Ocean acidification

Furthermore, if calcium carbonate were precipitating in an inorganic ocean the carbonate forming reactions would be drawn to the right constantly forming hydrogen ions and decreasing the pH unless there were some mechanism consuming hydrogen ions at the surface. Thus, the pH of the oceans is falling in the short term. The oceans are becoming more acidic. The overall reaction is

For every mole of that dissolves in the ocean there is an increase by two equivalents of in the absence of a mitigating factors. The important factor to consider it the buffering capacity of the multiple equilibria of and . This effect is illustrated in Figure 1.

By Elizajans - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79625305

Figure 1 The uptake of CO₂ and the two acidic equilibria are shown. The figure also indicates one of the crucial effects of acidification, which is the loss of stability of the CaCO₃ shells of diatoms, forams and other photosynthetic organisms.

Sample calculation of ocean acidification

Calculate the change in pH if 50% of the atmospheric CO₂ emitted each year were absorbed by the ocean by the Henry’s law equilibrium for the next 30 years. You may assume that the change in CO₂ is 2 ppm and then use the mass of the atmosphere to calculate the amount of CO_2. The volume of the oceans is 1.347 x 10¹⁸ m³. Assume that the pH of the ocean today is pH = 8.1 and ignore buffering.

Answer: Since there are 6.5 x 10¹⁶ moles of CO₂ in the atmosphere, the amount of CO₂ is in 30 years at a rate of 2 ppm per year is 60 ppm of this amount or 3.9 x 10¹² moles. The result is the production of two moles of H⁺ for each mole of CO₂ absorbed from the atmosphere, which results in 7.8 x 10¹² moles. Given the mass of the ocean the total volume is 1.35 x 10²¹ L. Thus, the new [H⁺] = 5.78 x 10^-9 M. The current concentration of H⁺ at pH 8.1 is [H⁺] = 7.94 x 10^-9 . Thus, the total [H⁺] = 1.37 x 10^-8 M. This would give rise to a pH of 7.8. We can compare this crude calculation to the value given in Table 1, which is pH = 7.95 in the year 2050. The difference of 0.15 pH units is most likely due to the inclusion of buffering in the model used in the table.

By Karbonatsystem_Meerwasser_de.svg: User: BeArderivative Author: Meiyuchang

Public Domain, https://commons.wikimedia.org/w/index.php?curid=11698714

Figure 2 The buffering due to the amphoteric ion is shown. The blue swath represents the region of the pH of the current ocean the predicted change in pH due to CO₂ uptake.

Figure 2 represents the buffering in the ocean due to the presence of both and . As discussed above the concentration of is relatively high and the ocean is supersaturated in (aq) and . The reasons for this are first that riverine fluxes wash and from surface limestone throughout the world. The carbon compensation depth (CCD) causes to redissolve at a depth of ca. 5000 m, preventing deposits of limestone in the deep ocean. Both the CCD and the buffering are fortunate from the point of view maintaining ocean pH. If all of the were to precipitate it would immediately cause the pH of the ocean to fall below pH < 5.0 and cause the death of all life in the ocean. However, even with the buffering the pH of the ocean has been falling systematically since the beginning of the industrial revolution in 1800. The result has been a fall in the pH of the ocean pH ~ 8.2 at the dawn of the industrial revolution to less than pH < 8.1 today. The predicted changes will have pH ~ 8.0 or even less by 2050 and pH ~ 7.8 by 2100. pH ~ 7.8 is near a tipping point where the shells of diatoms and forams would dissolve. These organisms, spread throughout the ocean, produce approximately 40% of the O₂ we breathe. Needless to say the tipping point of losing ocean photosynthesis would be a major threat to many species on land including humans.

Table 1.The ocean pH from pre-industrial times to 2100 are provided based on existing data and modeling.

Average surface ocean pH
Time	pH	pH change relative to pre-industrial	Source	H^+ concentration change relative to pre-industrial
Pre-industrial (18th century)	8.179		analysed field
Recent past (1990s)	8.104	−0.075	field	+ 18.9%
Present levels	~8.069	−0.11	field	+ 28.8%
2050	7.949	−0.230	model	+ 69.8%
2100	7.824	−0.355	mode	+ 126.5%

References

1. Orr, James C.; et al. (2005). "Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms" Nature. 437: 681–686

2. Key, R. M.; Kozyr, A.; Sabine, C. L.; Lee, K.; Wanninkhof, R.; Bullister, J.; Feely, R. A.; Millero, F.; Mordy, C.; Peng, T.-H. (2004). "A global ocean carbon climatology: Results from GLODAP". Global Biogeochemical Cycles. 18 (4): GB4031.

3. Hall-Spencer, J. M.; Rodolfo-Metalpa, R.; Martin, S.; et al. (July 2008). "Volcanic carbon dioxide vents show ecosystem effects of ocean acidification". Nature. 454: 96-99