Can Our Oceans Survive the Acid Attack?

Atmospheric carbon dioxide levels are rising fast, global temperatures are increasing, sea levels are encroaching and now the oceans are becoming more acidic. But is it really all such doom and gloom?

As a scientist researching climate change, I am interested in putting everything into perspective, and lucky for us, there is so much work taking place in all fields of climate science at the moment that our knowledge is expanding faster than ever.

One of the most interesting and hotly debated questions at the moment is: how acidic will the oceans become and what effect will this have on the creatures that live within them?

The oceans began acidifying around the same time as the Industrial Revolution. As carbon dioxide was pumped into the atmosphere owing to the burning of fossil fuels, some of it diffused into the oceans.

Since most of the surface ocean has a lower concentration of carbon dioxide than the atmosphere, it acts like a sponge, dissolving the carbon from the air. Over two hundred years, the ocean as a whole has only lowered in pH by 0.1 of a unit. This seems small but the pH scale is logarithmic, which means that sea water is now actually 30 per cent more acidic than before the Industrial Revolution began.

Acidifying the ocean is a very similar chemical process to producing carbonated drinks - except the ocean does not become fizzy... I have a very vivid memory of my parents dropping one of my baby teeth into a glass of fizzy drink and watching it dissolve; though I cannot say it put me off drinking carbonated drinks, it certainly demonstrated their acidic nature.

The chemistry behind this process is fairly simple; when CO₂ dissolves it reacts with a water molecule to produce carbonic acid (H₂CO₃). This dissociates into a bicarbonate ion (HCO₃^-), and an acidic hydrogen ion (H⁺) which causes the water's acidity level to rise. Fortunately, the ocean happens to be loaded with plenty of other negative ions, which can help mop up any excess hydrogen ions - but only up to a point.

Just like teeth, shells are made of readily-dissolvable material, most often calcite or in a few cases, aragonite, which are both forms of calcium carbonate. So animals that live within shells are vulnerable to any change in acidity. But how vulnerable they are also depends on where they live.

We can divide the oceans into three main environments; shallow coastal habitats, polar oceans and the rest, simply called "open ocean", where the majority of life-forms are microscopic and inhabit only the very top layer of sea water.

The shallow shelf areas are hugely productive. Here we find most of the large calcifying organisms; corals, brittle stars, star fish, molluscs and sea urchins. They can be quite vulnerable to changes in their habitat as they do not have much space to adapt; they require lots of light and a narrow range in temperature, so sudden changes can really stress certain species.

Corals, in particular, are thought to be at risk because they make their structure out of aragonite, which is even easier to dissolve than calcite. A group from Columbia University, in Arizona, have predicted that some coral species will almost halve the amount they can calcify by the time CO₂ concentrations reach twice the pre-industrial revolution level; according to the Intergovernmental Panel on Climate Change (IPCC), this could be as early as 2045. Sea urchins and brittle starts are also at risk because the acidity affects the larval stage of their life cycle, although studies so far have found that some species are more at risk from rising temperatures than acidification.

However, there is a flip side to living near the coast. The natural acidity of these habitats varies quite a lot depending on the location. Species near estuaries and river mouths live happily in more acidic (lower pH) conditions than normal seawater, so these species are less likely to be affected by rising CO₂ levels. This could give them a competitive advantage and they may even thrive in the future.

Polar species are predicted to be hit first. This is because CO₂ dissolves most readily in cold water. You can observe this with fizzy drinks - less gas is released when you open a can that's been the fridge compared with one left out in the warm. So the polar oceans will become more acidic more quickly.

There are teams of scientists funded by a European initiative called EPOCA, who are currently doing experiments with whole ecosystems in the Arctic. They envelop a parcel of sea water, along with all its inhabitants, in a giant 17m long tube called a mesocosm. They can then change the CO₂ concentration, simulating higher atmospheric levels, while otherwise keeping the environment similar to the surrounding Arctic water. These experiments are still in their infancy, but will hopefully yield useful results and providing a more well-rounded view by virtue of assessing the responses of multiple species simultaneously.

The open ocean covers by far the largest area, but the inhabitants are mostly microscopic plants and animals. Many laboratories have been involved in culturing these tiny creatures under differing CO₂ conditions and, so far, the results are contrasting. First, because plants take up carbon dioxide and use it to photosynthesise, these species tend to flourish in elevated CO₂ conditions. But their hard-shelled animal counterparts, however, a group known as the coccolithophorids, may fare less well.

Recent experiments growing coccolithophorids demonstrate the need for such studies to replicate the natural world as closely as possible. For example, in one study seawater was acidified with dilute hydrochloric acid but plants reportedly failed to thrive. But another study used carbonic acid in place of hydrochloric, thereby simulating CO₂-driven acidification, and the plants grew even better than under normal conditions.

So the jury is still very much out as to how much open ocean calcifying plants will suffer from increasing atmospheric CO₂, and as with all habitats the effects will be species specific. One argument for coccolithophorid adaptation is they have a high degree of genetic diversity within each species, this may allow them to evolve quickly to changing environments.

A really important question is not just how each individual species will be affected, but whether ecosystems will be able to adapt? But this is a difficult question to answer when we still do not know that much at a species level. There are several groups using computer models to assess impacts on ecosystems. Their aim is not just to include environmental pressures like temperature and CO₂ change, but also human interaction, such as fishing. So far, these models have not been very good at assessing shelf areas, mainly because they vary so much, and have the highest biodiversity. But since these models rely on actual scientific data, the more experiments are conducted, the better the models become.

In summary, ocean acidification is a daunting problem we will have to face. It has occurred several times before in Earth's history, so not only can we use experiments to predict the outcome of the ocean, we can also learn from geological records which preserve past environmental conditions and species response.

It is extremely important that laboratory experiments replicate real environments, not only testing CO₂ changes, but also temperature rises, competition for nutrients and sea level change - and this is no small task. We are only now learning that some areas of the ocean and different species will be affected more than others, and with ongoing world wide science collaborations and experiments, we will hopefully be able to tackle this problem head on.