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Ocean acidification

Ocean acidification

Idea

As the concentration of carbon dioxide in the atmosphere increases, it is making the oceans more acidic, in a phenomenon known as ocean acidification. This in turn can hurt calcareous organisms such as shellfish and coral reefs. For more, see:

Here is a graph of carbon dioxide concentration in the water, and the pH of the water, in the ocean at Station Aloha near Hawaii:

Seasonal variations are evident: there is more carbon dioxide in the atmosphere in winter, when Northern Hemisphere plants are photosynthesizing less. But there is an overall trend toward lower pH, meaning more acidic ocean water. 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. Thus, ‘acidification’ refers to a drop in pH, rather than a drop below pH 7.

The http://www.climatechange2013.org/eport) put out by the IPCC in 2014 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.

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. For aragonite, a less stable form of calcium carbonate, undersaturation becomes widespread in these regions at atmospheric CO2 levels of 500–600 ppm.

Here is a figure from a draft of the IPCC AR5 report, discussed on the Azimuth Blog by Steve Easterbrook:

This is a 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 (best case, in blue) and RCP8.5 (“business as usual”, in red). Black (grey shading) is the modelled historical evolution using historical reconstructed forcings. The numbers indicate the number of models used in each ensemble.

Details

Apparently the ocean is more acidic now than any time in the last 650,000 years. According to the Wikipedia entry on ocean acidification, the basic numbers are as follows:

Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104, a change of −0.075 on the logarithmic pH scale which corresponds to an increase of 18.9% in H+ (acid) concentration. By the first decade of the 21st century however, the net change in ocean pH levels relative pre-industrial level was about -0.11, representing an increase of some 30% in “acidity” (ion concentration) in the world’s oceans.

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.

Measurement of ocean pH

There are three points frequently used by those claiming ocean acidification is not happening, along with some vague commentary:

  1. Measurements of historical pH seem excessively precise.

    This appears to be on the assumption that measurements were made at the time, rather than being back-inferred from ratios of 11B{}^11 B in deposited material. Is the accuracy of this technique confirmed in the literature anywhere?

  2. Ocean pH vaies greatly geographically, diurnally and historically.

    It’s not logically impossible for a system which exhibits fluctuations to nonetheless be sensitive to changes in the mean, particularly if they are rapid. Is there a reference that confirms that mean pH changes matter even in the presence of fluctuations? The historical graphs used on this “The Ocean Acidification Fiction” webpage show signficant local minima, eg, in the 1930s. Are these correct and if so, is there an accepted explanation?

  3. Some recent papers/press releases don’t distinguish clearly between measurements and model predictions of ocean pH.

    It’s always in the eye of the beholder if a paper states what it’s doing in loud enough terms to prevent the reader (or onwards reporter) getting the wrong impression. I don’t know if this criticism is generally valid.

Points 1 and 2 seem analogous to similar points that are raised for CO 2CO_2, and presumably an oceanographer/chemist (which I’m not) could rate their validity and context.

I’ve seen some additional claims that the correlation between decreasing pH and “bad things happening” isn’t there, but let’s figure out the pH issues first. – David Tweed

Here is a diagram that shows the changes in ocean acidity from pre-industrial times up to 2000:

Effects

In addition to its effect on coral reefs, which is discussed in another article here, increased ocean acidity will also impact other sea life:

  • Jacqueline Ruttiman, Sick seas, Nature 442 (2006), 978-980.

For example, coccolithophores are tiny algae and other sea creatures that have calcium carbonate shells:

While they’re hard to see and you might never have heard of them, there are lots of them, so they’re important both for creating oxygen and as food for other ocean life. A quote from the above article:

In a fjord in southwest Norway, Riebesell has set up an outdoor laboratory consisting of a raft with what look like giant milk cartons moored to it. The containers, known as ‘mesocosms’, are 50-litre vessels filled with coccolithophores—photosynthesizing plankton, or phytoplankton, with carbonate coverings. Riebesell immerses the coccolithophores into tanks that are aerated with the projected levels of carbon dioxide in the next 50 and 100 years. He calls them “the oceans of the future”. The coccolithophores’ outer casings—tiny hubcaps known as coccoliths—are made of the carbonate mineral calcite. Riebesell found that exposing coccolithophores to three times the present-day atmospheric level of carbon dioxide caused nearly half of their protective coating to disintegrate. Such changes don’t bode well for one of the ocean’s most abundant types of phytoplankton bad for the food webs they sustain. By aggregating on the surface of waste from the upper levels of the ocean (mainly fish droppings), the coccoliths help it to sink down to the seabed communities, which recycle its nutrients into the ocean. Weakening the coccoliths could have knock-on effects on nutrient cycling.

Also:

With more attention on the problem, a new possibility has raised its head. Ocean acidification might not just run in parallel with global warming—it could amplify it. The chalky coccolithophores, when blooming, lighten the surface of the oceans, which means more sunlight is reflected into space. Reduce their number and even if other phytoplankton take their place, that lightness will be gone. Coccolithophores are also responsible for many of the clouds over oceans. They produce a lot of dimethylsulphide, which accounts for much of the aerosolized sulphate in the atmosphere above the oceans. Sulphate particles act as ‘seeds’ around which cloud droplets grow. Remove them, and you could remove a significant fraction of the world’s clouds, warming the planet yet further.

References

  • Ocean acidification, Wikipedia.

  • P. Goodwin, R. G. Williams, A. Ridgwell and M. J. Follows, Climate sensitivity to the carbon cycle modulated by past and future changes in ocean chemistry, Nature Geoscience 2 (2009), 145–150. doi:10.1038/ngeo416

  • A. Ridgwell and D. N. Schmidt, Past constraints on the vulnerability of marine calcifiers to massive carbon dioxide release, Nature Geoscience 3 (2010), 196–200. doi:10.1038/ngeo755

  • C. Rollion-Bard, D. Blamart, J. Trebosc, G. Tricot, A. Mussi and J.-P. Cuif, Boron isotopes as pH proxy: a new look at boron speciation in deep-sea corals using 11^11B MAS NMR and EELS. Geochimica et Cosmochimica Acta 75 (2011) 1003.

Abstract: Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)3 and tetrahedral borate ion B(OH)4-. One of the key assumption in the use of boron isotopic compositions of carbonates as pH proxy is that only borate ions, B(OH)4-, are incorporated into the carbonate. Here we investigate the speciation of boron in deep-sea coral microstuctures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (11B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordination species, but in different proportions depending on the coral microstructure, i.e. centres of calcification versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportions of B(OH)3 and B(OH)4- determined by NMR and our previous ion microprobe boron isotope measurements, we propose a new equation for the relation between seawater pH and boron isotopic composition in deep-sea corals.

Apparently this is “an information outlet on ocean acidification provided by EPOCA, the European Project on Ocean Acidification”.

category: oceans