Ocean acidification: the forgotten cost of climate change
Ocean acidification is the forgotten consequence of climate change. The oceans have reduced the rate at which temperatures rise on land by absorbing carbon dioxide and thermal energy, but this comes at a terrible cost: the cost of ocean acidification. It is a cost which is relegated and ignored in the public discourse on climate change, despite the catastrophic damage to marine ecosystems, and the livelihoods of millions, that it will inflict, now, and in the future.
The oceans are the largest sink of carbon dioxide, containing 3.8 x 1013 tons of CO2; annually, they absorb between 25-30% of all CO2 emitted into the atmosphere. In principle, this is life-saving, but the problem is that dissolved CO2 (aq) reacts, in microseconds, with water, and produces carbonic acid, H2CO3. The ions that make up carbonic acid tend to dissociate in water, due to the electrostatic attraction between the dipoles of the water molecules, producing bicarbonate (HCO32-) and hydrogen (H+) ions.
The overall ionic reaction is shown in the following equation (missing out carbonic acid, and instead showing its constituent ions):
Aqueous carbon dioxide is increasing in its proportions due to increased anthropogenic emissions associated with climate change. The reaction is reversible, and so Le Chatelier’s Principle applies to it: an increase in volume of carbon dioxide, will cause the reaction to produce more bicarbonate and hydrogen ions to return the system – in this case the sea – to equilibrium, the point at which both sides of the reaction are happening at the same rate.
However, in doing so, this causes an increase in the concentration of hydrogen ions, otherwise viewed as protons. An acid is essentially a donator of protons (H+), hence this process increases acidity of the ocean, decreasing the pH.
A decrease of 0.11 in pH might not sound significant; however, the logarithmic scale on which pH is measured means that this decrease is equivalent to the oceans having become 30% more acidic, compared to pre-Industrial levels, which is an unprecedented change in oceanic chemistry.
This is easier to visualise when graphed, as below. When the models of OA are extrapolated forward, the current estimates suggest that by 2100 if we do not curb CO2 emissions, the acidity of the ocean will have increased by 150%.
For millennia, seawater has been kept in equilibrium with the weathering of calcite-based rocks (to yield both carbonate and bicarbonate ions: ) counteracted by acidic volcanic gases (such as CO2) bubbling into the ocean. Seawater is also kept within its narrow range by buffer solutions, such as aqueous sodium, magnesium, and calcium salts, but the scale of the carbon dioxide dissolving into the oceans means these mechanisms can no longer control the increasing acidity of the oceans.
This acidity is already having fatal consequences for the ecology in our oceans, and the people these ecosystems support.
The biggest threat is to calcifying species. The problem is two-fold: the increase in the forward reaction means that the concentration of the carbonate ion is falling, and consequently the saturation of CaCO3 is decreasing. Consequently, calcifying species must expend more energy to build up their exoskeletons, and for many organisms, especially in their juvenile stage, this is too big a burden, leading to disrupted growth and death. Secondly, the hydrogen ions can react with their calcium carbonate exoskeletons and shells, forming carbonic acid, which corrodes their protective skeletons.
a magnified section of coral exoskeleton, showing the importance of CaCO3
Corals are severely affected because they form exoskeletons of aragonite around each individual coral polyp, an unstable form of calcium carbonate which is the most likely to decay when it reacts with hydrogen ions. When the saturation of calcium carbonate in the waters decreases, it becomes harder for corals to assimilate it into their skeletons; the calcification rate of coral in one study decreased by 14% between 1995 and 2005 alone, and the reduction in CaCO3 saturation may have even worse effects in the microalgae, known as zooxanthellae, which are endosymbiotic with coral, and provide 90% of coral’s energy through photosynthesise. If they are negatively affected, ocean acidification could trigger mass coral bleaching.
