Acid test for the seas
This topic is sponsored by the Australian Government Department of Climate Change.
The basic facts on ocean acidification.
Chemists have known for a long time that a beaker of water sitting in a lab will absorb carbon dioxide from the air and turn acidic. Would it happen at a larger scale? If we greatly increased the concentration of carbon dioxide in the world's atmosphere, for example, would the oceans become a vast acid bath? What would be the ecological effects? Over the next century or so, we are going to find out.
Increasing atmospheric carbon dioxide
For more than two hundred years, the human race has been releasing large quantities of carbon dioxide and other greenhouse gases into the atmosphere. It has been doing this in two main ways: by burning fossil fuels, such as coal, oil and natural gas, and by clearing and burning vegetation, such as forests.
Carbon dioxide, methane, water vapour, nitrous oxide and ozone are known as greenhouse gases because they trap heat in the atmosphere, like glass does in a greenhouse; in this way they help warm the Earth. From 1750 to 2007, the concentration of atmospheric carbon dioxide increased from 278 to 383 parts per million, or by about 38 per cent; with 22 per cent occurring in the past 50 years. This might not seem much, but most climate-change scientists agree that it has been enough to heat up the Earth by an average of about 0.7 ºC. This also might not seem like much, but if the concentration of greenhouse gases in the atmosphere continues to increase, average surface temperature of the Earth could rise by as much as 6 ºC by 2100. Life in our already-sunburnt country would become very sticky indeed.
Not all the carbon dioxide released into the atmosphere stays there; some of it – about a third of total human-induced emissions – has been absorbed by vegetation during photosynthesis and a similar amount has been soaked up by the ocean. We're lucky that it has, or global warming would be happening much more quickly than it is.
The oceans are naturally alkaline, or basic, with a pH of about 8.2. When carbon dioxide dissolves in sea water it forms carbonic acid (H2CO3), which releases hydrogen ions (H+), lowering the pH and making it more acidic. Scientists estimate that the additional carbon dioxide in the atmosphere and the subsequent absorption of some of this by the oceans has lowered oceanic pH by about 0.1 units since 1750. They also estimate that the oceans will continue to absorb the excess carbon dioxide present in the atmosphere and that oceanic pH will fall by a total of about 0.5 units by the end of this century, bringing it down to about 7.7. This is still slightly basic – so we won't be creating a vast acid bath. But the pH scale is logarithmic, which means that even a decline of half of one unit will mean a several-fold increase in the concentration of hydrogen ions.
Plankton at risk
One of the victims of these extra hydrogen ions could be a type of phytoplankton called coccolithophores, one of the most abundant single-celled algae in the ocean. They are found in the upper, sunlit layers of the sea and play a vital ecological role. Coccolithophores produce a large proportion of the planet's oxygen, sequestering huge quantities of carbon and providing the primary food source for many of the ocean's animals. Coccolithophores use calcite, a form of calcium carbonate, to form tiny plates, or scales, on their exterior. Calcium carbonate starts to dissolve as pH declines: ocean acidification could therefore have a harmful effect on the abundance of coccolithophores and, consequently, on the health of the oceans and the planet.
Further ocean acidification could also be damaging for corals, such as those in the Great Barrier Reef. Corals are constructed with the skeletons of countless generations of small animals called anthozoans, which, in turn, are made largely of calcite, or another form of calcium carbonate, called aragonite. Aragonite is less stable than calcite, and so will be even more susceptible to lower oceanic pH. Ocean acidification could limit the formation of new corals, weaken existing corals and could also exacerbate the problems associated with coral bleaching.
An unsaturated ocean
As well as being highly soluble in even weak acids, calcium carbonate also dissolves at low temperatures and at high pressures. Scientists have discovered what they call a 'saturation horizon' in the ocean, above which the water is 'supersaturated' in carbonate ions (CO32-). This means that there is so much calcium carbonate in the water that organisms that use it (or other forms of carbonate) can flourish. Below this horizon, where the pressure is higher and the temperature lower, calcium carbonate tends to dissolve and is not so readily available to calcifying organisms. With the increasing oceanic absorption of carbon dioxide, the saturation horizon is starting to move upwards, shrinking the habitat of calcifying organisms such as coccolithophores and coral.
Cold water is naturally less saturated in carbonate ions, so Arctic and Antarctic waters will become less hospitable for the calcifying organisms more quickly. Cold-water corals, which often occur deep in the ocean, will also be highly vulnerable. In some parts of the world, the saturated zone is expected to completely disappear if the amount of carbon dioxide absorbed by the oceans continues to rise.
The long goodbye
The acidification of the ocean is not a short-term phenomenon; the ocean has a huge capacity to absorb more carbon dioxide and will continue to do so for hundreds and even thousands of years. The reversal of this process will also take thousands of years, if evidence from another era of high atmospheric carbon dioxide and oceanic acidity, called the Paleocene-Eocene Thermal Maxium (PETM), is a guide.
The PETM occurred about 55 million years ago, probably due to the release of thawing methane deposits trapped in seafloor sediments. Scientists drilled cores in the ocean floor to examine the sedimentary layers deposited during the PETM. They found that the acidification of the ocean eventually caused the calcium carbonate that had accumulated on the seafloor over millennia to dissolve. This reversed the acidification process and allowed the concentration of carbonate ions to again increase to a point where calcite and aragonite started to re-form in the surface layers. The process took about 100,000 years; in the meantime, marine biodiversity declined sharply.
Limiting the damage
Scientists are working hard to find ways of reducing the atmospheric concentrations of greenhouse gases. It would help if we could develop economically viable sources of energy that don't release greenhouse gases. Perhaps we could also sequester carbon dioxide already in the atmosphere, which means capturing and storing it in a place where it can't leak back into the atmosphere. Storage schemes that have been proposed include pumping carbon dioxide into old oil or gas wells (more-or-less returning it to where it came from) and speeding up the absorption of carbon dioxide by phytoplankton by 'fertilising' the ocean with iron (Box 1: Iron fertilisation of the oceans). Other possible methods of reducing the acidity of the ocean have been proposed including the dumping of chalk into the sea to neutralise the acid.
Climate mitigation efforts will probably include a combination of these and other measures. The global carbon dioxide experiment is well under way. How we minimise its dangers and deal with its effects will be an acid test for our species.
Posted January 2008.