Extreme Science: Does Sea Ice Speed Up Ocean Acidification?
This is the third in a series of blog posts from scientists taking part in the third Catlin Arctic Survey.
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 — any brand will do. Put the seashell in the fizzy drink and leave it for a few days. Then have a look and see how much of the shell is still there.
You will see that it is starting to dissolve away.
A similar process is happening in the oceans today. Carbon dioxide (CO2) in the atmosphere is being absorbed by the oceans. When CO2 dissolves in water it forms carbonic acid. Fizzy drinks are also carbonated — they have CO2 bubbled into them, and this makes them acidic. In the oceans, cold waters absorb CO2 more rapidly, and so the process of ocean acidification affects the coldest seas, such as the Arctic Ocean, the most.
The carbonic acid in the oceans doesn’t stay as carbonic acid for very long. It quickly breaks down into bicarbonate (HCO3-) and a hydrogen ion (H+). Acidity is the measure of hydrogen ions in a liquid. So as the number of hydrogen ions increases we say it is becoming more acidic, and we communicate this using the pH scale.
On the pH scale, the oceans are actually basic — the current average pH level of the oceans is about 8.2 (compared to freshwater which has an average pH of about 7). By continually adding more CO2, and increasing the number 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 CO2 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 CO2 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 at which CO2 is being added, and so 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 the oceans' pH level. Models predict that the pH level will continue to lower (become more acidic) to 7.8 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, our 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 go straight to the 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. Regulating pH levels 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 faster here than anywhere else, but there is still a lot we don’t understand about how the sea ice and associated processes affect how CO2 is taken up by the Arctic Ocean. For example, scientists had assumed that the sea ice acted like a lid, which would stop CO2 from going into the ocean during winter, altering the acidification process.
But new information is telling us that it is not that simple. The research being carried out here at the Catlin Arctic Survey Ice Base is aimed at finding out more about the transfer of CO2 through sea ice, what this means in terms of ocean acidification and how acidification, or changes in these processes, might affect the organisms that live in and under the sea ice.
Helen Findlay has a doctorate in biological oceanography and works at Plymouth Marine Laboratory (PML) in the United Kingdom. She is currently one of a team of scientists working at the Catlin Ice Base off Ellef Ringnes Island on the edge of the Arctic Ocean north of Canada, where she is studying ocean acidification and the ability of organisms to cope with a changing environment.