Ocean Acidification: The Other Carbon Problem

1. Ocean Acidification:The Other Carbon Problem Spring 2012, Lecture 12

2. Carbonic Acid • Carbon dioxide combines with water in the atmosphere to form carbonic acid • CO2 + H2O → H2CO3 • It can also dissolve in ocean water directly from the atmosphere • Carbonic acid is present in all carbonated beverages – cola, beer, champagne… • It is a weak acid, but is produced in enormous quantities • Rainwater is naturally acidic because it contains some carbonic acid

3. Acidity Scale • pH.swf • Acid is usually measured on a scale called the pH scale, which ranges from 0 (highly acidic) to 7 (neutral) to 14 (highly alkaline), as the figure shows • Natural rainwater has a pH of around 5.5-5.6, but this varies from place to place • pH is defined as: • pH = - log [H+] , where [H+] is the hydrogen ion concentration

4. Addition of Carbonic Acid to the Ocean • The increase in carbon dioxide levels in the atmosphere also means that more carbonic acid is added to the ocean • Carbonic acid is more soluble in cold water than warm, so more carbon dioxide is added to polar oceans • That is gradually reducing the pH of the ocean, with several unfavorable consequences

5. CO2 Enters the Ocean • Part 1: Half of the CO2 introduced into the atmosphere since the start of the Industrial Revolution has been absorbed by the ocean • Part 2: It combines to form carbonic acid

6. Carbonate Reduction • Part 3: Carbonic acid raises the hydrogen ion concentration and reduces the carbonate ion (CO32-) • Organisms need carbonate ion to form shell material, so this hinders that activity • pH values have been reduced by more than 0.1 pH units since the start of the industrial revolution • By the end of the 21st century, it is estimated pH will fall another 0.3-0.4 units

7. Continuing Damage • Carbon dioxide already in the atmosphere will ensure that this problem will continue to grow worse for at least a century and probably longer • As more carbon dioxide is added to the atmosphere, the effects will increase

8. Effects on Life • As waters become more acidic, organisms like plankton, coral, and mollusks will have trouble building and maintaining internal skeletons or exoskeletons • This will also affect the ocean food chain • Plankton are the base of the food chain so any reduction in planktonic abundance will deplete available food for almost all higher organisms

9. Global Variation in Marine pH • pH varies from region to region • Areas with lower pH values (more acidic) are often the result of the upwelling of cold, deep, CO2 rich waters • The west coast of Africa and, especially, of South America show this effect

10. Warm-Water Coral Reefs • Calcification rates of warm-water corals may drop by 50-60% • This will have adverse effects on the structure of the reef • Growth depends on the corals growing faster than the reef is eroded • Weak structures will suffer more erosion from storms and large waves

11. Cold-Water Coral Reefs • These reefs provide vital habit for commercially important fish species • They often serve as a “nursery” providing habitat and protection for young fish until they grow large enough to have a chance to survive in open waters • Conservative estimates are that 50-70% could find themselves under threat by the end of the century • Some scientists say the end will come much quicker

12. Plankton • The term “Plankton” covers a variety of species • Some are unaffected by increased acidity • Others, such as the coccolithophores (single-celled algae) show marked decreases in calcification rates when exposed to acidic waters • Plankton are also adversely affected by increasing water temperature

13. Plankton and Water Temperature • An article in the July 29, 2010 issue of Nature by Boyce et al. claimed that plankton biomass has been declining at a rate ∼ 1% of the global median per year • This work was based on satellite data, as well as observations of ocean transparency from Secchi dish observations dating from 1899

14. The Importance of Plankton • Plankton account for approximately half the production of organic matter on Earth

15. Plankton Biomass as a Climate Feedback • A reduction in plankton abundance thus affects the entire marine food chain • It also reduces the oceans ability to absorb carbon dioxide, since the plankton use carbon dioxide to create organic matter, and excrete oxygen as a bye-product • This creates another positive feedback for climate change

16. Geoengineering Climate Change Using Plankton • One group has proposed artificially bringing cold, nutrient rich waters to the surface in order to increase plankton production and reduce carbon dioxide in the atmosphere • The video on the next slide discusses this idea

17. “The Power of Plankton”

18. Invertebrates • Mollusks, such as mussels and clams, appear to suffer from thinning of their shells • Juveniles are more susceptible than adults • This could cause population depletion

19. Sea grasses • Some work indicates that sea grasses grow better in high CO2 waters • Grass offers a valuable feeding and spawning site for a variety of species • Some of these are commercially valuable fish • More research will be needed to determine overall affect on the marine food chain

20. Effect on the Carbon Cycle • Exchange of carbon dioxide between the atmosphere and the ocean has been a sink for CO2, reducing the amount of CO2 in the air • It is estimated that 50% of global CO2 currently enter the oceans • The oceans absorb carbon in two main ways - physically and biologically

21. Physical Absorption • CO2 dissolves into cold ocean water near the poles • It is carried to the deep ocean by sinking currents, • The deep ocean residence time is hundreds of years • Thermal mixing eventually returns the water to the surface, and the ocean emits carbon dioxide into the atmosphere in tropical regions • This natural carbon pump moves carbon from the atmosphere into the sea for storage

22. Biological Absorption • The biological absorption of CO2 involves phytoplankton • They use sunlight, water and dissolved CO2 to produce carbohydrates and oxygen • When the plankton, or the sea animals that eat the plankton die, they sink to the ocean floor • A small percentage of the carbon in the creatures' remains is eventually buried and stored in the sediment

23. Feedback • Ocean acidification will likely reduce plankton blooms in the ocean • This will result in less CO2 absorption in the ocean, with more CO2 remaining in the atmosphere, creating a positive feedback • This will warm the oceans further, possibly leading to even less absorption

24. Heat Transfer in a Cold Ocean • When the oceans are cold, the upper layers are relatively dense • It is easier to mix surficial layers with underlying waters • Mixing transfers both chemicals and heat • This means dissolved CO2 and heat will be more easily distributed through a large volume of ocean

25. Heat Transfer in a Warmer Ocean • When the ocean surface warms, it becomes less dense • The density difference between surficial and deeper waters is greater • This impedes mixing, which means dissolved CO2 and heat will be trapped in the surface layer • The ocean will gradually become stratified

26. Thermal Stratification • If the ocean is warm enough, it may begin to behave like many lakes • Layers of different temperature will form, creating a thermally stratified ocean • If the temperature differences are sufficient, thermal stratification can be stable over long periods • Stratification inhibits mixing, and may finally cut off mixing entirely

27. Effects of Stratification • Lack of mixing prevents upwelling of cold, nutrient rich waters, which will lower the biologic productivity of the ocean • Deeper waters will have less oxygen, and portion may be anoixic • Stratification will inhibit CO2 absorption from the atmosphere even further, leading to greater warming

28. Mixing Times for Heat • When the earth is cooling, the oceans mix easily, and heat, or lack thereof, is quickly distributed through the ocean • When the earth is warming, the oceans will mix with increasing difficulty, and heat is concentrated in the surface layers

29. Ocean Acidification Video • Dr. Graham Phillips of the Australian Broadcasting Corporation (ABC) interviews Dr. Will Howard and Dr. Charlie Vernon.

30. NOAA Ocean Acidification • Dr. Jane Lubchenco has been the administrator of NOAA since March 20, 2009 • Her specialty is marine ecology