Ocean acidification: the other CO2 problem

Carbon dioxide emissions are altering the chemical balance of the oceans, as acidification threatens the marine ecosystems that provide livelihoods to millions around the world. Carol Turley explains.

Marine ecosystems make up approximately two thirds of the world’s surface, carry out about 50% of global primary production and support great biodiversity. Oceans also play an important role in transfer of heat around the planet and in determining weather systems and climate at sea and on land. Oceans are also key in the cycling and storage of the earth’s elements. For example, the oceans are the largest reservoir of carbon (other than that in rocks) on the planet, around 19 and 54 times greater than that in the terrestrial biosphere or in the atmosphere, respectively. Marine ecosystems also provide livelihoods for millions of people through fisheries, aquaculture, transport, tourisms and recreation. In essence, marine ecosystems play a large role in providing the earth’s life support system.

The same manmade carbon dioxide (CO2) that is the major greenhouse gas causing climate change is also altering the chemical balance of the oceans. This – the other half of the CO2 problem – has received little attention until recently, but it may turn out to be as serious as the more familiar part.

The surface waters of the oceans have already taken up over 500 gigatonnes (Gt) of CO2, about half of all that generated by burning fossil fuels and cement manufacturing since 1800. By absorbing all of this additional CO2 the oceans have buffered the effects of atmospheric climate change. But there is a cost to the oceans that has just recently emerged. Carbon dioxide reacts with seawater to form a weak acid (carbonic acid) and results in a greater seawater acidity (expressed as a reduction in pH). Surface ocean pH has already declined by about 0.1 since pre-industrial times. This may not sound much, but pH is measured on a logarithmic scale and measures the amount of hydrogen ions (H+) in the water; a 0.1 reduction, in fact, means that the amount of H+ has increased by 30%. If this trend continues and we were to burn all available fossil-fuel reserves, ocean pH would plummet and acidity would increase. pH would rise by as much as 0.4 units from its current level (around pH 8.1) by the year 2100 and 0.67 by 2300. For ocean chemistry to return to that of pre-industrial times, it would take tens of thousands of years while surface oceans gradually mix with deep waters and react with the calcium carbonate sediments, through their dissolution finally raising pH again.

Figure 1 Past (white diamonds, data from Pearson and Palmer, 2000) and contemporary variability of surface ocean pH (grey diamonds with dates). Future predictions are model derived values based on IPCC mean scenarios (Turley et al. 2006).

Such a reduction in pH is far greater than the annual variation that organisms currently experience and has not occurred for at least 420,000 years, probably for the past tens of millions of years. Marine organisms have therefore had a constant pH environment in which to evolve. About 55 million years ago ocean pH did decline to levels we can expect to see at 2300, which resulted in the extinction of many marine bottom dwelling calcifying (shell-producing) organisms, even though, in this case, it took thousands of years for the pH to fall. The current decline in ocean pH will happen far more rapidly, over the course of a couple of centuries or even decades. It is not surprising, then, that scientists are concerned with not only the level of decline in ocean pH, but also the speed at which it will happen.

An increase in seawater CO2 results in a decrease in both pH and in the amount of carbonate ions, which lowers the saturation of calcium carbonate minerals, making it more difficult for calcifying organisms to make calcium carbonate shells, skeletons and liths (small platelets). Currently most surface waters of the world’s oceans are saturated with calcium carbonate minerals. However, recent studies predicting future calcium carbonate saturation, using the Intergovernmental Panel on Climate Change (IPCC) “business-as-usual” scenario of fossil-fuel burning, show that the aragonite, a mineral from of calcium carbonate used by corals to make their hard skeletal reefs, will be so low in tropical waters where CO2 has doubled that coral calcification will be reduced by 20 to 60%, meaning the framework of the reefs may be weakened and more erodible. Warm water corals also suffer from another global warming impact: coral bleaching caused by rising sea surface temperatures. Our current understanding would suggest that corals could become rare on tropical and sub-tropical reefs by around 2050 because of raised sea temperature and declining aragonite saturation. Coral reef ecosystems harbour a huge number of species and are the most diverse of marine habitats. They are also important socio-economically to the tourism and fishing industries, and have a role in protecting shores from waves.

