Ocean acidification

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March 23, 2010, 7:27 pm
May 31, 2021, 9:24 am

Ocean acidification relates to a decrease in ocean pH as a result of the uptake of sulfur dioxide (SO2), nitrogen dioxide (NO2) and carbon dioxide (CO2) in the ocean. Sulfur dioxide and nitrogen dioxide are each about 100 times more powerful as an ocean acidification agent compared to carbon dioxide. Or, as Steding asserts: "Because carbonic acid is a relatively weak acid, the ability of carbon dioxide alone to generate true acid rain is very limited. Acid rain is caused by industrial emissions of sulfur dioxide and nitrogen oxides (which form much stronger acids when equilibrated in rainwater). (Steding, 2014)

Surface ocean pH is estimated to have decreased from approximately 8.25 to 8.10 between 1751 and 2021 and may reach 7.85 in 2100; note that all of these levels are basic, not acidic. Reduction of ocean pH also has an impact of increasing polar ice melt where there is direct contact between ice sheets and ocean, such as the Western Antarctic Ice Sheet. The term "ocean acidification" relates to the decrease in pH and does not imply that the pH of ocean surface waters will become acid (below 7.0) at any time.

Sulfur dioxide ocean acidification

Sulfur dioxide is emitted into the atmosphere from natural sources, pesticide use and combustion of fossil fuels. While North America and Europe have dramatically reduced man made emissions of SO2 in the last century, Asia continues a rapid rate of growth in these emissions, producing over half the world's SO2 emissions, or more than 60 million tons per year; India, China and Russia contribute the great majority of these emissions. Sulfur enters oceans and soil via direct deposition and also through acid rain. Western nations have greatly reduced air pollution emissions of sulfur dioxide in recent years; however, China, with its growing use of coal burning, is an expanding producer of sulfur dioxide into the global atmosphere. This is of notable concern, since manufacture of electric vehicle batteries is accomplished by considerable amounts of coal burning, and hence sulfur dioxide and carbon dioxide emissions.(Dai et al, 2019)

Oxides of nitrogen acidification

Nitrogen dioxide and certain other oxides of nitrogen enter the oceans via acid rain and deposition. NO2 causes and aggravates respiratory disease in mammals, including humans. Moreover, atmospheric NO2 forms acid rain, since atmosperic vapor combines with NO2 to form the strong acid, nitrous acid, which in turn cause ocean and freshwater acidification. The pH of acid rain caused by NO2 can be any value below 5.6; a value of 4.2 is considered common for acid rain generated by oxides of nitrogen or sulfur dioxide. Note that this acidity strength is roughly fifty time the acidity strength of pre industrial rain dominated by naturally occurring CO2 in the atmospere.

CO2 and carbonate chemistry

Hitimeseries2 med.jpg
The partial pressure of CO2 (pCO2) increases in the atmosphere (Atmospheric composition and structure) due to anthropogenic inputs of carbon dioxide through fossil fuel burning, deforestation and production of cement. China produces the greatest CO2 emissions of any country, in excess of 10000 million tons per year, and it is the country with the highest rate of growth of CO2 emissions; the EU and USA have emissions rates that are declining. CO2 has increased by 32% between 1880 and 2000 (280 vs. 370 µatm) leading to changes in the Earth's temperature and in the functioning of terrestrial ecosystems.

Penetration in the ocean

The world’s oceans currently absorb approximately 79 million tons of carbon dioxide (CO2) released into the atmosphere every day. Over the past 250 years, the world's oceans have absorbed about one third of the CO2 released due to anthropogenic activities.

Changes in the carbonate chemistry

The increase of pCO2 in the surface ocean affects the seawater carbonate system. It lowers the pH, decreases the availability of carbonate (Marine carbonate chemistry) (CO32-) ions and lowers the saturation state of the major shell-forming carbonate minerals (Table):

CO2 + CO32- + H2O → 2HCO3-

Carbonate chemistry of surface seawater. Total alkalinity, pCO2, salinity and temperature were fixed and used to derive all other parameters using the seacarb software and the constants of Merbach. pH is expressed on the total scale. TA was held constant at its pre-industrial value from the late 1800s onward.

