New IPCC Report (Part 5)

guest post by Steve Easterbrook

(5) Current rates of ocean acidification are unprecedented.

The IPCC report says:

The pH of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. […] It is virtually certain that the increased storage of carbon by the ocean will increase acidification in the future, continuing the observed trends of the past decades. […] Estimates of future atmospheric and oceanic carbon dioxide concentrations indicate that, by the end of this century, the average surface ocean pH could be lower than it has been for more than 50 million years.

(Fig SPM.7c) CMIP5 multi-model simulated time series from 1950 to 2100 for global mean ocean surface pH. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red). Black (grey shading) is the modelled historical evolution using historical reconstructed forcings

(Fig SPM.7c) CMIP5 multi-model simulated time series from 1950 to 2100 for global mean ocean surface pH. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red). Black (grey shading) is the modelled historical evolution using historical reconstructed forcings. [The numbers indicate the number of models used in each ensemble.]

Ocean acidification has sometimes been ignored in discussions about climate change, but it is a much simpler process, and is much easier to calculate (notice the uncertainty range on the graph above is much smaller than most of the other graphs). This graph shows the projected acidification in the best and worst case scenarios (RCP2.6 and RCP8.5). Recall that RCP8.5 is the “business as usual” future.

Note that this doesn’t mean the ocean will become acid. The ocean has always been slightly alkaline—well above the neutral value of pH 7. So “acidification” refers to a drop in pH, rather than a drop below pH 7. As this continues, the ocean becomes steadily less alkaline. Unfortunately, as the pH drops, the ocean stops being supersaturated for calcium carbonate. If it’s no longer supersaturated, anything made of calcium carbonate starts dissolving. Corals and shellfish can no longer form. If you kill these off, the entire ocean food chain is affected. Here’s what the IPCC report says:

Surface waters are projected to become seasonally corrosive to aragonite in parts of the Arctic and in some coastal upwelling systems within a decade, and in parts of the Southern Ocean within 1–3 decades in most scenarios. Aragonite, a less stable form of calcium carbonate, undersaturation becomes widespread in these regions at atmospheric CO2 levels of 500–600 ppm.

You can download all of Climate Change 2013: The Physical Science Basis here. Click below to read any part of this series:

  1. The warming is unequivocal.
  2. Humans caused the majority of it.
  3. The warming is largely irreversible.
  4. Most of the heat is going into the oceans.
  5. Current rates of ocean acidification are unprecedented.
  6. We have to choose which future we want very soon.
  7. To stay below 2°C of warming, the world must become carbon negative.
  8. To stay below 2°C of warming, most fossil fuels must stay buried in the ground.

Climate Change 2013: The Physical Science Basis is also available chapter by chapter here:

  1. Front Matter
  2. Summary for Policymakers
  3. Technical Summary
    1. Supplementary Material


  1. Introduction
  2. Observations: Atmosphere and Surface
    1. Supplementary Material
  3. Observations: Ocean
  4. Observations: Cryosphere
    1. Supplementary Material
  5. Information from Paleoclimate Archives
  6. Carbon and Other Biogeochemical Cycles
    1. Supplementary Material
  7. Clouds and Aerosols

    1. Supplementary Material
  8. Anthropogenic and Natural Radiative Forcing
    1. Supplementary Material
  9. Evaluation of Climate Models
  10. Detection and Attribution of Climate Change: from Global to Regional
    1. Supplementary Material
  11. Near-term Climate Change: Projections and Predictability
  12. Long-term Climate Change: Projections, Commitments and Irreversibility
  13. Sea Level Change
    1. Supplementary Material
  14. Climate Phenomena and their Relevance for Future Regional Climate Change
    1. Supplementary Material


  1. Annex I: Atlas of Global and Regional Climate Projections
    1. Supplementary Material: RCP2.6, RCP4.5, RCP6.0, RCP8.5
  2. Annex II: Climate System Scenario Tables
  3. Annex III: Glossary
  4. Annex IV: Acronyms
  5. Annex V: Contributors to the WGI Fifth Assessment Report
  6. Annex VI: Expert Reviewers of the WGI Fifth Assessment Report

8 Responses to New IPCC Report (Part 5)

  1. Bruce Smith says:

    Are there any mitigation strategies for ocean CO2 (or some related chemical form it turns into or causes) as distinct from atmospheric CO2? For example, are there any proposed schemes for “direct removal of CO2 from the ocean”?

    • John Baez says:

      I’ve heard someone argue that removing CO2 from water is much more efficient than removing it from the air. I’d be interested to know more about if this is true, and if so, why. Could it simply be because water in equilibrium with air holds much more CO2 per cubic centimeter than the air? Or is it also because lots of chemical reactions work better in water?

      Here is something relevant:

      • April Flowers, New CO2 removal technique produces green fuel, offsets ocean acidification, Redorbit, 29 May 2013.

      and the original paper:

      • Greg H. Raua, Susan A. Carroll, William L. Bourcier, Michael J. Singleton, Megan M. Smith, and Roger D. Aines, Direct electrolytic dissolution of silicate minerals for air CO2 mitigation and carbon-negative H2 production, Proc. Nat. Acad. Sci. 110 (2013), 10095–10100.