In polar and sub-polar waters, aragonite is predicted to become marginal or undersaturated (so low that it will become corrosive to shells and they will dissolve) by 2100. All of the Southern Ocean – the ocean around the Antarctic – and large parts of the Arctic will suffer from aragonite undersaturation and decreasing calcite saturation. Organisms that use aragonite and calcite (another calcium carbonate mineral) to make their shells such as pteropods (the sea butterfly) and shellfish, which form an important part of the food web, will have difficulty surviving in these waters. Whales and salmon and are amongst the animals that eat pteropods, while mammals like walruses feed on shellfish.  

The importance of deep cold water corals as a habitat and their substantial geographic distribution is only just emerging. So is concern over corals’ vulnerability to the shoaling of the aragonite saturation horizon: below this horizon aragonite is undersaturated, above it aragonite is saturated. The horizon is currently hundreds or even thousands of metres deep, but as the surface oceans take up more and more CO2, it will move upwards towards the sea surface. In high polar latitudes, it may even surface this century, meaning those waters will be undersaturated and corrosive to organisms such as deep cold water corals.

Figure 2 Satellite image of a coccolithophore bloom off southwest England. The milky whiteness of the waters is caused by the calcite liths or platelets made by the microscopic calcifying marine algal coccolithophore, Emiliania huxleyi, by extracting the calcium carbonate from the seawater. Millions of years ago, blooms like this shed their liths which fell to the seabed and in time created massive chalky deposits like the White Cliffs of Dover. Courtesy Peter Millar, Plymouth Marine Laboratory.

Figure 3 Scanning electron microscope image of Emiliania huxleyi (diameter 5 μm), showing the liths made from calcium carbonate. Courtesy Patrizia Ziveri and professor Harry Elderfield FRS, Department of Earth Science, University of Cambridge.

Microscopic plants called coccolithophores produce blooms that are so extensive they can be seen from space (Figure 2). They are currently thought to be the largest producers of calcite on the planet. When they die their calcium carbonate platelets (Figure 3), which are known as “liths”, rain down to the ocean floor where over geological time they are buried and can form vast structures such as the white cliffs of Dover, on the coast of southeast England. The liths also act as “ballast” making the organic part of the particles sink faster to the deep sea bed and thus help transfer organic carbon before it has time to be recycled and respired to CO2 in the surface of the ocean. Counter intuitively, the process of calcification itself results in the production of CO2. So, on one hand the blooming and sinking of the organic part of these microscopic plants draws down CO2, and on the other the laying down of calcium carbonate plates through calcification reduces CO2 draw down. Understanding the balance of these different components of this “biological pump” in a future high-CO2 ocean will be a challenge and important to understanding the exchange of carbon between the oceans, atmosphere and sediment. This is especially important as scientists have shown that the ability of one important coccolithophore species to form calcite liths was impaired when grown at CO2 concentrations expected by the end of the century, so much so that the calcification was reduced and liths deformed. The impact of this on the extent and direction of the biological pump is of concern, since this has a feedback into the earth’s carbon cycle.

The study of the impact of altered ocean chemistry on marine organisms is still in its infancy, and scientists are currently using controlled laboratory experiments or seawater and seabed mesocosms (large volume natural enclosures) dosed with future CO2 concentrations, as well as complex ecosystem models, to predict future impacts. Most calcifying organisms studied to date, representing the major marine calcifying groups (coccolithophores, pteropods, foraminifera, corals, calcareous macroalgae, mussels, oysters, echinoderms and crustacean), show reduced net calcification rates in response to elevated CO2. At Plymouth Marine Laboratory, mesocosms are being used to look at the impact of a high CO2 ocean on animals that live on the seabed and within the sediments and their biodiversity and biogeochemistry. Some of these animals, which burrow and plough through the sediments (such as starfish, sea urchins and shellfish), play a significant role in maintaining the biodiversity and important chemical feedback processes to the overlying seawater, and help sustain primary production. Some of them are economically important for shell fisheries and some are important food for commercial fish.

