What is ocean acidification?
Ocean acidification is caused by carbon dioxide in the atmosphere dissolving into the surface waters of the oceans. This is a natural process, as the atmosphere and the oceans remain in a gaseous equilibrium. This means that the more carbon dioxide we put into the atmosphere, the more carbon dioxide dissolves into the oceans.
When carbon dioxide dissolves in sea water it instantly separates into bicarbonate ions (HCO3-) and hydrogen ions (H+). This increase in hydrogen ions results in a lower pH, or an increase in acidity. The oceans have a natural buffering system, i.e. they have a mechanism that attempts to limit pH change. This mechanism is the carbonate ion which bonds with the hydrogen ion forming bicarbonate, thus preventing the hydrogen ion from decreasing pH. When the rate of carbon dioxide dissolution into the oceans outpaces the natural buffering mechanism, however, ocean acidification occurs.
Not quite a Hollywood scenario
The term "ocean acidification" is perhaps a misleading one. The oceans will never become an acid. Their pH will never drop below 7. You will never be able to throw a bad guy into the sea and watch his skin melt off due to ocean acidification. But that doesn't mean it's not serious for the critters living in the oceans.
pH is a logarithmic scale. This means that a small change in the pH number actually means a large change in real terms. Before the Industrial Revolution (c.1750) the average sea surface water pH was about 8.1, whereas now its 8.0 pH. This drop in pH represents a 30% increase in acidity. A further reduction to pH 7.8 is an increase of 150% in acidity.
It's not all about the measurement
However, it's not the pH of the oceans that is directly causing concern. The ocean's natural buffering mechanism uses carbonate ions. This means that as sea water acidity rises, the amount of carbonate left in the oceans decreases. Carbonate is an incredibly important molecule with everything from corals to shellfish to plankton using it to form their shells and skeletons.
With less carbonate available, the organisms struggle to build their carbonate skeletons and when they do succeed that carbonate is more likely to dissolve back into the sea water. If they cannot form their skeletons, they die. Larval (baby) forms are more likely to be affected by this as they are smaller (and so have a larger surface area) and have thinner shells (so less carbonate needs to dissolve to be disastrous).
Undersaturated and corrosive
Once the surface waters reach a pH of 7.9 the sea is said to be undersaturated with respect to aragonite, the least stable form of carbonate. Undersaturation means that any shells which are formed are very likely to be dissolved back into the sea water and that water is said to be corrosive to calcitic skeletons. Aragonite is used by snails, sea urchins and starfish, and a number of other shell fish in their shells. These corrosive effects can be reversed by decreasing the acidity and raising the pH; this can be done by adding shell and carbonate material (such as chalk) to an area, but is not practical on a large scale.
Has this happened before?
pH is a logarithmic scale. This means that a small change in the pH number actually means a large change in real terms. Before the Industrial Revolution (c.1750) the average sea surface water pH was about 8.1, whereas now its 8.0 pH. This drop in pH represents a 30% increase in acidity. A further reduction to pH 7.8 is an increase of 150% in acidity.
It's not all about the measurement
However, it's not the pH of the oceans that is directly causing concern. The ocean's natural buffering mechanism uses carbonate ions. This means that as sea water acidity rises, the amount of carbonate left in the oceans decreases. Carbonate is an incredibly important molecule with everything from corals to shellfish to plankton using it to form their shells and skeletons.
With less carbonate available, the organisms struggle to build their carbonate skeletons and when they do succeed that carbonate is more likely to dissolve back into the sea water. If they cannot form their skeletons, they die. Larval (baby) forms are more likely to be affected by this as they are smaller (and so have a larger surface area) and have thinner shells (so less carbonate needs to dissolve to be disastrous).
