of calcifying organisms such as coccolithophores and coral.

Cold water is naturally less saturated in carbonate ions, so Arctic and Antarctic waters will become less hospitable for the calcifying organisms more quickly. Cold-water corals, which often occur deep in the ocean, will also be highly vulnerable. In some parts of the world, the saturated zone is expected to completely disappear if the amount of carbon dioxide absorbed by the oceans continues to rise.

The long goodbye

The acidification of the ocean is not a short-term phenomenon; the ocean has a huge capacity to absorb more carbon dioxide and will continue to do so for hundreds and even thousands of years. The reversal of this process will also take thousands of years, if evidence from another era of high atmospheric carbon dioxide and oceanic acidity, called the Paleocene-Eocene Thermal Maxium (PETM), is a guide.

The PETM occurred about 55 million years ago, probably due to the release of thawing methane deposits trapped in seafloor sediments. Scientists drilled cores in the ocean floor to examine the sedimentary layers deposited during the PETM. They found that the acidification of the ocean eventually caused the calcium carbonate that had accumulated on the seafloor over millennia to dissolve. This reversed the acidification process and allowed the concentration of carbonate ions to again increase to a point where calcite and aragonite started to re-form in the surface layers. The process took about 100,000 years; in the meantime, marine biodiversity declined sharply.

Limiting the damage

Scientists are working hard to find ways of reducing the atmospheric concentrations of greenhouse gases. It would help if we could develop economically viable sources of energy that don't release greenhouse gases. Perhaps we could also sequester carbon dioxide already in the atmosphere, which means capturing and storing it in a place where it can't leak back into the atmosphere. Storage schemes that have been proposed include pumping carbon dioxide into old oil or gas wells (more-or-less returning it to where it came from) and speeding up the absorption of carbon dioxide by phytoplankton by 'fertilising' the ocean with iron (Box 1: Iron fertilisation of the oceans). Other possible methods of reducing the acidity of the ocean have been proposed including the dumping of chalk into the sea to neutralise the acid.

Climate mitigation efforts will probably include a combination of these and other measures. The global carbon dioxide experiment is well under way. How we minimise its dangers and deal with its effects will be an acid test for our species.

Related Nova topic:

Coral bleaching – will global warming kill the reefs?

Box 1: Iron fertilisation of the oceans

Ocean acidification is expected to make life difficult for phytoplankton such as coccolithophores and diatoms that use carbonate in their skeletal structures. But in a strange twist, some scientists believe that greatly increasing the growth of these organisms could help solve the problem of too much carbon dioxide in the atmosphere.

Phytoplankton are abundant in most parts of the ocean, but there are some 'dead spots' – in the deep equatorial waters of the Pacific, for example, as well as in the Gulf of Alaska and the Southern Ocean – where they are relatively scarce, even when plenty of sunlight and nutrients are available. Scientists debated the reason for this scarcity for many years. Some thought that it was simply predation by zooplankton that kept the phytoplankton in check and prevented them from blooming. Others thought it might be the lack of a micronutrient, iron.

In a series of experiments designed to test the latter hypothesis, scientists tipped a few tonnes of iron into the ocean. The phytoplankton loved it, increasing their natural abundance in the experiment areas by up to 30-fold. This extraordinary result led many to speculate that 'iron fertilisation' could help mitigate global warming. The idea would be to dump thousands of tonnes of iron into the phytoplankton-poor parts of the ocean, thus stimulating phytoplankton blooms. These would use carbon dioxide dissolved in seawater from the atmosphere to build their skeletal structures and then, when the organisms die, they would deposit carbon – in the form of calcium carbonate or silicon carbonate – in the lower reaches of the ocean, effectively removing it from the carbon cycle for hundreds or thousands of years.

Iron fertilisation is not just an idea: it happens naturally on a grand scale. Wind-blown dust has been a significant source of oceanic iron for millions of years. More dramatically, volcanic eruptions can deliver huge quantities of iron into the ocean in short periods, causing sudden pulses of phytoplanktonic growth. The eruption of Mt Pinatubo in the Philippines in 1991 is a modern example: it deposited an estimated 40,000 tonnes of iron dust into oceans worldwide, stimulating growth in phytoplankton that caused a measurable (although temporary) decline in the concentration of atmospheric carbon dioxide.

But the idea of artificial iron fertilisation is also controversial. Some scientists say it would not have a significant long-term impact on carbon dioxide levels. Others fear that such ecological engineering could have unintended consequences, such as the creation of toxic algal blooms, oxygen depletion in the deep ocean and increasing the rate of ocean acidification. Both advocates and opponents seem to agree, though, that artificial fertilisation, even on a grand scale, is no iron bullet for ending global warming.

