Carbon Dioxide and Ocean Acidification

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Ocean acidification is often referred to as ‘the other carbon dioxide problem’. We’re all quite rightly concerned about the effects that rising atmospheric carbon dioxide levels may have on climate, and the potential consequences of climate change are well documented: more frequent instances of extreme weather, and higher global average temperatures to name but two. Ocean acidification gets comparatively less press, and as such is sometimes misunderstood – but its effects could be equally serious.

Let’s start with the basics. Seawater is slightly alkaline. Carbon dioxide can dissolve in seawater and initially this would appear to be a positive point, as it lowers the amount of carbon dioxide in the atmosphere. However, it can also cause ocean acidification, decreasing the alkalinity of seawater, a process that is increased by the increasing levels of carbon dioxide being produced by human activities.

Acidity and alkalinity can be measured on the pH scale. Even if you’re not a chemist by trade, you might well remember this scale from school chemistry lessons, but just as a reminder, it measures acidity and alkalinity on a scale that runs from 0 to 14 (though it’s worth noting that pH values outside this range are actually possible too). Under normal conditions, values below 7 are acidic, and values above 7 are alkaline. Present day seawater has a pH of 8.1, making it slightly alkaline.

Acidity stems from the presence of hydrogen ions in a solution, which is where carbon dioxide comes in. Carbon dioxide from the atmosphere can dissolve in and then react with seawater. The product of this reaction is carbonic acid, which quickly dissociates into its component ions: a hydrogen ion and a bicarbonate ion. The hydrogen ions produced by this process are the culprits behind ocean acidification.

Ocean acidification as a result of this process is a well-documented phenomenon; in fact, the pH of the ocean has already dropped since the pre-industrial era. Back in the early 1800s, seawater’s pH was approximately 8.2, so it’s dropped by 0.1 pH units since then to its present-day value of 8.1.

It’s easy to look at these numbers and think that scientists are making a mountain out of a molehill. 0.1 pH units sounds like a pretty insignificant change in the grand scheme of things. However, to fully comprehend what this means, you have to realise that the pH scale is a logarithmic scale. To put that in more straightforward terms, it means that a drop of one pH unit would represent hydrogen ion concentration multiplying by ten, and that measly sounding 0.1 pH unit drop actually represents a 25% increase in hydrogen ion concentration.

That’s since pre-industrial times, but humans have been increasing the amount of carbon dioxide in the atmosphere through fossil fuel combustion and deforestation since then. The amount of carbon dioxide in the atmosphere is still increasing – just last year we passed the 400 parts per million of carbon dioxide in the atmosphere. More carbon dioxide in the atmosphere means more carbon dioxide available to dissolve in the oceans.

Current projections suggest that average ocean surface pH will drop to around 7.7 by 2100, a decrease which would represent around a 150% increase in acidity from present day levels. Suddenly the numbers involved don’t seem so small. Of course, this is just a projection; while we’re unlikely to cut emissions sufficiently to completely arrest the ocean’s falling pH this century, action to reduce emissions could still mitigate the damage. Then again, the incoming US president thinks climate change is a Chinese hoax, so it remains to be seen if meaningful steps to combat the problem and in turn ocean acidification will be taken.

You might well wonder what the problem is with ocean acidification. After all, though the ocean’s pH is dropping, it’s still alkaline after that drop, and to actually turn it acidic (below pH 7, remember) would require a colossal amount of carbon dioxide in the atmosphere – far more than there is currently or even will be 100 years into the future. It’s a concern that the pH is dropping, sure, but is there really any consequence?

Sadly, the answer is yes. The direct effects of ocean acidification may not be immediately felt by humans, but unsurprisingly by the organisms that inhabit the ocean. And effects on them could have knock on effects that impact our lives too.

One of the groups of animals most affected by ocean acidification will be calcifying organisms. These are animals such as clams, oysters, and crustaceans that pull carbonate ions from seawater to form the calcium carbonate that makes up their shells. This is possible because calcium carbonate is supersaturated in ocean surface waters, meaning the calcifying organisms are able to precipitate it out as solid calcium carbonate under the right conditions.

The problem lies in some of the other chemistry that carbonate ions can undergo in seawater. They can combine with hydrogen ions, forming bicarbonate ions, reducing the concentration of the carbonate ions. In a way, this is good, as it removes some hydrogen ions from the water, mitigating some of the increased acidity produced by dissolved carbon dioxide. This is a problem for calcifying organisms though, because if the water around them is undersaturated with respect to carbonate, the usually insoluble calcium carbonate that makes up their shells starts to dissolve.

Research has shown that for many calcifying organisms, calcification rates will be reduced by decreased ocean pH levels. It’s not a universal truth – some calcifying organisms have oddly shown increased calcification rates in the same conditions, the reasons for which still aren’t entirely clear. Coral is also a calcifying organism, and is also affected by ocean acidification, though assessments of its impact vary in the research literature. What is clear is that a wide range of organisms could be affected, in unpredictable ways.

Effects on crabs and oysters might not seem that significant from a human perspective. However, calcifying organisms are at the root of a number of marine food webs. If ocean acidification affects the populations of these organisms, it could have knock on effects further up the food chain, including on species of fish that are widely caught for food. A reduction in the population of these fish would mean economic problems for commercially fishing.

It’s not just calcifying organisms that are affecting, either. Many marine organisms use chemical signalling for a range of reasons, be it detecting predators, settlement, or reproduction. Studies have shown that ocean acidification can cause chemical changes in the signalling molecules used, which can in turn affect their detection. This could modify the behaviour of certain species, and have further detrimental effects.

A lot of those who deny that ocean acidification poses a problem claim that the ocean has seen similar acidification previous, albeit millions of years in the past. They are correct to point this out – but it’s not correct that it didn’t cause problems previously. In fact, ocean acidification has been linked to Earth’s greatest extinction event, 255 million years ago, during which almost all marine species vanished.

During the second phase of the Permian mass extinction, huge volcanic eruptions in Siberia spewed out vast quantities of carbon dioxide. Over 10,000 years, pH levels in the oceans dropped by 0.7 units, and though other factors were also likely at play, it’s thought that this ocean acidification played a significant role in helping wipe out many marine species. This change took place over 10,000 years, whereas the ocean’s pH might drop by 0.4 units in just 100 years’ time – an unprecedented rate of change, and one that makes it unlikely that many organisms will be able to adequately adapt to the changing conditions.

The classic fallback option for those who deny that ocean acidification is an issue is to state that ‘Nature always balances itself out in the end”. Eventually the complex chemical systems involved will find a new balance, but this balancing act takes tens of thousands of years – and there’s no requirement for all of the ocean species, or even humankind, to emerge unharmed on the other side. We know ocean acidification is happening, because ocean surface pH has already dropped since pre-industrial times. What we need to do now is take action to mitigate it as much as possible.

 

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