Ocean acidification: An experiment to try at home

Ocean acidification: An experiment to try at home

You are probably already aware of the concerns surrounding global warming, caused by the release of carbon dioxide (CO2) from burning fossil fuels. Rising temperatures threaten to cause changes in sea level, ocean circulation and even dissolved oxygen levels. But how do CO2 emissions affect the chemistry of our oceans?

It is thought that the oceans have absorbed up to half of the CO2 emitted by burning fossil fuels. When CO2 dissolves in water, it combines with water molecules to form carbonic acid. So the more CO2 the oceans absorb, the more acidic seawater will become. Organisms like gastropods, bivalves, corals and some plankton have shells or skeletons made of calcium carbonate, which is affected by acidity. See for yourself what happens when a shell is exposed to acid with this simple experiment:

  1. Take two clean glasses or jam jars (no lids!). Fill one with water and one with white vinegar. The vinegar is our acid.
  2. Find two similar pieces of seashell, e.g. from a mussel or cockle. Put one in water and one in vinegar.
  3. Observe what happens to the one in vinegar – what can you see?
  4. Leave the shells for 24 hours and then compare.

You should see that the shell in water hasn’t changed much, but the one in vinegar has started to erode and break down. The acidic vinegar has attacked the calcium carbonate shell, and caused it to start dissolving. The bubbles observed in step 3 consist of carbon dioxide, a by-product of this reaction. Why not Tweet a photo of your experiment? Please tag with #FLOceans. Here are my photos – I used limpet, cockle and abalone shells:

Effervescence (fizzing) due to carbon dioxide production
Effervescence (fizzing) due to carbon dioxide production
The shells exposed to acid on the right are severely corroded after 24 hours.
The shells exposed to acid (on the right) are severely corroded after 24 hours

Although the oceans won’t become as acidic as the vinegar in this experiment, only a small change will start to affect delicate organisms. We are already starting to see these changes, for example at the ALOHA station in the Pacific Ocean, just north of Hawaii. This graph shows how pH has decreased over the last 25 years at the station (pH decrease = acidity increase):

Not only does higher acidity erode existing calcium carbonate, but it also makes it difficult for organisms to produce more of this material. One study showed that this causes malformations in calcifying plankton. The top images here show normal development, and the bottom images show development under high dissolved CO2 conditions:

Image credit: Riebesell et al. (Nature 407(6802), 364-367, 2000)
Image credit: Riebesell et al. (Nature 407(6802), 364-367, 2000)

The global consequences of ocean acidification could be extremely serious – the destruction of coral reef habitats and modification of ecosystems may have severe implications for fisheries, aquaculture, tourism and coastal communities. Ultimately the only way to slow the rise in ocean acidity is to reduce the amount of CO2 in the atmosphere, either by cutting emissions or by removing it using controversial climate engineering.

For more information on ocean acidification, check out http://www.oceanacidification.org.uk/.

Reference:

Billé, R., et al. Taking action against ocean acidification: A review of management and policy options. Environ. Manage. 52(4), 761-779 (2013).

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