Meghan Hodges
The ocean is the world's greatest carbon sink, absorbing about 30% of the carbon dioxide released into the atmosphere. This occurs at the sea surface, where carbon dioxide is dissolved from the air into the seawater. However, increasing carbon dioxide concentrations in the atmosphere, caused by human activities such as burning fossil fuels, are resulting in the ocean absorbing more carbon dioxide. This is causing ocean acidification, which is having detrimental effects on marine life and ecosystems, such as shellfish and coral reefs, and reducing the ocean's ability to absorb carbon dioxide.
| Characteristics | Values |
|---|---|
| Amount of carbon dioxide absorbed by the ocean | The ocean absorbs about 30% of the carbon dioxide released in the atmosphere. |
| Ocean acidification | The ocean's pH has decreased from 8.1 to 7.8 in the last decade, making it more acidic. |
| Impact on marine life | Ocean acidification is harming shellfish and other marine life, including coral reefs. |
| Carbon sink | The ocean is the world's greatest carbon sink, absorbing excess heat and energy from rising greenhouse gas emissions. |
| Carbon exchange | More than 150 billion tonnes of carbon dioxide move back and forth between the ocean and atmosphere annually. |
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What You'll Learn
Ocean acidification
The ocean absorbs about 30% of the carbon dioxide (CO2) released into the atmosphere. As levels of atmospheric CO2 increase from human activity such as burning fossil fuels and changing land use, the amount of carbon dioxide absorbed by the ocean also increases. The ocean has absorbed about 29% of global CO2 emissions since the end of the preindustrial era. In the last decade (from 2008-2017), about 40 gigatons of emissions of heat-trapping gases were dumped into the atmosphere annually.
When carbon dioxide is absorbed by seawater, a series of chemical reactions occur, resulting in an increased concentration of hydrogen ions, which increases acidity and lowers the pH. This process has far-reaching implications for the ocean and its inhabitants. Ocean acidification is already impacting many species, especially organisms like oysters and corals that make hard shells and skeletons by combining calcium and carbonate from seawater. As ocean acidification increases, available carbonate ions bond with excess hydrogen, resulting in fewer carbonate ions for calcifying organisms to build their shells and skeletons.
The pH of surface ocean waters has fallen by 0.1 pH units since the Industrial Revolution, which may not sound significant, but the pH scale is logarithmic, so this change represents an approximate 30% increase in acidity. Projections show that by the end of this century, ocean surface waters could be more than twice as acidic as they were at the end of the previous century if carbon emissions are not reduced. Algae and seagrasses may benefit from higher CO2 conditions in the ocean, as they require CO2 for photosynthesis, and there are studies examining if growing seaweed can slow ocean acidification.
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Impact on marine life
The ocean has always acted as a carbon sink, absorbing excess heat and energy released from rising greenhouse gas emissions. However, the increasing levels of carbon dioxide in the atmosphere from the burning of fossil fuels in cars and power plants are causing the ocean's chemistry to change. Oceans are naturally alkaline, but the interaction with carbon dioxide is making them more acidic. In the past 200 years alone, ocean water has become 30% more acidic—faster than any known change in ocean chemistry in the last 50 million years.
This increase in acidity lowers the amount of carbonate in seawater, a building block of the calcium carbonate that many marine organisms use to grow their skeletons and create coral reefs. As a result, these organisms will grow more slowly, or their skeletons will become less dense, similar to osteoporosis in humans. This means that reefs are threatened because corals may be unable to build them as fast as erosion wears them away. This threat is compounded by warming-induced mass bleaching events, which occur when unusually warm temperatures cause coral to expel the colourful microscopic algae that provide coral polyps with food.
Calcifying organisms, including common marine plankton such as pteropods (a type of snail), are directly threatened by rising carbon dioxide levels. A report by the National Center for Atmospheric Research predicts that the amount of carbon dioxide in the atmosphere will double or triple by the end of the century, which will dramatically alter marine life.
The shellfish industry is also at risk, with a study in the United States of America finding that the industry could lose more than US$400 million annually by 2100 due to ocean acidification.
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Carbon sinks
The ocean's ability to absorb carbon dioxide is crucial for mitigating the impacts of climate change. As greenhouse gas emissions increase, the ocean's capacity to absorb carbon dioxide is reduced, leading to detrimental changes in marine and terrestrial life. The ocean's absorption of carbon dioxide also contributes to its acidification, which has negative consequences for marine ecosystems, such as coral reefs and shellfish.
The ocean carbon pump is a key mechanism in the ocean's ability to act as a carbon sink. It consists of a biological pump that transfers surface carbon towards the seabed through the food web and a physical pump that results from ocean circulation. In the Polar Regions, dense water flows towards the Deep Sea, dragging down dissolved carbon. The cold temperatures in these regions facilitate the absorption of atmospheric CO2, making the Polar Regions particularly important in the carbon cycle.
