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Explain the causes and effects of ocean acidification.

Causes of Ocean Acidification

The primary cause of ocean acidification is the absorption of large amounts of carbon dioxide (CO₂) by the oceans. As humans burn fossil fuels (such as coal, oil, and gas) for industry and transportation, CO₂ is released into the atmosphere. Since the Industrial Revolution (around 1750), approximately one-third to one-half of this CO₂ has been absorbed by the oceans.

As more carbon dioxide enters the atmosphere, the ocean, as a natural carbon sink, absorbs more carbon dioxide. When CO₂ gas dissolves into the ocean, it reacts with water to form carbonic acid (HCO₃^-), which lowers the water’s pH, making the ocean more acidic.

CO₂ + H₂O + CO₃^2- ➡ 2HCO₃^-

Scientists estimate that the pace of ocean acidification since the Industrial Revolution has been approximately 100 times more rapid than during the past 650,000 years. The average pH of seawater has declined from 8.19 to 8.05, corresponding to a 30% increase in acidity. Explore the graph below. You can see that the annual and monthly average of oceanic pH at the Aloha monitoring station in Hawaii has decreased significantly since 1988:

Oceans and forests are both carbon sinks, but as deforestation has increased, forests haven't been able to absorb as much carbon, exacerbating the problem of ocean acidification.

Additionally, certain natural processes can affect ocean acidity, such as hydrothermal vents, which are underwater hot springs that release acidic substances. However, the current rate of ocean acidification is much faster than natural occurrences in the past, largely due to human-induced CO₂ emissions.

Essential Knowledge

Ocean acidification is the decrease in pH of the oceans, primarily due to increased CO₂ concentrations in the atmosphere, and can be expressed as chemical equations.

As more CO₂ is released into the atmosphere, the oceans, which absorb a large part of that CO₂, become more acidic.

Anthropogenic activities that contribute to ocean acidification are those that lead to increased CO₂ concentrations in the atmosphere: burning of fossil fuels, vehicle emissions, and deforestation.

Effects of Ocean Acidification

Ocean acidification can have widespread effects on the marine ecosystem, which can cascade onto terrestrial ecosystems. The trigger for these cascading effects has to do with how carbonic acid reacts with calcium carbonite, which forms mollusc shells, calcareous algae, and coral, arthropod, and echinoderm skeletons. These organisms make their hard shells and skeletons by combining calcium and carbonate from seawater. As ocean water becomes more acidic, these types of organisms begin to have trouble maintaining their shells, skeletons, and other calcium carbonate structures. If the pH is too low, it can slow the growth of their shells and skeletons and can even begin to dissolve shells faster than they can form.

Infographic explaining how ocean acidification affects marine life. — *The oceans absorb a large amount of carbon from the atmosphere, which can cause ocean acidification because the carbon dioxide reacts with other salts in the water, lowering the pH of the ocean water.*

If this happens to mollusks, arthropods, and echinoderms, they can have trouble maintaining the integrity of their shells or skeletons. This can disrupt food webs, fisheries, and ecosystems. However, if this happens to corals, it causes a cascade of problems because these corals are the foundation of coral reef ecosystems. Coral reefs are built by multiple generations of Cnidarians known as corals that secrete hard skeletons. Corals have a symbiotic relationship with a type of algae known as zooxanthellae, which is why they are found in the photic zone.

Decreasing pH can cause coral bleaching, which often results in coral death. The death of coral reefs could decrease the protection reefs provide against storms, reduce tourism, decrease biodiversity, and risk the stability of marine food chains. Because coral reefs serve as nurseries for many commercially important species, fisheries that are dependent on these fish could collapse, costing jobs and reducing a critical food supply for coastal communities.

Essential Knowledge

Ocean acidification damages coral because acidification makes it difficult for them to form shells, due to the loss of calcium carbonate.

Mitigating Ocean Acidification

All is not lost! Scientists have created several models for ocean acidification and the models show that we can drastically slow ocean acidification if we work together to create sustainable solutions and reduce carbon dioxide emissions.

An international team of climate scientists, economists, and earth systems modelers have built a range of new "pathways" that examine how global society, demographics, and economics might change over the next century. They are collectively known as the "Shared Socioeconomic Pathways" (SSPs). NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) has created several fully coupled climate and earth system models that can utilize the SSPs to glimpse into the future. Explore the three models below. As you can see, we have the chance to decide to make real changes and mitigate some of these effects.

Sustainable Model 🌿 (click to reveal)

In this dataset, the ocean-atmosphere exchange of carbon dioxide is shown, using GFDL’s ESM4 model between 2015 and 2100. SSP1-1.9 is a very low greenhouse gas emissions scenario where carbon dioxide emissions cut to net zero around 2050. The resolution of the climate model is 1/4 degree for the oceans and 1 degree for the atmosphere.

