Ocean Acidification


 Ocean Acidification – The ‘Other’ Carbon Dioxide challenge…

Sara Smith

 Images Rowena Mynott


We’ve all heard about global warming and climate change, it can be quite a complex issue.  The more we know the more there is to discover! Scientists tell us that our planet is warming at quite an alarming rate. There are others who are quite vocal sceptics who debate the science unwilling to accept that it’s something we should be worried about. Whether you agree or not, what is not debatable is that since the industrial revolution, when we started burning fossil fuels, we humans have released massive amounts of carbon dioxide into the atmosphere. Aside from creating that warm blanket of gas in our atmosphere, emitted carbon is threatening the Earth with the greater, until recently unrealised, problem of ocean acidification.

Let’s take it back to basic high school science. All gases will dissolve into water.

As carbon dioxide is released and levels increase in the atmosphere, some of this carbon dioxide is dissolved into the water of our oceans. This absorption continues as the ocean attempts to regain the equilibrium of carbon in different forms over the globe. By absorbing this carbon, the oceans have removed much of the potential depth of the carbon dioxide blanket we have created for our atmosphere.

So far the oceans have absorbed a third of the anthropogenic (human produced) carbon emitted since the industrial revolution (525 billion tonnes). On average, our oceans have removed one tonne of carbon per person every year from our atmosphere and two thirds of this carbon has been absorbed by the Southern Ocean alone.  Our oceans are still absorbing 22 million tonnes of carbon dioxide every single day.

Some inhabitants of our oceans use this carbon to produce calcium carbonate shells (molluscs and crustaceans) and hard structures (such as coral and limestone). Plants and phytoplankton use the carbon dioxide in photosynthesis to produce their own energy and oxygen (phytoplankton are the greatest consumers of carbon dioxide producing 50% of the world’s oxygen through photosynthesis). Carbon can also be stored deep in the ocean in the form of waste as dead plants and animals sink to the depths carrying carbon with them. Over time, this dead organic material is compressed under layers becoming what we call fossil fuels. The human population’s huge appetite for this fuel and the process of extracting it from the Earth has disrupted the natural carbon cycle, releasing more into the atmosphere.

By doing this however, we’ve basically managed to change the chemistry of our oceans.

Prior to the industrial revolution, the ocean had a carbon dioxide concentration of 280 parts per million (ppm). This increased to 385 ppm in 2010 which has led to a drop in the pH* of our oceans of 0.1.

It is estimated that by 2050 the concentration of carbon dioxide in the oceans will sit at over 500ppm and by 2100, at over 800ppm, in turn causing a drop in pH of up to 0.5.

A low pH represents an acidic environment whereas a high pH represents a more alkaline environment. Since the last ice age, our oceans have remained at a stable pH of 8.2; we have now managed to reduce that to 8.1. This may not sound like much, but as pH is measured on a logarithmic scale (like earthquakes with the Richter scale) it actually represents a 30% increase in hydrogen ions. The predicted 2100 drop to a pH of 7.7 equates to a doubling of hydrogen ions in the oceans.

As the oceans become more acidic, the ability for animals to produce calcium carbonate is reduced. As it becomes more difficult to calcify, more energy is required for this calcification. In turn the productivity of phytoplankton (the basis of all food chains) is reduced and fewer organisms reproduce.

Acidic oceans will also potentially dissolve calcium carbonate structures, not only the coral reefs and the calcium carbonate sinks in our deep oceans but also calcium carbonate structures of the plankton.

All marine organisms alive today have evolved in the previously stable pH range. Life processes such as growth, metabolism and reproduction occur easiest within a small pH range. Once this pH is changed, each of these processes become more difficult, requiring more energy as it is more stressful for the marine organisms.

As these organisms have to spend more energy just to survive, less is available to reproduce or even to photosynthesize. The increased stress may make these species unable to tolerate this change, thus threatening their existence.

