This article was updated on the 11th September 2019.
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In addition to climate change, the rising levels of atmospheric carbon dioxide and its potential impact on the chemistry of the ocean on Earth is a pressing environmental concern. Approximately half of the carbon dioxide emitted into the atmosphere from human activities (like burning fossil fuels) in the 250 years since the Industrial Revolution has dissolved into the ocean.
An increase in the concentration of carbon dioxide stored in oceans results in the water becoming more acidic. Recently, scientists have started to verify changes in the pH levels of the ocean and observe the impacts of chemical changes on marine animals and plants.
Oceans and the Carbon Cycle
The atmospheric concentration of carbon dioxide in the natural carbon cycle represents a balance of fluctuations between the atmosphere, terrestrial biosphere and oceans. However, human activities, such as the combustion of fossil fuels, have resulted in a new fluctuation of carbon dioxide into the atmosphere. Some of this carbon dioxide has remained in the atmosphere and some is believed to have been absorbed by terrestrial plants while the majority share has been consumed by the oceans.
Carbon dioxide reacts with water when it is dissolved to form a balance of non-ionic and ionic chemical species: dissolved free carbon dioxide, bicarbonate, carbonate and carbonic acid. Factors such as the temperature of seawater and its alkalinity govern the ratio of these chemical species.
How Does the Water Change?
When carbon dioxide is dissolved in water, it generates carbonic acid (the same weak acid found in carbonated drinks). The increase in the levels of carbonic acid interferes with the water’s calcium carbonate formation. Calcium carbonate is a major structural component of the shells of a number of important planktonic organisms. The increase in acidity also reduces the availability of nutrients in the ocean.
What Effect will Ocean Acidification Have?
Calcite and aragonite are stable in surface waters under normal conditions because the carbonate ion is at supersaturating concentrations. But as pH levels in the ocean fall, the concentration of the carbonate ions also falls. This causes the carbonate to become under-saturated. Structures constructed using calcium carbonates become more susceptible to dissolution. Research has already indicated that corals, foraminifera, coccolithophore algae, pteropods and shellfish experience reduced calcification or enhanced dissolution when exposed to elevated levels of carbon dioxide.
As the formation of calcium carbonate becomes more infrequent, it will become more difficult for some planktonic organisms (key components in the food chain) to form shells. These organisms are also important for the removal of carbon from the surface waters to the Deep Ocean and the release of oxygen into the air.
The respiration in fish, an important metabolic process, is impaired by ocean acidification because a decrease in the pH level reduces the efficiency of oxygen exchange in their gills.
Ocean acidification, along with rising sea temperatures caused by global warming, is also causing coral bleaching which is killing coral reefs at an unprecedented rate. In 2016, coral bleaching in the Great Barrier Reef, the Earth’s largest organic structure, killed between 29% and 50% of the reef.
How Acidic will the Ocean Get?
The pH levels of seawater are slightly alkaline and have historically remained at around 8.2. However, the emission of carbon dioxide from human activities has resulted in the pH of the surface water of oceans to drop by 0.11 pH units. This is equivalent to a 30% increase in acidity. The pH is expected to fall by 0.5 pH units (a 320% increase in acidity) by the year 2100 if the emission of carbon dioxide is not restrained.