Undersaturated and corrosive
Once the surface waters reach a pH of 7.9 the sea is said to be undersaturated with respect to aragonite, the least stable form of carbonate. Undersaturation means that any shells which are formed are very likely to be dissolved back into the sea water and that water is said to be corrosive to calcitic skeletons. Aragonite is used by snails, sea urchins and starfish, and a number of other shell fish in their shells. These corrosive effects can be reversed by decreasing the acidity and raising the pH; this can be done by adding shell and carbonate material (such as chalk) to an area, but is not practical on a large scale.
Has this happened before?
Scientists think ocean acidification has happened before, approximately 200 million years ago at the end of the Triassic. Huge volcanic eruptions released large volumes of carbon dioxide over a relatively short timescale (<100,000 years), and the rise in temperatures also mobilised the frozen methane clathrates. Methane also removes carbonate from the oceans in the same manner as carbon dioxide.
This volcanic-induced ocean acidification, as shown by the global disappearance of carbonate rock of that age, resulted in a mass extinction where an estimated 80% of species disappeared and an absence of coral reefs for around 8-10 million years. Eventually the sea surface waters returned to a normal pH as the natural buffering system caught up and large-scale sea circulation transported the carbon dioxide to the deep sea where it was neutralised by sediments, but this took approximately 10,000 years.
So what lies ahead?
This volcanic-induced ocean acidification, as shown by the global disappearance of carbonate rock of that age, resulted in a mass extinction where an estimated 80% of species disappeared and an absence of coral reefs for around 8-10 million years. Eventually the sea surface waters returned to a normal pH as the natural buffering system caught up and large-scale sea circulation transported the carbon dioxide to the deep sea where it was neutralised by sediments, but this took approximately 10,000 years.
So what lies ahead?
The rate of carbon dioxide release from the volcanoes at the end of the Triassic is small compared to the current anthropogenic rate of release. Since 1750 carbon dioxide concentration in the atmosphere has increased from approximately 280 ppm (parts per million) to 400 ppm with a corresponding drop in sea surface pH of 0.1. It has been estimated that by the time carbon dioxide levels reach 560 ppm the surface waters of the Southern Ocean will be undersaturated with respect to aragonite and the pH reduced to 7.9.
Cold water can absorb more carbon dioxide more easily than warm water so northerly and southerly oceans are going to be more adversely affected. Already in parts of the Arctic Ocean the pH has dropped briefly to 7.9 and some species (such as pteropods, a swimming snail) are struggling to form their shells. The growth rate in corals is also dropping as the pH decreases. The organisms which are going to be worst affected are some of the most crucial - those that make up a large part of the plankton which forms the base of the ocean's food chain. Most oceanic organisms spend the larval part of their life cycle in the plankton, so many many species are going to be affected.
Members of the public at the Science Museum event were asking me "So what can we do to prevent this?". My response could not be a positive one. It's already happening and the carbon dioxide concentration in the atmosphere is still increasing. One way to combat ocean acidification may be to add large amounts of carbonate material, dug up from quarries on land or created in a laboratory, to the oceans but both of these activities add more carbon dioxide to the atmosphere and are very costly. So I guess the question is, how much are we willing to do to save our oceans, and is it already too late?
Cold water can absorb more carbon dioxide more easily than warm water so northerly and southerly oceans are going to be more adversely affected. Already in parts of the Arctic Ocean the pH has dropped briefly to 7.9 and some species (such as pteropods, a swimming snail) are struggling to form their shells. The growth rate in corals is also dropping as the pH decreases. The organisms which are going to be worst affected are some of the most crucial - those that make up a large part of the plankton which forms the base of the ocean's food chain. Most oceanic organisms spend the larval part of their life cycle in the plankton, so many many species are going to be affected.
Members of the public at the Science Museum event were asking me "So what can we do to prevent this?". My response could not be a positive one. It's already happening and the carbon dioxide concentration in the atmosphere is still increasing. One way to combat ocean acidification may be to add large amounts of carbonate material, dug up from quarries on land or created in a laboratory, to the oceans but both of these activities add more carbon dioxide to the atmosphere and are very costly. So I guess the question is, how much are we willing to do to save our oceans, and is it already too late?
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