Unit glacial preindustrial 1990 2065 2100
Temperature °C 13.7 14.7 15.7 16.7 17.7
Salinity - 35 34.3 34.3 34.3 34.3
Total alkalinity mol kg-1 2.356 10-6 2.302 10-6 2.302 10-6 2.302 10-6 2.302 10-6
CO2 partial pressure (seawater) µatm 200 280 360 560 706
[CO2] mol kg-1 7.796 10-6 1.063 10-5 1.326 10-5 2.002 10-5 2.452 10-5
[HCO3-] mol kg-1 1.714 10-3 1.787 10-3 1.851 10-3 1.961 10-3 2.006 10-3
[CO32-] mol kg-1 2.620 10-4 2.105 10-4 1.846 10-4 1.398 10-4 1.216 10-4
Dissolved inorganic carbon mol kg-1 1.984 10-3 2.008 10-3 2.049 10-3 2.121 10-3 2.152 10-3
pH - 8.31 8.18 8.09 7.93 7.84
[H+] - 4.940 10-9 6.577 10-9 8.101 10-9 1.181 10-8 1.446 10-8
Calcite saturation - 6.2 5.0 4.4 3.3 2.9
Aragonite saturation - 4.0 3.2 2.8 2.2 1.9

The values shown in the table are global values for ocean surface waters. Changes will be much more pronounced in areas such as the Southern Ocean, which will become undersaturated with respect to aragonite in 2050. Data collected at several time-series stations fully validate the above changes in the carbonate (Marine carbonate chemistry) chemistry]derived from thermodynamic calculations.

Any additional CO2 is reduces ocean pH, and also affects carbonate chemistry through the reduction of the carbonate ions, aragonite and calcite, which are used by many marine organisms to build their external skeletons and shells. If the current trends in CO2 emissions continue to increase due to human activities, by 2100, the pH of surface seawater could decrease by about 0..34 units from pre-industrial times. This change in the chemistry of the oceans is quantifiable for a given level of atmospheric NO2, SO2 and CO2.

Biological and biogeochemical consequences

Pteropod Limacina Helicina. Photo courtesy of Russ Hopcroft, University of Alaska, Fairbanks.
Biologists generally agree that an optimum pH for marine organisms is in the range of 6.5 to 9.0; thus, there is not a global concern for biological organism destabilization from any projected marine pH change. Although changes in the carbonate (Marine carbonate chemistry) chemistry are well known, the biological and biogeochemical consequences are much less well constrained for several reasons. First, very few processes and organisms have been investigated so far (research in this area only began in the late 1990s). Second, most experiments were carried out in the short-term (hours to weeks), effectively neglecting potential acclimation and adaptation by organisms. Third, the interaction between pCO2 and other parameters poised to change, such as temperature, concentration of nutrients and light, are essentially unknown.

It is not anticipated that oceanic primary production will be directly affected by these changes in carbonate chemistry because most primary producers use carbon concentrating mechanisms that rely on bicarbonate rather than CO2. Note, however, that primary production of some species is likely to be stimulated. It is now well established that shell or skeleton growth through calcium carbonate precipitation of most calcifiers will be greatly affected by changging ocean pH. Short-term experiments on corals, benthic macroalgae, planktonic algae and protists, mollusks and sea urchins have shown that calcification decreases by up to 57% at the pCO2 levels expected in 2100. Note, however, that some calcifiers either do not show any response to increasing pCO2 or exhibit a bell-shaped response curve with an optimum rate of calcification at pCO2 values close to current ones and rates that decrease at pCO2 values below and above the current values.

Most modeling studies have aimed at predicting the consequences of the decline in the rate of calcification on the global carbon cycle and its feedback on atmospheric (Atmospheric composition and structure) CO2. This is not an easy task because two opposite feedbacks are involved. First, calcification is a source of CO2:

Ca2+ + 2HCO3- → CaCO3 + CO2 + H2O
Pteropodpics1 med.jpg
hence a decline in calcification is a negative feedback on atmospheric CO2. The magnitude of this feedback is poorly known because future changes in calcification are poorly understood due to biological variability, very limited data on interaction between pCO2 and other parameters which will also change (e.g., temperature, nutrient and light), and the lack of information on potential remediation by acclimation processes by organisms or selection and evolution of resistant genotypes. Second, the CO2 generated by calcification is a function of pCO2. The calcification equation above is correct in freshwater (Freshwater biomes) but the current ration of CO2 generated per mole of CaCO3 precipitated is about 0.6 in 'standard' seawater and will increase as seawater pCO2 will increase.