Related sites


  • LearnNet (Royal Society of Chemistry, UK)

  • Learn: Atmospheric science explorers (The University Corporation for Atmospheric Research, USA)

  • Rising tides (Goddard Space Flight Center, USA)
    • Carbon cycle activity – students investigate the exchange of carbon dioxide between the atmosphere and the ocean’s surface.

  • Ecosystems at risk (Sydney Aquarium, Australia)

  • SeaWorld Adventure Parks (USA)
    • Growing coral – students get to grow crystals that form in a similar way to how coral polyps create their calcium carbonate cups.

  • The iron story (Monterey Bay Aquarium Research Institute, USA)
    • The biological pump – students learn about the processes involved in the biological pump, and how the availability of nutrients affect the amount of carbon dioxide that will be sequestered in the deep oceans.

  • Science upd8 (UK)

Further reading

Australian Academy of Science
August 2010
The Science of Climate change: Questions and Answers.
A document produced by the Australian Academy of Science that aims to summarise and clarify the current understanding of the science of climate change for non-specialist readers.

Australasian Science
September 2008, page 15
Corals take up acid
Reports on a study showing coral tissues are twice as acidic as previous estimates.

June 2008, page 10
Fish getting lost as oceans acidify
Proposes that ocean acidification may be affecting the sense of direction of fish.

May 2008, pages 31-33
The acid test (by Stephen Luntz)
Looks at the effect of ocean acidification on coral reefs.

November/December 2007, page 14
Iron seeding may not sink CO2
Looks into the idea of seeding the oceans with iron to combat global warming.

Australian Antarctic Magazine
Issue 10, 2006, pages 26-27
Ocean acidification: A newly recognised threat
Provides background information on ocean acidification.

12 November 2009
Ocean acidification impacts coastal rivers (by Holly Hight)
Looks at impact of ocean acidification on coastal ecosystems.

12 November 2008
Southern Ocean dangerously acidic (by Octavia Cade)
Reports that ocean acidification may reach a tipping point by as soon as 2030.

December 2008/January 2009, pages 77-83
Oceans of acid (by John Pickrell)
Explains the effects of ocean acidification on marine organisms.

23 October 2007
Oceans losing ability to absorb carbon
Reports on study which shows a drop in the absorption of atmospheric CO2 by the world's oceans.

Earthwatch Institute
Climate change: Mitigation – carbon capture and storage
Provides information on ecosystems that sequester carbon and act as carbon sinks.

No. 147, 2009, page 5
Unprecedented slowdown in coral growth
Reports on the effects of ocean acidification and warming on coral growth in the Great Barrier Reef.

No. 141, 2008, page 7
Concerns raised about ocean fertilisation for carbon credits
Highlights concerns raised over fertilising oceans to remove carbon dioxide.

3 June 2008
UN decision puts brakes on ocean fertilization (by Jeff Tollefson)
Reports on an agreement preventing major ocean fertilization projects until scientists understand the impacts.

New Scientist
2 February 2009
Acid oceans no laughing matter for clownfish (by Rachel Nowak)
Describes the effects of ocean acidification on clown fish.

5 September 2008
Climate change could stop corals fixing themselves (by Devin Powell)
Claims that coral populations living in waters low on calcium carbonate due to ocean acidification are less able to repair damage.

June 2008
Acidic 'champagne sea' nothing to celebrate for corals (by Catherine Brahic)
Describes the effect of reduced pH on oceanic life.

14 June 2008, page 7
Is ocean seeding dead in the water? (by Michael Reilly)
Reports that ocean seeding could increase levels of a toxic acid.

14 May 2008, page 16
A sprinkle of limestone could help oceans absorb CO2 (by Kate Ravilious)
Describes a strategy for reducing ocean acidification.

3 October 2007
Chemical 'sponge' could filter CO2 from the air (by Catherine Brahic)
Explores a way of extracting CO2 from the atmosphere using a simple chemical process.

29 September 2007, page 4
Gaia scientist endorses plan to halt climate change
Reports on the concept of pumping nutrient rich deep water from the oceans as a potential cure for global warming.

12 September 2007, pages 42-45
Can 'fertilising ' the ocean combat climate change? (by Emma Young)
Discusses the use of iron seeding of the oceans to combat climate change.

5 September 2007, page 22
Oceans not safe from safe rain
Reports on the impact of acid rain on the oceans.

22 August 2007, page 16
Fossil-fuel hangover may block ice ages (by Fred Pearce)
Investigates the suggestion that emissions from the burning of fossil fuels may prevent the onset of the next ice age.

