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Ocean Acidification

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In addition to climate change, another concern about increasing atmospheric CO2 is its potential impact on the chemistry of the world’s oceans. Higher CO2 concentrations cause ocean waters to become more acidic. In a more acidic ocean, calcium carbonate, the foundation of the shells and skeletons of many aquatic organisms, starts to dissolve.

In the past 5-10 years, scientists have begun to demonstrate changes in ocean chemistry and to observe the impacts of chemical changes on marine plants and animals. To better understand this phenomenon, let’s briefly look at the global carbon cycle and the carbonate system of seawater.

The Global Carbon Cycle

The world’s carbon is spread throughout several different “pools”. Some is located in the Earth’s atmosphere as CO2. Other pools of carbon are found in the Earth’s biosphere (all living plants and animals), as dead biomass (remnants of life, such as logs and plant litter on land and dissolved organic compounds in soil and in seawater), as the seawater carbonate system (defined later in this article), and as solid carbonate rocks and organic matter in the solid Earth.

Except for the latter (solid Earth), carbon moves fairly easily between its different pools. Plants, for example, take up carbon (usually as CO2) from the air or water through the process of photosynthesis. In return, respiratory activities of biota (plants, animals, and microbes) release CO2 back into the environment. CO2 is also exchanged between air and water where they are in contact with each other. These “mobile” carbon pools and their carbon contents are listed in Table 1. The seawater carbonate system is by far the largest pool, dwarfing all the others.

Pool Quantity (Gt)
Atmospheric CO2
720
Seawater carbonate system
37,400
Biosphere on land
800
Dead biomass on land
1,200
Biosphere in sea
2
Dead biomass in sea
1,000

Table 1. Pools of mobile carbon in the Earth system, listed in units of gigatons (Gt) where 1 Gt = 1015 grams (1015 = 1 million billion). There are a number of estimates of these pools with slight variations; the ones given here are from Falkowski et al, 2000 (see References section for information).

The uptake and release of carbon by plants and animals (called fluxes) are on the order of 100 Gt per year. That is, both uptake and release by land biomass of about 120 Gt and uptake and release by marine biosphere of about 90 Gt each year (source of these fluxes, Schlessinger, 1997).

These uptakes and releases are fairly closely balanced, and it can be said that the natural global carbon cycle is in equilibrium. Anthropogenic (human-caused) carbon emissions to the atmosphere are on the order of 6 Gt per year; some of this is taken up by the land and sea, and about 50% stays in the atmosphere (see the “Increasing Atmospheric CO2” section). Much of the carbon that is taken up goes into the carbonate system of seawater.

The Carbonate System of Seawater and pH

Atmospheric gases are in equilibrium with surface waters of the Earth and hence there is exchange across the air-water interface. CO2 also reacts with seawater, becoming part of the carbonate system (also called the dissolved inorganic carbon, DIC pool) and assuming one of three chemical forms: dissolved CO2, bicarbonate ion, and carbonate ion. The three species are in equilibrium with each other in chemical reactions that chemists call “dissociation” reactions.

As discussed above, CO2 also reacts with the biosphere. The overall result is that CO2 from the atmosphere can exchange in and out of water, in and out of the biosphere, and undergo exchange reactions within the DIC pool. For example, when algae take up CO2 from the water during photosynthesis, more CO2 can be supplied from dissociation of bicarbonate ion and from atmospheric exchange. In a reverse direction, CO2 produced by biological respiration can go into the DIC pool and/or contribute to exchange from water to the atmosphere.

An integral part of the carbonate system is the pH (acid-base) buffer of seawater. pH is a measure of a solution’s acidity or alkalinity. Represented by a scale of 0 to 14, a pH less than 7 is acidic, and one greater than 7 is basic. pH is measured on a logarithmic scale, so a pH change of 0.1 indicates a 10% change in acidity or alkalinity.

If there are no significant changes in the global carbon cycle, the pH of water will be maintained through equilibrium. However, if there is a major change, such as an increase in atmospheric CO2, the seawater DIC pool with increase and the pH will drop (water will be more acidic). Studies now show that this is indeed happening. The pH of the world’s ocean has dropped about 0.1 pH units over the past several decades. With unabated CO2 emissions, scientists estimate that the pH will drop another 0.3 to 0.5 pH units by the year 2100.

Based on the increase of atmospheric CO2 that has already occurred and on future increases, scientists have modeled future seawater chemistry. They have used knowledge of ocean currents and mixing, so that projections can be made for surface and deep waters. Although it is fairly complicated, the illustration in Figure 1 has been displayed in many places to illustrate the modeling of future pH changes. The upper panel in the figure shows anthropogenic (from human activity) carbon emissions; they are modeled to reach a maximum by 2100 and then decrease. The middle panel shows atmospheric CO2 concentrations; which would peak in 2300 at about 1900 ppm (contrasted to the present day level of 380 ppm). The x-axis (horizontal) shows time, starting before the Industrial Era (1750) and then extending into the future until the year 3000. The bottom panel of the figure shows the pH in ocean waters from the surface to 4.5 km depth (4500 meters, about 15,000 feet) for the same time period.

Ocean Acidification Figure 1

Figure 1. Modeled changes in ocean pH from prior to the Industrial Era into the future, lower panel; note depth scale on the y-axis (vertical). The upper two panels respectively show the history of anthropogenic CO2 emissions and atmospheric CO2 concentration over the same time scale. The surface ocean pH decrease will reach its maximum magnitude of -0.77 units by 2250 and will drop thereafter due to ocean mixing with a stabilized atmospheric concentration. With normal ocean mixing, there is little drop in deep ocean pH until about 2500. From Caldiera and Wickett, 2003 (see References section for information).

