|2011-01-10 19:46:13||Ocean Acidification Isn't Serious|
What the Science Says:
The current debate on the connection between CO2 emissions and climate change has largely overlooked an independent and equally serious problem, the increasing acidity of our oceans. Last December, the respected journal “Oceanography” published projections (see graphic below) for this rising acidity, measured by falling pH [i], through to the end of the century [ii].
CO2 in the atmosphere has increased from 278 ppm in pre-industrial times to 390 ppm today. During this time, the amount of CO2 dissolved in the ocean has risen by more than 30%, decreasing the pH of the ocean by 0.11 units. As with CO2 and global warming, there is a lag between cause and effect. That means we are yet to see the worst of the problem. According to the Australian Antarctic Division of our Department of the Environment, “even if all carbon emissions stopped today, we are committed to a further drop of 0.1 to 0.2 pH units” [iii]. However, if CO2 is allowed to rise along a business as usual trajectory, they are concerned that pH will “fall by 0.5 pH units by 2100, a 320% increase in acidity”.
The close relationship between CO2 in the atmosphere, CO2 dissolved in the ocean, and the effect of the latter in falling pH, is illustrated by the graph [iv] below:
The chemistry of ocean acidification is well documented by McElroy [v]. CO2 dissolves in water to form carbonic acid. (It is worth noting that carbonic acid is what eats out limestone caves from our mountains.) In the oceans, carbonic acid releases hydrogen ions (H+), reducing pH, and bicarbonate ions (HCO3-).
CO2 + H2O <=> H+ + HCO3- (1)
The additional hydrogen ions released by carbonic acid bind to carbonate ions (CO32-) to form additional HCO3-.
H+ + CO32- <=> HCO3- (2)
Combining equations (1) and (2), and removing H+ from each side, we have:
CO2 + CO32- + H2O <=> 2 HCO3- (3)
“Since this is a chemical equilibrium, Le Chatlier’s principal states that a perturbation, by say the addition of CO2, will cause the equilibrium to shift in such a way as to minimize the perturbation. In this case, it moves to the right. The concentration of CO2 goes up, while the concentration of CO32- goes down. The concentration of HCO3- goes up a bit, but there is so much HCO3- that the relative change in HCO3- is smaller than the changes are for CO2 and CO32-. It works out in the end that CO2 and CO32- are very nearly inversely related to each other, as if CO2 times CO32- equaled a constant” [vi].
This relationship is presented graphically in the diagram at right. The blue column shows the fall in pH since pre-industrial times. Note that more acidic conditions caused by rising dissolved CO2 correlates with falling CO32-.
The reason these changes are of such concern is that corals and other marine creatures build their exoskeletons from limestone (calcium carbonate) by the following reaction:
Ca2+ + CO32- <=> CaCO3 (4)
The two main forms of calcium carbonate used by marine creatures are aragonite and calcite. Decreasing the amount of carbonate ions in the water therefore makes it harder for both aragonite users (corals, shellfish, pteropods and heteropods) and calcite users (foraminifera and coccolithophore algae) to build their exoskeletons.
The implications of all of this are frightening. For corals to absorb aragonite from seawater, the latter needs to be saturated in this mineral. Now a report from NOAA scientists found large quantities of water undersaturated in aragonite are already upwelling close to the Pacific continental shelf from Vancouver to northern California [vii]. Although the study only dealt with the area, the authors suggest that other shelf areas may be experiencing similar effects.
It is often said that a picture is worth a thousand words. In the Southern Ocean, there is grave evidence of harm to foraminifera. Even though these creatures use calcite, which is less soluble than aragonite, there are already clear signs of physical damage.
According to Dr Will Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, shells of one species (Globigerina Bulloides) are 30 to 35 percent thinner than shells formed prior to the industrial period [viii]. The photo above left shows a pre-industrial exoskeleton of this species obtained from sea-floor sediment. The photo above right shows a exoskeleton of a live specimen of the same species obtained from the water column in the same area in 2007. These stunning images were obtained by Dr Howard using an electron microscope. (An interview with Dr Howard was broadcast on the Catalyst television program). [ix] These and creatures like them are at the base of an ocean food chain, and they are already seriously damaged. If they are lost, it is not just biodiversity we are loosing, but our food supply as well.
For corals like those in Australia’s Great Barrier Reef, the outlook is grim. They are threatened with destruction on two fronts, both caused by CO2 emissions. Not only do increased ocean temperatures bleach coral by forcing them to expel the algae which supplies them with energy (see photo below) [x], but increased ocean CO2 reduces the availability of aragonite from which reefs are made.