16 May 2007
Climate myths: It's been far warmer in the past, what's the big deal? (by David Chandler)
Looks at the significance of the warmer periods in the Earth's history.

12 February 2007
CO2 being pushed deep into the oceans (by Catherine Brahic)
Reports on findings indicating that atmospheric carbon dioxide is being pushed deeper into the oceans.

6 December 2006
Warming oceans produce less phytoplankton (by Catherine Brahic)
Reports on study showing a persistent decrease in phytoplankton as the Earth's oceans warm up.

5 August 2006, pages 28-33
Ocean acidification: The other CO2 problem (by Caspar Henderson)
Provides a general overview to ocean acidification.

9 July 2005, page 15
Sea life in peril as oceans turn acid (by Rowan Hooper)
Reports on conclusion from the first review of studies on the effects of ocean acidification.

30 June 2005
Marine crisis looms over acidifying oceans (by Rowan Hooper)
Looks at the effects of increasing acidification of the oceans on marine life and possible ways of neutralising the acidity.

18 June 2005, page 19
Ancient glimpse of seas' bleak future (by Jeff Hecht)
Reveals the emission of gigatonnes of carbon dioxide into the atmosphere 55 million years ago led to rapid global warming.

16 February 2002, page 16
Don't rely on plankton to save the planet (by Nicola Jones)
Reports on model which shows applying iron to the ocean would be an inefficient method of removing atmospheric carbon dioxide.

17 October 2007
Acid oceans threaten corals
Looks at the threat to corals as the world's oceans become more acidic.

18 May 2007
Climate change weakens carbon sink
Reports on evidence of the Southern Ocean carbon sink weakening as a result of climate change.

6 April 2007
Reef 'at risk in climate change'
Explores the risks to the Great Barrier Reef from climate change.

Scientific American
30 August 2006
Anemic phytoplankton absorb less carbon than thought
Presents results from study which showed phytoplankton in the Pacific Ocean have low iron levels and soak up less CO2.

March 2006, pages 38-45
The dangers of ocean acidification (by Scott Doney)
Investigates the repercussions of ocean acidification on marine life.

Vol. 13, No. 1, 2007
Contains a number of articles on climate change and ocean acidification, including:
  • Impacts of climate change on the Southern Ocean
  • Salt and vinegar – a taste of climate change and ocean acidification
  • Climate change impacts on coral reef ecosystems

Useful sites

Antarctic Climate and Ecosystems Cooperative Research Centre (Australia)

Ocean uptake of carbon dioxide: Are the oceans acidifying? (Public Lectures, 7 July 2005, Australian Academy of Science)

Transcript of a lecture given by Dr Steve Widdicombe. Discusses the acidification of oceans.

Ocean acidification (Oceana)

Provides information on ocean acidification and a link to an extensive, clearly written report on the same topic.

The ocean acidification network

Contains information on the acidity of oceans, sequestration of carbon in the ocean and how ocean acidification affects marine life.

Australian Broadcasting Corporation

  • Oceans, carbon dioxide and acidity (Science Show, 28 February 2009)
    An interview discussing ocean acidification, measurement of ocean carbon dioxide and proposals to remove it through ocean fertilisation.

The iron hypothesis (Palomar College, USA)

Discusses the iron hypothesis proposed by John Martin.


diatoms. A common type of phytoplankton which have cell walls made of silica.

logarithmic scale. A scale of measurement in which an increase or decrease of one unit represents a ten-fold change in the quantity measured.

parts per million (ppm). This is a way of expressing very dilute concentrations of substances. Just as per cent means out of a hundred, so parts per million or ppm means out of a million. Therefore 500,000 ppm is the same as 50 per cent, because 500,000 is half of a million.

pH. The pH scale is used to measure the strength of acids and bases (or alkalis). The acid strength in the human stomach is about pH 2. Alkalis such as caustic soda and basic household cleaners have a pH of about 12 to 14. Neutral is pH 7, (ie, neither acidic or alkaline). The scale is logarithmic, so pH 4 is ten times as acidic as pH 5 and pH 2 is ten times as acidic as pH 3, and so on.

photosynthesis. The biochemical process in which some microorganisms use energy from light to synthesise carbohydrates from carbon dioxide and water. Photosynthesis can be shown as:

CO2 + H2O + energy® [CH2O] + O2

phytoplankton. Plankton that possess plant-like characteristics.

zooplankton. Plankton that possess animal-like characteristics.

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Posted July 2009.

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