Ocean Acidification as a topic has only come to light in scientific circles in the last ten years; it is a very new concept to researchers. Of the recent research that has been conducted, additional problems have also been observed:


×        Fish larvae will often react to an increase in acidity, it obscures their senses and some species are actually drawn to predators rather than instinctively being more wary.

×        Heavy metals become more soluble in the oceans and can therefore increase in the tissues of marine organisms

×        Additional problems have been observed in research on a number of species including metabolic changes, decreased immunity, decreased reproductive success and even decreased gene expression

×        Coral reefs are unable to produce their calcium carbonate skeleton and the change in pH is highly detrimental to the survival of the tiny zooxanthellae (organisms that have a symbiotic relationship with corals and provide 98% of the coral’s food)

×        Animals in the ocean that use calcium carbonate as their structural exoskeleton (clams, snails, oysters, scallops, sea stars, urchins, crabs, prawns, lobsters and of course coral reefs) have all displayed reduced growth and reduced reproductive success in more acidic conditions. Most important of these are plankton, the basis of all our food chains

×        Some species, such as krill (at the lower level of the food chain in the ocean and the primary food for our beloved humpback whales), have displayed very limited tolerance to the predicted levels of carbon, with less than 0.1% survival rate.


As mentioned above, phytoplankton produces 50% of all oxygen on the planet via photosynthesis. Their shells are made up of calcium carbonate and they utilise the carbon in the water column to produce their skeletons. At the ‘normal’ pH level, they can do this very easily. As more carbon dioxide dissolves into the ocean however, carbonic acid is produced, there are more hydrogen ions and the pH decreases. Consequently, it becomes harder for these organisms to produce their skeletons. Research has shown that their shells are thinner and more brittle. Furthermore, they require more energy to produce in the first place, leaving less energy for other essential processes such as growth and reproduction.

As plankton form the base for all food chains, the ecosystem is affected from the bottom up. If there is less food, fewer animals are able to survive and reproduce.

Our polar regions, having cooler waters, absorb more carbon than the tropical regions of our planet. In some regions of the northern hemisphere (e.g., northern coast of California) the waters are already acidic enough that they are dissolving calcium carbonate shells.


Why is this time different?

These higher pH levels have been present in our oceans in the past, however all organisms now present on our planet have evolved in the more stable environments seen since the last ice age. It is therefore predicted that these organisms may be unable to adapt and evolve to the rate of change that we are now seeing.

Four of the five great mass extinctions were associated with rapidly acidifying oceans, however this rate of increase that we are observing now is ten times faster than before the last mass extinction.

Populations of marine animals are already under stress from human impact and are not likely to handle further stress. For example, of the seven species of sea turtles that exist today, all of which survived the extinction of the dinosaurs, six species are now listed as endangered.



What do we do?

We need to give our ocean ecosystems the best chance to survive and withstand the hardship whilst we attempt to fix the problem. A healthy system has more chance of surviving.

Primarily we should reduce the carbon emissions that we continue pumping on a daily basis into our atmosphere. Supporting alternative energy sources including solar and wind power can achieve this.

Less than 1% of the world’s oceans are protected in any way, the other 99% are able to be fished, polluted, mined and exploited for oil. It is imperative that we support marine protected areas and protect more of our oceans.

If we reduce the amount of stress on our fish populations and give them a better chance to survive, they will be more likely to handle the other stresses we are throwing at them. There is a great deal of illegal and unsustainable fishing happening in our oceans; know the facts and start making the smart choice to support sustainable fishing practices. Vote with your wallet! With your pets, make the choice not to feed them fish.

Without the ocean, there is no life on Earth. The ocean is our life support system providing not just the air we breathe; it is the basis of our food chains, providing us with food and the water we drink. It even drives our climate. It is time we take better care of it.

If you want to know more, the short documentary, “The Acid Test” is available free, online. [RM1]


*pH = the amount of hydrogen ions in water


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