Recent data suggest that elevated CO2 substantially increases nitrogen fixation by the cyanobacterium Trichodesmium, which could fundamentally alter the nitrogen cycle (Marine nitrogen cycle) and potentially drive some oceanic regimes towards phosphorus limitation.

Changes in carbonate (Marine carbonate chemistry) chemistry can affect many processes based on redox chemistry but such consequences have not been explored yet.

Source: NOAA

References

  1. Barcelos e Ramos,J., Biswas H., Schulz K., G, LaRoche J. & Riebesell U., 2007. Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium. Global Biogeochemical Cycles 21, GB2028. doi:10.1029/2006GB002898.
  2. Doug Steding ( Fact Check: Are Carbon Emissons Contributing to Acid Rain? Science, Law and the Environment.
  3. Caldeira K. & Wickett M. E., 2005. Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean. Journal of Geophysical Research- Oceans 110, C09S04. doi:10.1029/2004JC002671.
  4. Qiang Dai, Jarod Kelly, Linda Gaines and Michael Wang (2019) Life Cycle Analysis of Lithium-Ion Batteries for Automotive Applications. Systems Assessment Group, Energy Systems Division, Argonne National Laboratory, DuPage County, Argonne, IL 60439, USA. Batteries 2019, 5(2), 48; https://doi.org/10.3390/batteries5020048
  5. Houghton J. T. et al., 2002. Climate Change 2001: The Scientific Basis. Cambridge: Cambridge University Press.
  6. Hutchins D. A., Fu F.-X., Zhang Y., Warner M. E., Feng Y., Portune K., Bernhardt P. W. & Mulholland M. R., 2007. CO2 control of Trichodesmium N2 fixation, photosynthesis, growth rates, and elemental ratios: Implications for past, present, and future ocean biogeochemistry. Limnology and Oceanography 52:1293-1304.
  7. Jacobson M. Z., 2005. Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry. Journal of Geophysical Research- Atmosphere 110, D07302. doi:10.1029/2004JD00522.
  8. Berresheim, H.; Wine, P.H. and Davies D.D. (1995). "Sulfur in the Atmosphere". In Composition, Chemistry and Climate of the Atmosphere, ed. H.B. Singh. Van Nostrand Rheingold ISBN 0-442-01264-0
  9. US EPA, OAR (2016-07-06). "Basic Information about NO2". US EPA. Retrieved 2020-07-03.
  10. Kleypas J. A., Feely R. A., Fabry V. J., Langdon C., Sabine C. L. & Robbins L. L., 2006. Impacts of ocean acidification on coral reefs and other marine calcifiers: a guide for future research. 88 p. Boulder, Colorado: Institute for the Study of Society and Environment (ISSE) of the University Corporation for Atmospheric Research (UCAR).
  11. Langer G., Geisen M., Baumann K. H., Klas J., Riebesell U., Thoms S. & Young J. R., 2006. Species-specific responses of calcifying algae to changing seawater carbonate chemistry. Geochemistry, Geophysics, Geosystems 7, Q09006. doi:10.1029/2005GC001227.
  12. Orr J. C. et al., 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437:681-686.
  13. Sabine C. L. et al., 2004. The oceanic sink for anthropogenic CO2. Science 305(5682):367-371.
  14. Santana-Casiano J. M., González-Dávila M., Rueda M. J., Llinás O. & González-Dávila E. F., 2007. The interannual variability of oceanic CO2 parameters in the northeast Atlantic subtropical gyre at the ESTOC site. Global Biogeochemical Cycles 21, GB1015. doi:10.1029/2006GB002788.
  15. The Royal Society, 2005. Ocean acidification due to increasing atmospheric carbon dioxide. 60 p. London: The Royal Society.\
  16. Likens, G. E., T. J. Butler, and D. C. Buso. 2000. Long- and short-term changes in sulfate deposition: effects of the 1990 Clean Air Act Amendments. Biogeochemistry. (52)1:1-11.

External links

  1. Ocean acidification Ocean Acidification International Coordination Centre (OA-ICC)
  2. Impacts of ocean acidification on coral reefs and other marine calcifiers - a guide for future research
  3. The ocean acidification network
  4. EUR-Oceans fact sheet
  5. U.S. Environmental Protection Agency. What is Acid Rain?

Citation

Jean-Pierre Gattuso & C. Michael Hogan (2011, updated 2021). Ocean acidification. Encyclopedia of Earth. National Council for Science and Environment. Washington DC. Retrieved from http://editors.eol.org/eoearth/wiki/ocean_acidification