      Looks intriguing! Obviously you can’t make more energy this way than you put in, but still, they claim:

      Using nongrid or nonpeak renewable electricity, optimized systems at large scale might allow relatively high-capacity, energy-efficient (<300 kJ/mol of CO2 captured), and inexpensive (<$100 per tonne of CO2 mitigated) removal of excess air CO2 with production of carbon-negative H2.

      • nad says:

        yes looks like a interesting method.

        <300 kJ/mol of CO2 captured
        On a first glance it looks indeed considerable “efficient” like when compared to the removal of CO2 from air.
        It would be interesting to know in that context how much usuable energy per Mol is on average produced per CO2 production, here are some first calculations. Don’t know how reliable they are.

      • Bruce Smith says:

        Very interesting. Are proper incentives in place to encourage investment in such things if they would be good? For example, uniformly applied appropriate taxes on emissions (and negative taxes on mitigations) would be ideal, but don’t exist; but maybe some less ideal approximation (like some form of “carbon pollution credits” for mitigations, broadly defined) could realistically exist soon?

        • John Baez says:

          I sort of doubt proper incentives are in place anywhere. The European Union Emission Trading Scheme is fairly well-developed, but the Wikipedia article says:

          Currently, the EU does not allow CO2 credits under ETS to be obtained from sinks (e.g. reducing CO2 by planting trees). However, some governments and industry representatives lobby for their inclusion. The inclusion is currently opposed by NGOs as well as the EU commission itself, arguing that sinks are surrounded by too many scientific uncertainties over their permanence and that they have inferior long-term contribution to climate change compared to reducing emissions from industrial sources.

          This is potentially tragic; a better compromise would be to risk some small mistakes, but not allow them to grow into huge businesses until there’s more certainty.

          The problem is that there’s been some gaming of the trading scheme and regulators are struggling to avoid more.

      • nad says:

        The wollastonite experiments seem though to bind Calcium to CaSO4, which may have (?) an impact on the formation of CaCO3 and thus on corals and shellfish.

  2. Given that geologic sequestration of carbon dioxide at long scales is, I believe, principally mediated by it being bound up in carbonates and then subducted at trenches, could Steve or someone would be able to comment on how decreasing oceanic pH might impair this process? A related question — whether or not the oceans will “defizz” as temperature increases — was answered in Harte’s CONSIDER A SPHERICAL COW/CONSIDER A CYLINDRICAL COW pair of books (I forget which) — and the answer is not for a long time, and not until the oceans get really warm. Still another question is how much oceanic uptake of carbon dioxide from atmosphere is impaired by its increasing mean temperature. The chemistry for calculating that is also probably in Harte, but I don’t think he does it. Moreover, Pierrehumbert in his Section 8.4 explains a set of complicated interrelationships which I don’t completely understand, where carbonate ions in solution impede carbon dioxide uptake and, so, apparently, at some point, oceans refuse to accept more carbon dioxide.

    • John Baez says:

      I can’t answer your question, but wish I knew more about these issues, so thanks for the references.

      My own interest comes in part from trying to understand the CO2-temperature feedback loop that seems to be important for understanding the Earth’s glacial cycle. It’s obvious how higher atmospheric CO2 concentrations increase temperature: the greenhouse effect. But it seems that perhaps higher temperatures also increase atmospheric CO2, creating a feedback loop that amplifies the rather small Milankovitch cycles. And according to Didier Paillard this is not fully understood. The obvious mechanism—warmer liquids can hold less dissolved gas—seems to be insufficient. Or so he said.

      The interactions between carbonic acid, carbonate ions and bicarbonate ions are discussed here:

      Ocean acidification: details, Azimuth Library.

      Let me quote:

      Here is a plot showing the concentration of various ions as a function of pH:

      The key processes involved are these. Without carbon dioxide, these processes are continually occurring in water:

      H2O ↔ OH + H+

      H2O + H+ ↔ H3O+

      In the first process, a hydrogen nucleus H+, with one unit of positive charge, gets removed from one of the H2O molecules, leaving behind the hydroxide ion OH and a positively charge proton H+. In the second, this H+ gets re-attached to the other H2O molecule, which thereby becomes a hydronium ion, H3O+. As the diagrams indicate, for each of these reactions, a reverse reaction is also present

      With carbon dioxide present, these reactions are also important:

      CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

      HCO3 ↔ H+ + CO32-

      The first reaction is the formation of carbonic acid H2CO3 from water and carbon dioxide. The next reaction is the splitting of carbonic acid into a hydrogen ion and a negatively charged bicarbonate ion, HCO3. In the third reaction, the bicarbonate ion further ionizes into an H+ and a doubly negative carbonate ion CO32-.

      To see how these reactions lead to the graph above, see:

      • Stephen E. Bialkowski, Carbon dioxide and carbonic acid.

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