Thus, recent observations and theory are consistent in showing that increasing atmospheric CO2 has caused a small, but significant, increase in the largest mobile carbon pool of the world ocean, the DIC. In conjunction with this, a small drop in pH has been seen in the ocean and predictions indicate a larger pH drop in the immediate future with projected continual increase in carbon emissions.

Solid Carbonates and Biological Calcification

While the seawater DIC pool is the largest “mobile” pool of the carbon cycle, solid carbonate rocks are a much larger pool (60,000,000 Gt vs 37,400 Gt for the DIC pool), but one that is less “mobile”. Unlike the seawater DIC pool, most of this solid pool is far removed from surface Earth reactions.

Most solid carbonates resulted from biological activities of organisms that make calcium carbonate skeletal materials. These marine “calcifying” organisms include microscopic unicellular micro-organisms (both plant and animals), small planktonic mollusks, and benthic mollusks (such as snails, clams and oysters), and corals and coralline algae of tropical and subtropical reefs. Over millions of years, thick deposits of carbonate rocks accumulated and became buried deep below the surface of the Earth. Ancient veins of these deposits can often be seen in mountains where geological uplifting has occurred.

Carbonate rock continues to be produced today in shallow ocean waters. However, geochemical models predict that increased atmospheric CO2 and the resultant ocean acidification will lead to a situation where the chemical stability of calcium carbonate biominerals will change. Combining experiments with chemical reactions in the laboratory with models, it is possible to make reasonable estimates of future chemical reactions in the ocean.

Figure 2 uses the concept of “calcium carbonate saturation state” to illustrate impacts of increased CO2 on coral reefs. High saturation state values indicate optimal conditions for coral reef survival; low numbers indicate poor conditions. In the research paper from which Figure 2 was taken, the authors estimated saturation states for different atmospheric CO2 concentrations (corresponding with various time periods, including pre-Industrial Era, present day, and future projections). The figure shows just two frames from the original publication; one evaluates the pre-Industrial period conditions and one projects conditions in the year 2070.

Ocean Acidification Fig 2a

Ocean Acidification Figure 2e

Figure 2. The upper panel shows calculated carbonate saturation state for pre-industrial times (atmospheric CO2 = 280 ppm). Under those conditions, all Pacific and Caribbean regions are in an optimal state for carbonate rock production. The lower panel shows calculated carbonate saturation state for 2070 (CO2 = 517 ppm). Under those conditions, most tropical reef areas are in a marginal state; a small area of Caribbean is in an adequate state. Figure after Guinotte et al, 2003 (see References section for information).

This modeled projection indicates that with increased atmospheric CO2, ocean acidification will lead to a situation where calcium carbonate biominerals will no longer be stable in much of the subtropical and tropical ocean areas where coral reefs are prominent today. In addition to other threats which are causing a demise of corals (e.g. pollution, coral bleaching, reef destruction due to development), ocean acidification may make it virtually impossible for corals and other coralline plant and animals to synthesize their carbonate skeletal materials.

While these are projections based on models, experimental measurements made by marine chemists and ecologists over the past few years do show that calcium carbonate production in the ocean is declining. In addition, experiments with increased atmospheric CO2 confirm that the lower carbonate saturation state will greatly decrease oceanic carbonate production.

What the Past Tells Us

To emphasize the projected future carbonate situation, paleoceanographic evidence indicates that there was a massive carbonate disappearance in geological history. This occurred in the Paleocene-Eocene Thermal Maximum (PETM), 55 million years ago. Sediment cores show that there was an abrupt event where no carbonates survived in the sediment record while carbonates are abundant directly below and above (earlier and later periods respectively). The record indicates that the carbonate disappearance was sudden, occurring in less than 10,000 years and that it recovered naturally in about 100,000 years. The exact cause of the PETM is not well explained but it is thought that there was a relatively sudden massive release of methane gas which contributed to global warming and that the methane was oxidized to CO2 causing ocean acidification.

An excellent article about ocean acidification has been written for the lay public by Elizabeth Kolbert in the New Yorker. This article can be found on line at:

http://www.newyorker.com/fact/content/articles/061120fa_fact3

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References for Background and Further Reading on Ocean Acidification

Caldeira, K., and Wickett, M.E. 2003. Anthropogenic carbon and ocean pH. Nature, 425: 365-368.

Doney, S. C. 2006. The dangers of ocean acidification. Scientific American. March 2006. pp. 58-65.

Falkowski, P., Scholes, P.J., Boyle, E. et al. 2000. The global carbon cycle: A test of our knowledge of Earth as a system. Science 290: 291-296.

Guinotte, J.M., Buddemeier, R.W., and Kleypas, J.A. 2003. Future coral reef habitat marginality: temporal and spatial effects of climate change in the Pacific basin. Coral Reefs 22: 551-558.

Kleypas J. A., Buddemeier R. W., Archer D., Gattuso J. P., Langdon C. and Opdyke, B. N. 1999. Geochemical consequences of increased atmospheric carbon on coral reefs. Science 284: 118-120.

Kolbert, E. 2006. The darkening sea. The New Yorker. November 20, 2006. 66-75.

Sabine, C. L., Feely, R. A., Gruber, N., et al. 2004. The oceanic sink for anthropogenic CO2. Science 305: 367-371.

Schlessinger, W.H. 1997. Biogeochemistry: An Analysis of Global Change, 2nd Edition. Academic Press (San Diego)

Zachos, J.C., Rohl, U., Schellenberg, S.A. et al. 2005. Rapid acidification of the ocean during the Paleocene-Eocene Thermal Maximum. Science 308: 1611-1616.


Prepared by Jonathan H. Sharp with assistance from Ferris Webster, Joseph Farrell, John Wehmiller, Ron Ohrel and Douglas White.

     
Updated 02/13/2007


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