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CO2: silent enemy of the ocean

By Synte Peacock

Our early 21st century lives are completely dependent on fossil fuels. We are currently burning so much coal, oil and gas on the planet that we are adding some 32 billion tons of carbon dioxide to the atmosphere each year. About half of this carbon dioxide stays put in the atmosphere, causing all manner of problems as it slowly warms the planet. The rest is taken up by photosynthesis on land, or absorbed by the ocean.

The ocean, however, doesn’t just quietly suck it up. Rather, it is responding to this extra burden of CO2 in a strange way: by changing its chemistry. As carbon dioxide is absorbed in seawater, a chemical reaction occurs.

Carbon dioxide (CO2) reacts with water (H2O) to produce something called carbonic acid (H2CO3):

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This carbonic acid breaks apart, or “dissociates” in water, releasing both hydrogen ions (H+) and bicarbonate (HCO3):

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So the first surprising thing that happens when you add CO2 to seawater is that you end up with a lot of additional hydrogen ions floating around.

Some of these free hydrogen ions (H+) can combine with carbonate ions (CO32-) in the water to form more bicarbonate (HCO3).

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So the carbonate ions (CO32-) – which there weren’t that many of to begin with – get used up. Bicarbonate (HCO3) – which there is plenty of already, thank you very much – increases.

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Image by Joanie Kleypas

The pH is a measure of how acid or alkaline something is. The pH scale ranges from 0 to 14, with values below 7 being considered “acidic” and above 7 being considered “alkaline”. Pure water has a pH of 7. Neither acid nor alkaline, but right in the middle, completely neutral. Add free hydrogen ions to a substance and you will move the pH down towards more acidic values (you may still be on the alkaline side of the scale, but if the pH is going down, you are trending towards more acidic values). So as carbon dioxide is dissolved in seawater and it sets off the chemical reactions above, H+ ions are released and the pH tends towards more acidic values. The average pH of the ocean is actually slightly above 8, so slightly alkaline. Since 1850, it has shifted towards lower pH (more acidic) values by about 0.1 pH units.

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Image by Joanie Kleypas

A shift of 0.1 pH units might not sound like much, but it corresponds to a 30% increase in the concentration of hydrogen ions. It is a large enough change that we are already starting to see the first signs of these large-scale CO2-induced changes in ocean chemistry.

Over the course of the 21st century, the amount of CO2 we put into the atmosphere (assuming we stay on our present course to oblivion) will lower the ocean pH by another 0.3 to 0.5 units. Ocean pH has changed in the past, but never, barring catastrophes like meteor impacts, anything like as quickly as it is changing today. To survive large changes in water chemistry, creatures need time to adapt and to evolve to their new environment. Tens to hundreds of thousands of years might be enough; for all but the most rapidly evolving micro-organisms, a few decades certainly is not.

Under a natural (non-human) state of affairs, there is also a negative feedback that kicks in when the ocean becomes too acidic, to prevent a “runaway acid-bath”. On geological timescales, as carbon dioxide increases, so too does the weathering of rocks. The breakdown of continental rocks releases the minerals calcium (Ca2+) and carbonate (CO32-) into the ocean, thus replenishing the stuff that is ‘lost’ to the increasing H+ ion. This natural buffer prevents the oceans from ever moving too far towards the acid side of the pH scale, thus keeping things in check. The process is so effective that even with giant natural swings in atmospheric CO2, the acidity of the ocean can remain relatively unaffected, as the feedback kicks in to balance things out. The problem for us is that this feedback process takes place over thousands of years, not decades. There is no natural process that we know of that can bring ocean chemistry back into check in a matter or decades.

You might be wondering what all the fuss is about. If an esoteric thing called a carbonate ion decreases, so what? Well, we care about carbonate ions, because this is the stuff that shells are made of. When carbon dioxide in seawater goes up, carbonate ion concentration goes down (and vice versa). Take the carbonate away and you start to dissolve existing shells, and you also make it very difficult to build new shells.

There are two common forms of calcium carbonate: calcite and aragonite (both have the same chemical formula, CaCO3). Each of these forms has a different solubility in seawater (solubility measures how easily it will dissolve, and depends on the amount of carbonate present, and the depth). In the upper ocean there is a lot of carbonate floating around, so these minerals tend to form easily. In the deeper ocean, where there is less free carbonate, these minerals tend to dissolve. The temperature of water is another factor we need to think about, because temperature determines how much CO2 water can hold – the colder the water, the more CO2 it can absorb from the atmosphere. More dissolved CO2 means less CO32-, and therefore the tendency to dissolve carbonate minerals is more prevalent in polar waters.