Lower rates of coral calcification mean that reefs are built up more slowly, or in some cases not at all, and accompanied by rising sea levels, many reefs are becoming deeper, and can therefore photosynthesise less effectively. Combined with the myriad of other threats to coral reefs – such as seafloor dredging, coral bleaching and die-back, marine pollution, and the crown-of-thorns starfish – coral reefs are dying and eroding, leading to reef erosion. This disintegration reduces the ability for fringing reefs to act as breakwaters, shoaling storm waves further from the shore dispersing their energy, and exacerbating coastal flooding. Acidification may even cause the disintegration of coral atolls, to the point at which they can no longer support human inhabitation, as is happening in the Maldives.
Coral reefs provide refuges and breeding grounds for 25% of all marine life, and what is concerning is that the slower rates of coral growth due to ocean acidification are flattening coral reefs, reducing the number and diversity of habitats for juvenal larvae of many species, and the adult for safety and food. Furthermore, the reproductive abilities of coral itself is likely to be affected as well, although at present, scientific studies conflict over the severity of this. The effects on deep-sea coral have hardly been studied, but the slower rates of growth and longer periods between reproduction mean that the effects of acidification will be amplified in these rich communities.
Ocean acidification will also affect larger calcifying species in coastal areas, such as bivalves, like clams, oysters, scallops, and mussels, which have important commercial implications, in addition to the habitats that may be lost, such as oyster reefs.
The ocean is also likely to become a much more confusing place for tropical fish to navigate due to acidification. Their ear bones, known as otoliths, are made of calcium carbonate, and in some studies tropical fish have been shown to become confused and unable to navigate coral reefs in CaCO3 depleted waters. Lower CaCO3 saturations also disrupts a key brain receptor, preventing certain neurotransmitters functioning, which creates an inability in certain species to detect predator fish, and disrupt their schooling, likely to lead to more predation and changes to the eco-system. For cetaceans, a 0.3 pH decrease could amplify sounds in the ocean by 70%, confusing the calls between pods, and the echolocation dolphins use to find prey.
Other less glamorous, but potentially even more critical, calcifying species will also be negatively affected. Half of all photosynthetic activity on Earth occurs in phytoplankton, and many species (coccolithophores) form CaCO3-based shells to protect themselves. Ocean acidification is likely to hinder the formation of these shells, leading to reductions in phytoplankton populations. One controversial study suggests that phytoplankton populations have declined by 40% since 1950, although not solely at the hands of ocean acidification; others place it at less than this, but nevertheless critical.
the incredible calcium carbonate shell of a phytoplankton
Phytoplankton support, as autotrophic primary producers, entire food chains, preyed upon by krill, and in turn fish and then cetaceans, particularly in the pelagic – or open ocean. Almost all fish larvae prey initially on plankton, and changes to phytoplankton due to ocean acidification are likely to cause huge shifts in marine ecology, especially in the more delicate Arctic regions, which will have cascading effects right up to apex predators.
For their biomass, phytoplankton are incredibly efficient at permanently sequestering CO2 in calcite-rocks on the seabed when they die; ocean acidification could make the oceans less efficient as carbon sinks in the future.
the disintegration of a pteropod shell due to ocean acidification
Of all plankton species, pteropods (sea butterflies – a type of floating snail) are the most studied scientifically. Pteropods shells have been shown to dissolve in more acidic water in as little as 30 days at pH 7.8, as the following images show. Declining populations of pteropods, especially in their juvenal stages, will hit northern fish populations hard; up to 60% of a salmon’s diet is on pteropods, for example. Calcifying zooplankton support many of the world’s fisheries, such as the krill in the Barents Sea, and salmon, cod, and herring in the North Atlantic, especially in delicate regions of the Arctic. All species, including the millions of humans for whom these fisheries are their livelihoods, are likely to be affected by an acidifying ocean.
Ocean acidification is a pressing yet ignored problem, partly because of the inconvenient truth that without it climate change would be a significantly worse. It is going to become an acute problem for our fragile marine ecosystems, and one that is entirely anthropogenic.