In this dataset, the ocean-atmosphere exchange of carbon dioxide is shown, using GFDL’s ESM4 model between 2015 and 2100. SSP2-4.5 is an intermediate greenhouse gas emissions scenario where carbon dioxide emissions continue around current levels until 2050, then decrease but do not reach net zero by 2100. The resolution of the climate model is 1/4 degree for the oceans and 1 degree for the atmosphere.

A GIF showing oceanic pH from 2015-2100. By 2100, oceanic pH has decreased to approximately 8.0. — ***The sustainable model predicts that by 2100, oceanic pH has decreased to approximately 8.0, which is only about a quarter of the decrease predicted in the worst-case scenario.***

In this scenario, the world shifts gradually, but pervasively, toward a more sustainable path, emphasizing more inclusive development that respects perceived environmental boundaries. Management of the global commons slowly improves, educational and health investments accelerate the demographic transition, and the emphasis on economic growth shifts toward a broader emphasis on human well-being. Driven by an increasing commitment to achieving development goals, inequality is reduced both across and within countries. Consumption is oriented toward low material growth and lower resource and energy intensity.

Middle of the Road 🛣️ (click to reveal)

In this dataset, the ocean-atmosphere exchange of carbon dioxide is shown, using GFDL’s ESM4 model between 2015 and 2100. SSP2-4.5 is an intermediate greenhouse gas emissions scenario where carbon dioxide emissions continue around current levels until 2050, then decrease but do not reach net zero by 2100. The resolution of the climate model is 1/4 degree for the oceans and 1 degree for the atmosphere.

A GIF showing oceanic pH from 2015-2100. By 2100, oceanic pH has decreased to approximately 7.5. — ***The middle of the road model predicts that by 2100, oceanic pH has decreased to approximately 7.5, which is about half the decrease predicted in the worst-case scenario.***

In this scenario, the world follows a path in which social, economic, and technological trends do not shift markedly from historical patterns. Development and income growth proceed unevenly, with some countries making relatively good progress while others fall short of expectations. Global and national institutions work toward but make slow progress in achieving sustainable development goals. Environmental systems experience degradation, although there are some improvements and overall, the intensity of resource and energy use declines. Global population growth is moderate and levels off in the second half of the century. Income inequality persists or improves only slowly and challenges to reducing vulnerability to societal and environmental changes remain.

Fossil-Fueled Development 🏭 (click to reveal)

In this dataset, surface pH is shown as projected by the SSP5 scenario, using GFDL’s ESM4 model between 2015 and 2100. SSP5-8.5 is a very high greenhouse gas emissions scenario - and unlikely to happen - where carbon dioxide emissions triple by 2075. The resolution of the climate model is 1/4 degree for the oceans and 1 degree for the atmosphere.

A GIF showing oceanic pH from 2015-2100. By 2100, oceanic pH has decreased to 7.3. — **The worst-case scenario predicts that by 2100, oceanic pH has decreased to 7.3, which indicates a 10-fold decrease in pH because the pH scale is logarithmic.**

In this scenario, the world places increasing faith in competitive markets, innovation, and participatory societies to produce rapid technological progress and development of human capital as the path to sustainable development. Global markets are increasingly integrated. There are also strong investments in health, education, and institutions to enhance human and social capital. At the same time, the push for economic and social development is coupled with the exploitation of abundant fossil fuel resources and the adoption of resource and energy intensive lifestyles around the world. All these factors lead to rapid growth of the global economy, while global population peaks and declines in the 21st century. Local environmental problems like air pollution are successfully managed. There is faith in the ability to effectively manage social and ecological systems, including by geo-engineering if necessary.

While some species will be harmed by ocean acidification, algae and seagrasses may benefit from higher CO₂ conditions in the ocean, as they require CO₂ for photosynthesis just like plants on land.

Research has shown that kelp, eelgrass, and other sea vegetation can effectively absorb CO₂ and reduce acidity in the ocean. By growing these plants in local waters, scientists believe it could help mitigate the damaging impacts of acidification on marine life. While marine plants can have local benefits, they are not a complete solution to global ocean acidification. The most effective way to address ocean acidification is still to reduce CO₂ emissions at the source. Nonetheless, conserving and restoring marine plant habitats can be a valuable part of a broader strategy to protect our oceans and the life within them.

AP Exam Tip

You should know that while the ocean has helped mediate the effects of climate change for many years, it has not been without consequences. You should know the causes and effects of ocean acidification. Think about the environmental, social, and economic impacts.

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