The acid sea

American embassy, Ottawa

One frequently neglected consequence of rising global concentrations of carbon dioxide is increasingly acidic oceans (though it has been mentioned here before). Since the Industrial Revolution, the world ocean has absorbed about 118 billion tons of anthropogenic CO2: half of total human emissions. Every day, another 20-25 million tonnes are being absorbed.

Before the Industrial Revolution, oceanic pH was about 8.179. Now, it is at 8.104. By 2100, it is projected to be 7.824. Because pH is a logarithmic scale, that is a bigger change than it seems to be. At the projected 2100 concentration, the shells and skeletons of corals, molluscs, and phytoplankton with aragonite shells begin to dissolve within 48 hours. James Orr et al, writing in Nature provide many more details:

In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

The effect of more acidic oceans on aragonite is part of why the Stern Review projects that coral reef ecosystems will be “extensively and eventually irreversibly damaged” at less than 450 ppm CO2 equivalent and less than 2°C of warming. Given how critical coral reefs are to overall oceanic ecosystems – including key commercial fish species – this should be of concern to everyone.

It is very hard to project what the consequences of all this will be. As with so many other climatic phenomena, the net impact for human beings probably has to do with the relative strength of positive and negative feedbacks and the corresponding resilience of ecosystems. What is certain is that the only way to prevent acidification is to signficantly cut CO2 emissions.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

12 thoughts on “The acid sea”

  1. US set to violate its standards on CO2 emissions
    11:21 24 September 2007 news service
    Catherine Brahic

    The US may violate its own standards on water quality by refusing to limit emissions of carbon dioxide, suggests a new study modelling ocean acidification.

    “About one-third of the CO2 from fossil-fuel burning is absorbed by the world’s oceans,” explains Ken Caldeira at Stanford University in California, US, who led the study.

    The CO2 lowers the pH of the ocean’s surface, a phenomenon known as ocean acidification. This is predicted to have dramatic consequences on marine life by dissolving the shells of tiny organisms and corals.

    If governments do nothing to limit greenhouse-gas emissions, CO2 levels in the oceans will rise to a point where, by 2050, ocean acidification will reach a level considered to be industrial waste by the US’s own standards, found the study to be published on 25 September.

  2. November 18, 2007
    Ocean Assification

    Last week, I heard zoologist Chris Harley speak on how climate change will affect intertidal diversity along our rocky shores. There was a typo in his poster (“acification” instead of “acidification”) and he said he would like to officially coin assification for what we’re collectively doing to the oceans (climate change, pollution, overfishing, etc.). Well, today there is greater consensus for ocean acifidication and, therefore, ocean assification.

  3. Ocean Acidification

    The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO2 concentrations lead to further acidification. Projections based on SRES scenarios give a reduction in average global surface ocean pH of between 0.14 and 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 shellforming organisms (e.g. corals) and their dependent species.

    From the IPCC 4AR SPM

  4. The other problem with CO2- Ocean Acidification

    By WhySharksMatter on science

    Most people have heard about how rising CO2 levels are resulting in a changing global climate. Fewer have heard about the other consequence of rising CO2 levels- when the CO2 is absorbed into the oceans, it disassociates into carbonic acid. This alters the pH of our world’s oceans, and it’s called “Ocean Acidification”. This changing ocean chemistry has many important and devastating consequences.

    Many marine organisms rely on complex chemical interactions with the ocean for survival, and these processes will be more difficult (if not impossible) in a more acidic ocean. One organism threatened by ocean acidification is corals (which take calcium carbonate out of seawater to make coral reefs. These reefs serve as home for thousands of unique life forms, and make up a huge part of the world’s ecotourism business- and if corals can’t make this reefs, these reef residents (as well as SCUBA diving businesses) are in big trouble. Crabs and other crustaceans also rely on ocean chemistry to make their protective shells, and without their shells they won’t be able to survive. Perhaps most devastating of all is that a more acidic ocean will make it impossible for pteropods to make their protective shells. Pteropods, also known as sea butterflies or sea angels, are a plankton species that serves as the base of many food chains. Without them, many commercially important fish populations could collapse. There is some good news- the same steps that we are taking to fight global warming will also help fight ocean acidification, since it’s really just another symptom of the same problem.

  5. Ocean acidification is speeding up
    15 February 2010, by Sara Coelho

    Carbon dioxide released from fossil fuels and dissolved in the ocean is making seawater more acidic and causing trouble for marine life. Now a new model suggests that seawater is acidifying at a rate that exceeds anything seen on Earth over the past 65 million years. The speed of change may in fact be faster than what marine animals can cope with.

    Dr Andy Ridgwell and colleague Dr Daniela Schmidt, from the University of Bristol, developed a model to compare current predictions of ocean acidification with what happened during a greenhouse gas event 55 million years ago, called the Palaeocene-Eocene thermal maximum (PETM).

    During this event, which saw a 5-6°C increase in surface water temperature, the oceans acidified and a massive amount of carbonate rocks was dissolved as a consequence. All this happened over thousands of years. It may look like a lot of time, but on a geological scale it was very sudden.

    The tiny animals and plants that make up the plankton at the surface of the sea did not suffer much during the event, possibly because they migrated to cooler waters or because they had time to adapt to the new conditions. Even so, the Palaeocene-Eocene acidification event was severe enough to cause the extinction of many benthic foraminifers, tiny organisms that live at the bottom of the sea protected by calcium carbonate shells.

    The mass extinction has been linked to the high levels of carbon dioxide dissolved in seawater, because it’s difficult for the foraminifers to build their shells if the ocean is too acidic on the seafloor.

  6. At a recent meeting of the Geological Society of London that was devoted to thinking about the Anthropocene and its geological record, Toby Tyrrell of the University of Southampton pointed out that pale carbonate sediments—limestones, chalks and the like—cannot be laid down below what is called a “carbonate compensation depth”. And changes in chemistry brought about by the fossil-fuel carbon now accumulating in the ocean will raise the carbonate compensation depth, rather as a warmer atmosphere raises the snowline on mountains. Some ocean floors which are shallow enough for carbonates to precipitate out as sediment in current conditions will be out of the game when the compensation depth has risen, like ski resorts too low on a warming alp. New carbonates will no longer be laid down. Old ones will dissolve. This change in patterns of deep-ocean sedimentation will result in a curious, dark band of carbonate-free rock—rather like that which is seen in sediments from the Palaeocene-Eocene thermal maximum, an episode of severe greenhouse warming brought on by the release of pent-up carbon 56m years ago.

  7. The variable people most worry about is called omega. This is a number that describes how threatening acidification is to seashells and skeletons. Lots of these are made of calcium carbonate, which comes in two crystalline forms: calcite and aragonite. Many critters, especially reef-forming corals and free-swimming molluscs (and most molluscs are free-swimming as larvae), prefer aragonite for their shells and skeletons. Unfortunately, this is more sensitive to acidity than calcite is.

    An omega value for aragonite of one is the level of acidity where calcium carbonate dissolves out of the mineral as easily as it precipitates into it. In other words, the system is in equilibrium and shells made of aragonite will not tend to dissolve. Merely creeping above that value does not, however, get you out of the woods. Shell formation is an active process, and low omega values even above one make it hard. Corals, for example, require an omega value as high as three to grow their stony skeletons prolifically.

    As the map above shows, that could be a problem by 2100. Low omega values are spreading from the poles (whose colder waters dissolve carbon dioxide more easily) towards the tropics. The Monterey report suggests that the rate of erosion of reefs could outpace reef building by the middle of the century, and that all reef formation will cease by the end of it.

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