Other experiments reveal links between ocean acidification and changes to egg fertilisation, embryo development, larval development and settlement, as well as changes to chemical cues that enable organisms to interact with each other and their environment. Changes to populations and their interactions within communities could well influence the relative composition, productivity, timing, location and predominance of the major functional groups and thereby affect the rest of the food web.

Ocean acidification is now a mainstream scientific concern for the majority of international marine research organisations. As the research into the impacts of ocean acidification emerges there will undoubtedly be impacts and adaptations that have not been addressed here. Understanding these, predicting what future marine ecosystems will look like and determining the feedbacks to the functioning of the earth’s life-support system will undoubtedly be one of the biggest challenges for marine scientists in future decades.

Surface ocean acidification is happening now and will continue as humans put more CO2 into the atmosphere. It is happening at the same time as the world is warming. Organisms and ecosystems are going to have to deal with a number of major rapid global changes at once unless we urgently introduce effective ways to reduce CO2 emissions. These changes are happening on human time scales that some of us, our children and grandchildren will experience. Avoiding even more serious ocean acidification is a powerful additional argument to that of future dangerous climate change for the urgent reduction of global carbon emissions. It is for this reason that Plymouth Marine Laboratory has also worked to bring this issue to the attention of stakeholders and policy makers at the national and international level and to encourage national and international research programmes on ocean acidification. For instance, the IPCC Fourth Assessment Report in 2007 included ocean acidification for the first time, stating, “The progressive acidification of oceans due to increasing atmospheric carbon dioxide is expected to have negative impacts on marine shell-forming organisms (e.g., corals) and their dependent species (very high confidence)”. The European Union also responded rapidly funding the European Project on Ocean Acidification (EPOCA), the United Kingdom Natural Environment Research Council has just announced a decision to fund a large-scale research programme on ocean acidification, Germany is looking to do the same and the United States has a bill passing through Congress to establish an ocean acidification research and monitoring plan.

As concerns over climate change grow, increasing numbers of geo-engineering solutions have been proposed. However, they often do not take into account or resolve the issue of ocean acidification. For example, the addition of sulphur dioxide into the stratosphere to deflect some of the sun's energy, or ocean pumps of deep water rich in nutrients to increase productivity and draw down CO2 do not take this into account, nor look at potential deleterious impacts on the marine environment, such as with the addition of quicklime to the oceans to soak up CO2, iron or urea fertilisation to increase ocean productivity and draw down CO2.

Figure 4 Trajectories for surface ocean pH decrease calculated for different atmospheric CO2 concentration profiles leading to stabilization from 450-1000 ppm and used by the IPCC assessment of climate change (2007) with the pH “guard rail” recommended by the WBGU (2006) inserted as a red line. 

Currently, expert opinion is that the only method of reducing the impacts of ocean acidification on a global scale is through urgent and substantial reductions in anthropogenic CO2 emissions. A guard rail, beyond which we should not cross, of a decrease in pH of 0.2 below the pre-industrial average value in any larger ocean region (nor in the global mean), has been recommended by the German Advisory Council on Global Change in order to avoid aragonite undersaturation in surface waters, disruption of calcification of marine organisms and the resultant risk of fundamentally altering marine food webs. In terms of CO2 atmospheric concentration (Figure 4) this would be just above the 450 ppm stabilisation scenario. The urgency of substantial controls of global CO2 emissions caused by burning fossil fuels in order to reduce the impacts of future ocean acidification is therefore evident, and a convincing argument to add to that of avoiding dangerous climate change. 


Carol Turley is head of science at the Plymouth Marine Laboratory.