The geological record provides further grounds for concern. To see where we are headed, let's look back to periods when pH fell to the levels projected for the end of the 21st Century. There have been several periods where pulses of CO2 have been injected into the atmosphere, from volcanic activity or melting of methane hydrates. One well known example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred around 55 million years ago. During this event, global temperatures increased by over 5°C over a time frame less than 10,000 years. This coincided with a massive release of carbon dioxide into the atmosphere, which led to ocean acidification. This change caused a series of biological responses, including the mass extinction of benthic foraminifera.
Looking further back, there are other examples of mass-extinctions coinciding with global warming and increases in atmospheric carbon dioxide. Examination of the mass extinction that occured 251 million years ago during the end-Permian find that the patterns of mortality are consistent with the physiological effects of elevated CO2 concentrations (along with the effects of global warming). 205 million years ago at the Triassic–Jurassic boundary, a sudden rise in the levels of atmospheric CO2 coincided with a major suppression of carbonate sedimentation, very likely related to ocean acidification. A similar situation occurred 65 million years ago during the Cretaceous–Tertiary extinction event. Most of the planktonic calcifying species became rare or disappeared.
Future acidification depends on how much CO2 humans emit over the 21st century. By the year 2100, various projections indicate that the oceans will have acidified by a further 0.3 to 0.4 pH units, more than many organisms like corals can stand. This will create conditions not seen on Earth for at least 40 million years.
It is time to wake up. Our planet is dying. I urge you to find the time to view a 20 minute documentary on the problem of ocean acidification produced by the international Natural Resource Defence Council. Simply go to www.acidtestmovie.com.
References and Notes
[i] pH is a measure of the acidity or alkalinity of a solution. It uses a negative logarithmic scale where a decrease of 1.0 units represents a 10-fold increase in acidity. In their natural state prior to industrialization, the oceans were slightly alkaline with a pH of 8.2 (see reference iii). Pure water has a pH of 7.0.
[ii] Feely R., Doney S., Cooley S. (2009). Present Conditions and Future Changes in a High-CO2 World. Oceanography 22, 36-47
[iii] Australian Antarctic Division, Ocean Acidification and the Southern Ocean, available at www.aad.gov.au/default.asp?casid=33583
[iv] Feely, Doney and Cooley, op. cit, using Mauna Loa data from the US National Oceanic and Atmospheric Administration and Aloha data from the University of Hawaii.
[v] Michael B. McElroy, The atmospheric environment: effects of human activity, Princeton University Press 2002, p. 148
[vi] RealClimate, The Acid Ocean – the Other Problem with CO2 Emission
[vii] Feely RA, Sabine CL, Hernandez-Ayon JM, Ianson D, Hales B (June 2008). Evidence for upwelling of corrosive "acidified" water onto the continental shelf. Science 320 (5882): 1490–2, available at http://www.sciencemag.org/content/320/5882/1490
[viii] Inter Press Service, Acid Oceans Altering Marine Life, available at http://ipsnews.net/news.asp?idnews=46055
[ix] Australian Broadcasting Corporation, Ocean Acidification – The Big Global Warming Story, downloadable at http://www.abc.net.au/catalyst/stories/s2029333.htm
[x] Great Barrier Reef Marine Park Authority, What is Coral Bleaching?, available at http://www.gbrmpa.gov.au/corp_site/key_issues/climate_change/climate_change_and_the_great_barrier_reef/what_is_coral_bleaching
A very good article.
May I suggest an insert between-
"Most of the planktonic calcifying species became rare or disappeared."
I suggest a brief note about the effects on algal blooms and bloom timings, which could affect the entire global marine food chain. That means us!
"Comparison of global bloom maps from remote sensing suggests another possible response to global change. Despite the intrinsic decrease of Ωcalcite and Ωaragonite in the cold high latitude waters, blooms of E. huxleyi appear to be moving northwards towards/into the Arctic Ocean, with satellite images showing new bloom areas in the eastern Bering Sea and the Barents Sea (Tyrrell and Merico, 2004)."
The source of the above is an excellent CO2 themed resource generally:
...earlier blooms may not sync up with the natural cycles of marine species which depend on them. For example, the peak of the bloom may occur while the creatures are still in their egg or larval life stages, and the organic matter produced may not be useable to them. The resulting mismatch in timing may explain the annual variability of fish stocks in the region.
"The spring bloom provides a major source of food for zooplankton, fish and bottom-dwelling animals," says Mati Kahru, lead author of the study. "The advancement of the bloom time may have consequences for the Arctic ecosystem." The concern for the researchers is that earlier blooms may spread to other parts of the globe and throw off marine ecosystems elsewhere. Food chains around the world could be affected.