The acid test

What is our use of fossil fuel doing to the ocean? (7 minute read)

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As surely as night follows day, water left to its own devices will run downhill. Well, that’s not entirely true – there are exceptions. After all, waves on a beach flow uphill, before they roll back down. If you dip a piece of toilet paper partly into water, you’ll notice that water does travel upwards into the paper. And then there are siphons, where water drains from one container to another via a tube. In the process, water actually does flow uphill for a short distance (for an explanation of this fascinating process, check out the video here).

But, regardless of these exceptions, water flowing downhill has a certain inevitability.

The flow of water downhill has been on my mind lately, as I’ve been writing about nitrogen and phosphate fertilisers. I was struck by the way that nitrates and phosphates get into the water and then flow down into the ocean, causing real problems there. It’s easy to forget how often what we do on land ends up affecting the ocean. However, once it’s pointed out, it seems obvious.

Less obviously, what we pump into the air affects the ocean too. When lead was widely used as an additive in petrol, tiny particles containing lead were released into the atmosphere and transported vast distances, sometimes thousands of kilometres. As a result, there was a noticeable increase in lead in sea water at the ocean’s surface, even in regions far from human activity, such as the southern Indian Ocean. Another dangerous pollutant, mercury, is released into the atmosphere by coal burning and small-scale gold mining. Mercury, too, can be carried thousands of kilometres through the air before ending up in the ocean – and then entering the food chain as the toxic methylmercury.

If we don’t already have enough reasons to stop burning coal, here’s another one – coal burning is the major source of mercury entering the atmosphere. In 2001, the US Geological Survey estimated that power plants in the United States emitted 45 tonnes of mercury into the atmosphere each year. In 2005, human activities released a total of 1930 tonnes of mercury into the atmosphere. Fossil fuel (mainly coal) burning for electricity and heating contributed 45% of that total.

Mercury is worrying, of course, but the main concern with coal is its role in climate change. We know that the increasing levels of carbon dioxide and some other gases in the atmosphere is causing the earth and the ocean to warm, but it turns out that carbon dioxide in the atmosphere is also finding its way into the ocean. Around 30% of the carbon dioxide released into the atmosphere is absorbed by the ocean. What impact is it having there?

Image credit: Melanie Newfield

The oceans are a crucial part of the global carbon cycle. Tiny marine plants, known as phytoplankton, absorb carbon dioxide during the process of photosynthesis, just as land plants do. And, just like land plants, they produce oxygen as a by-product of photosynthesis. In fact, at least half of the oxygen in the atmosphere comes from photosynthesis in the ocean, so marine plants are really important to us.

Carbon dioxide, then, is essential to the functioning of the ocean. As with land plants, increased carbon dioxide may lead to a greater rate of photosynthesis, pulling carbon dioxide out of the ocean and locking it up in the form of living creatures. But there’s another effect as well, and it’s not something that happens on land, so it’s worth a closer look.

To understand why carbon dioxide is causing problems in the ocean, it’s useful to understand something about the chemistry of acids and bases. We are all familiar with acidic compounds such as lemon juice and vinegar and most of us have probably heard of pH, which is the way that acidity is measured. The pH scale runs from 0 to 14 – a pH of 7 is neutral, a pH of less than 7 is acidic and a pH of more than 7 is basic. The scale of pH is not linear but logarithmic, which means that there is a tenfold difference between one number and the next. So a pH of 3, for example, is ten times as acidic as a pH of 4.

The pH of an acid or base is not an intrinsic property of a particular compound, but depends on the concentration of that compound in water. So, for example, a typical white vinegar has a pH of around 2.4. Dilute one part vinegar with ten parts of water, and the pH will rise to around 2.9. For a quick explanation of acids and bases, I’ve linked to a short video here.

When carbon dioxide dissolves in water, it forms an acid called carbonic acid. This is a fairly weak acid – sparkling water, which has a large amount of carbon dioxide dissolved in it, only has a pH of 5-6, so it’s around 1000 times less acidic than vinegar.

Before humans began burning fossil fuels, the oceans had a pH of 8.2, which is very slightly basic. But since we’ve been pumping extra carbon dioxide into the atmosphere, the pH has dropped to 8.1. This sounds like a tiny change, but because the pH scale is logarithmic, this change is actually quite large. By the end of this century, the ocean is expected to fall to a pH of 7.8 or even 7.7, the lowest that it has been in the last 20 million years.

Living creatures are quite sensitive to changes in pH. For marine creatures, one reason that pH is important is that many have body structures made of calcium carbonate. Shells for example, are made from calcium carbonate. Mostly, this is in the form of the mineral calcite, but some also have the mineral aragonite, which has a beautiful, soft lustre – it’s what pearls are made of. Corals, too, build shells from calcium carbonate. Each individual coral organism in a reef is tiny, an average of 1-3 millimetres, but together their shells form massive coral reefs, and that structure is mostly calcium carbonate. Some types of phytoplankton also have calcium carbonate shells.

As the ocean’s water becomes more acidic (or, rather, less basic, since the pH is above 7), there is less carbonate available in the water. In theory, this is likely to make life more difficult for marine creatures with calcium carbonate shells. It’s not that they are incapable of building shells under the changed conditions, but it takes more energy to do so.

But it’s one thing to have a theory about an environmental problem, and quite another to actually measure it. We are raising the temperature of the ocean, pouring in pollutants and over-fishing. Each of these makes life more difficult for marine creatures. Identifying the impact of these separately is tricky.

The first documented sign that ocean acidification is affecting marine organisms is with an organism called a pteropod, or, more poetically, sea butterflies. I’d never heard of sea butterflies before I began researching this article, but they are fascinating. They are a type of mollusc, which means that they are related to garden snails, as well as shellfish like mussels and oysters. Mussels and oysters live sedentary lives, while a snail moves slowly on its slimy, muscular foot. But sea butterflies float in the ocean water, and their foot has been transformed into two flaps, like the wings of a butterfly. They’re tiny, but they’re lovely – take a look at them swimming in this video.

Pteropod shells are made of calcium carbonite in the form of aragonite, which I mentioned earlier is the material pearls are made from. Aragonite is a less stable form of calcium carbonate, which makes pteropods more vulnerable than other marine creatures with tougher shells of calcite. So it’s not surprising that pteropods are the first marine creature where we have found signs of harm from ocean acidification. Scientists have found pteropods with weakened shells in areas of the ocean in Antarctica which were particularly acidic.

Overall, though, it’s hard to know just what effect the increasing acidity of the ocean is having, because there are so many changes going on in the ocean. We do know different parts of the ocean have different pH values, and so we know that marine life can adapt to different pH conditions. Perhaps they will adapt to the changes that we are causing.

But what is most worrying about the change in the ocean’s pH is not, so far, the amount of change – it’s the rate of change. The change in ocean pH is happening at a rate that is 10 to 100 times faster than any change in ocean chemistry in the last 100 million years. That fact, more than anything else, is what alarms me about ocean acidification. We don’t know what the effect will be of these rapid changes. It’s just another example of the way we are putting the earth’s environment to the test, without really understanding what we are doing.

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