If you want to understand how ocean acidification might impact some marine creatures you need to do two things. First go to the seaside and find a seashell. Then go to a shop and buy a fizzy drink. Put the seashell in the fizzy drink and leave it for a few days. Then have a look and see how much is still there. You will see that it is starting to dissolve away.

A similar process is happening in the oceans today. Carbon dioxide in the atmosphere is being taken into the ocean. When carbon dioxide dissolves in water it forms carbonic acid. Fizzy drinks are carbonated – they have carbon dioxide bubbled into them and this makes them acidic. Carbon dioxide is taken up into cold waters more rapidly and so the process of ocean acidification affects the coldest seas, such as the Arctic Ocean, the most.

By continually adding more carbon dioxide, and increasing the amount of hydrogen ions, the ocean is becoming more acidic – the pH level is dropping. The pH level will continue to decrease into the future as long as carbon dioxide keeps being taken up by the oceans.

Ocean pH is regulated by a process called ‘buffering’. Hydrogen ions react with calcium carbonate (both limestone and chalk are types of calcium carbonate). Continued buffering relies on there being enough chalk in the water to cope with the increased levels of carbon dioxide being dissolved into the seas, but this is a slow process and happens over thousands of years. So the rate of buffering at the moment does not match the rate of carbon dioxide addition and the ocean buffering system is not able to prevent the rapid decrease in pH that we are seeing in the oceans today. Since the beginning of the Industrial Revolution, there has already been a 0.1 drop in pH level. Models predict that the pH level will continue to lower (become more acid) to 7.7 in the next 100 years and to 7.4 in 300 years.

This may seem like a small amount but the impact can be quite large. For example, your and my blood pH is kept constant by processes in our bodies. If this pH level changed by 0.1 in either direction, it would be time to call an ambulance and take you straight to hospital. All organisms need to regulate their internal pH and marine creatures are no exception. The difference is that many marine organisms are more dependent on the ocean to act as a pH regulator. It is also important for those organisms that have shells made of calcium carbonate – they find it difficult to maintain their shells, which start to dissolve as the pH decreases (just like in a fizzy drink, only much slower).

The Arctic acts as a bellwether for acid levels in our seas and their impact on the marine ecosystem. Acidification is thought to happen here faster than anywhere else.

Ice Base Scientist Dr Ceri Lewis, from the University of Exeter, explains: "Within only a few decades, an increase in ocean acidity may cause seawater to become corrosive to the carbonate shells of the smaller marine creatures that are so abundant in our marine ecosystems, with potentially serious consequences for both them and the larger marine fish and mammals that rely on them for food.

Edited from an original blog on the Catlin Arctic Survey website by Dr Helen Findlay, April 2011.