Healthy coastal ecosystems, such as mangroves, seagrass beds, and salt marshes, also play a significant role in carbon sequestration. These ecosystems capture carbon during their development, storing it in their calcium skeletons. Mangroves, for example, store on average 1,000 tonnes of carbon per hectare in their biomass and soils, while also providing numerous other ecological and economic benefits.
While the ocean is an essential carbon sink, it is important to recognize that the concept of carbon sinks is controversial. Storing CO2 in carbon sinks can release steam water, contributing to the greenhouse effect. Additionally, the increase in greenhouse gas concentrations can modify physical, chemical, and biological equilibriums, affecting the ocean's ability to function as a carbon sink.
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Rising CO2 concentrations
The ocean absorbs about 30% of the carbon dioxide released into the atmosphere. As levels of atmospheric CO2 increase due to human activities such as burning fossil fuels and changing land use (e.g. deforestation), the amount of carbon dioxide absorbed by the ocean also increases. The ocean has absorbed around 25% to 29% of the carbon dioxide released into the atmosphere by humans in recent decades.
The ocean's average pH is now around 8.1, which is basic or alkaline. However, as the ocean continues to absorb more CO2, the pH decreases, and the ocean becomes more acidic. This process is known as ocean acidification. During the 200-plus years since the industrial revolution began, the pH of surface ocean waters has fallen by 0.1 pH units. This represents a 30% increase in acidity.
Ocean acidification has harmful effects on marine life, particularly shellfish and corals, as it reduces the availability of carbonate ions required for building their hard shells and skeletons. It also increases the growth of toxic algae, which can be harmful to humans. Additionally, ocean acidification, along with warming ocean temperatures, can interact to the detriment of marine ecosystems.
The rising concentration of carbon dioxide in the atmosphere is driving up ocean surface temperatures and causing ocean acidification. These changes in the ocean's chemistry slow its ability to uptake CO2, creating a vicious cycle. It is crucial to address the root cause of the problem by reducing unabated CO2 emissions from the burning of fossil fuels and transitioning to renewable energy sources.
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Human activity
Human activities, primarily the burning of fossil fuels, have led to increased levels of carbon dioxide (CO2) in the atmosphere, which is causing ocean acidification. The ocean absorbs about 30% of the CO2 released into the atmosphere, and as the atmospheric concentration of CO2 rises, so does the amount absorbed by the ocean. This has resulted in a decrease in ocean pH, making the seawater more acidic.
Since the Industrial Revolution, human actions have significantly contributed to the increasing concentration of CO2 in the atmosphere. The annual rate of increase in atmospheric CO2 over the past 60 years is about 100 times faster than previous natural increases. For example, the rise in the use of fossil fuels and the associated emissions from vehicles and industries have led to higher CO2 levels. Additionally, deforestation and changing land use practices have also played a role in increasing atmospheric CO2.
The burning of fossil fuels, such as coal, oil, and natural gas, for energy production, transportation, and industrial processes, is a major contributor to the rise in atmospheric CO2. As more fossil fuels are burned, higher levels of CO2 are released into the atmosphere. This increase in atmospheric CO2 has a direct impact on the ocean, as CO2 dissolves into the seawater, triggering a series of chemical reactions.
The increased concentration of CO2 in the ocean leads to an increase in hydrogen ions, which makes the seawater more acidic. This process is known as ocean acidification. Ocean acidification has significant implications for marine life, especially for organisms that rely on calcium carbonate to build their shells and skeletons, such as oysters, clams, and corals. As the ocean becomes more acidic, the availability of carbonate ions decreases, making it more difficult for these organisms to build and maintain their structures.
In addition to the direct impact on marine life, ocean acidification also has indirect effects on ecosystems and human communities. Marine ecosystems, such as coral reefs, provide habitat and resources for a diverse range of species, support fisheries, and offer coastal protection. As ocean acidification weakens these ecosystems, it can disrupt food webs and impact the economies and livelihoods of communities that depend on marine industries.
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Frequently asked questions
Carbon dioxide in the atmosphere dissolves into seawater. When carbon dioxide enters the ocean, it forms carbonic acid, which dissociates into bicarbonate ions and hydrogen ions. This process increases the concentration of hydrogen ions, making the ocean more acidic.
Ocean acidification impacts marine life, especially organisms that require calcium carbonate to build shells and skeletons, such as oysters and corals. The increased acidity of seawater reduces the availability of carbonate ions, making it more difficult for these organisms to build and maintain their shells.
As the ocean becomes more acidic, its ability to absorb carbon dioxide decreases. This is due to the change in the ocean's chemistry, specifically the increased concentration of hydrogen ions and the reduction of carbonate ions. The ocean's capacity to act as a carbon sink is diminished, impacting its ability to mitigate global warming.