There is an invisible horizon in the ocean that marks the boundary between waters that are corrosive to calcium carbonate, and those that are not (actually, there are two invisible horizons, one for each form of the mineral). In waters above this invisible horizon (that changes its depth with location), the critters are safe. There is enough spare carbonate ion here that they can continue life without fear of dissolution. These shallow waters are called “supersaturated”. In deeper waters (which don’t have enough spare carbonate ion) the waters are “under-saturated”, and the calcium carbonate dissolves. The boundary between the two is the “saturation horizon”. A large part of the reason for the deeper waters not having a lot of spare carbonate ions floating around is that there is a lot of dissolved CO2 down there: as creatures die and sink, they dissolve, releasing carbon dioxide back into the water column in the process.

As we continue adding CO2 to the ocean, the carbonate ions get gobbled up, and the invisible horizon, the “calcium carbonate saturation horizon”, rises up in the water column. The zone containing abundant carbonate ions gets squished into an ever-narrower zone. So a progressively smaller volume of the ocean is safe for life.  Because the ocean is not a simple layered fluid, but rather a vast and complex region of swirling currents and sinking and rising waters, the saturation horizon for the more sensitive aragonite is already at the surface in places. Such regions have no effective habitat left for organisms that make their living with these minerals. One example of this is off the west coast of North America, where deep currents pulled to the surface by the pattern of overlying winds are dragging up their carbonate-ion-deficient waters to the surface. Mollusks with aragonite shells who once made their homes here have now become chemical refugees, and have either perished or had to move on to more accommodating waters.

Creatures like foraminifera and coccolithophores are tiny plankton encased in calcite shells that float through the ocean in their countless billions. Others, such as corals and many mollusks choose aragonite to build their homes. Aragonite is some 50% more soluble in seawater than calcite, so if there is a deficiency of carbonate ions (CO32-) in the water column, it is the corals, and mollusks that will suffer first. In coming decades it is thought that the corals of the tropics and subtropics (which currently cover an area of some 1.28 million square kilometers) will suffer greatly – with implications not just for the reef structure itself, but for the host of organisms that make their living in the reefs. In addition to fish, sea urchins and sharks, some 1 billion people are dependent on coral reefs for their food and livelihoods. Tourists who like to swim and dive in these colorful waters fuel a coral reef tourism industry valued at some $30 billion annually.

It has been estimated that by the time atmospheric CO2 reaches some 550ppm (which will happen in the mid 21st century in the absence of major changes in the way we do business), calcification rates for corals will have decreased by 10-30%, and the situation will only worsen if CO2 continues to rise beyond that. The saturation horizon for aragonite will have risen by several hundred meters by the turn of the 21st century, and will have actually reached the surface in the Southern Ocean by this time.

So by 2100 we could be living in a world in which the entire Southern Ocean is under-saturated with respect to aragonite. Put simply, this means that many of those creatures with aragonite shells that find themselves in the Southern Ocean (the entire ocean south of about 40°S) at the end of this century will dissolve, a casualty of chemistry.

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Photo credit: National Geographic Images.

The shelled organisms might be the most obvious casualties of ocean acidification, but those without shells might also be affected. The simple fact that CO2 concentration is increasing might affect all stages of the life cycle of many other creatures – in ways we are only just beginning to understand. For example, the combination of increased CO2 and decreased pH could have an effect on the respiration of larger marine animals, which take up oxygen from the seawater, and expel CO2 through their gills. Large marine animals appear to be much more sensitive to changes in their ambient CO2 than are land lubbers. It is thought that an increase in the CO2 of ambient water could acidify body tissues and fluids, and change the ability of the blood to transport oxygen. This could lower the rate of respiration and of protein synthesis (which could in turn impact other essential processes).  It remains to be seen whether larger marine life-forms are able to adapt to the increase in CO2 we are about to hit them with, or whether their fate will be a less happy one.

It is likely that there will be winners and losers as we continue our giant chemistry experiment with the ocean. There will certainly be shifts in the relative abundance of species as CO2 goes up and carbonate ion goes down. While the multitude of effects will be very difficult to predict or accurately model, they probably won’t be pretty.

The threat from acidification is like a silent freight train that we have already put in motion. Is it too late to stop it? We can put the brakes on, but cannot reverse the changes that we’ve already made. The pH change that has occurred so far is essentially irreversible in our lifetimes. Nature will take care of it – in a few thousand years. But that might be too late for the stunning diversity of life that currently graces our shallow coralline seas.  And the more CO2 we keep adding to the atmosphere, the more the ocean will continue to take up, continuing the downward spiral in pH. If we reach the point where the tiny creatures that form the basis of food-chains start to dissolve, the effects could ripple through oceanic life.