Too Much Carbon Dioxide is Dangerous for the Planet

Earth’s atmosphere contains many gases. Some gases can hold the sun’s energy, warming up the atmosphere like a greenhouse. Because of that, they are called greenhouse gases. The greenhouse effect keeps the climate stable and allows life as we know it to exist. Carbon dioxide (CO₂) is one of the natural greenhouse gases in our planet’s atmosphere. However, for many decades now, humans have been directly releasing CO₂ into the atmosphere by burning fossil fuels and indirectly adding to this gas via deforestation. Because of this, scientists discovered that the temperature of Earth’s atmosphere is rapidly increasing. Fortunately, the ocean helps to balance the increasing CO₂ levels in the atmosphere by taking some of this CO₂ (this Frontiers for Young Minds article tells you how).

The ocean is a big soup of chemical elements. One of these elements is carbon, which is very important to marine life (learn more about carbon in this Frontiers for Young Minds article). There are four principal carbon-containing molecules in the ocean: CO₂, carbonic acid (a very weak acid used to give fizz to sodas), the bicarbonate ion (part of the baking soda used to bake cakes), and the carbonate ion (which helps create the shells of many marine animals). The increase of atmospheric CO₂ is also increasing the concentration of CO₂ in the ocean at a very similar pace. This increase is a big problem because it is turning the ocean acidic.

The ocean has absorbed about 26% of the human-produced CO₂ from the atmosphere over the last decade (2014–2023) [1]. When dissolved in seawater, a molecule of CO₂ gas reacts with a water molecule, forming the bicarbonate ion and releasing a hydrogen ion. In a second reaction, the bicarbonate ion can release another hydrogen ion and a carbonate ion (Figure 1). Because more CO₂ is entering the ocean due to human activities, more hydrogen ions are being generated. The increase in hydrogen ions makes the seawater more acidic. Scientists measure the seawater pH to determine its acidity. The pH of a solution indicates the concentration of hydrogen ions in that solution. The more hydrogen ions that are present in a solution, the lower the pH and the more acidic (like a lemon) the solution is. Conversely, the fewer hydrogen ions that are present, the more basic a solution is. In this case, the solution is seawater. The lowering of the ocean pH due to human activities is called ocean acidification, and it poses a threat to marine life.

The Marine Ecosystem is Becoming More Fragile

Ocean acidification is now measurable thanks to high-precision pH instruments and numerical models, which show that the ocean’s natural balance is changing from the North Pole to the South Pole. The consequences of centuries of releasing CO₂ into the atmosphere are also visible on marine organisms. Many marine organisms—like clams, sea snails, sea urchins, and starfish—are in danger. What do they have in common? Shells. These organisms are called calcifiers. They all use special ingredients in the water to build their shells and skeletons, just as humans use bricks to build houses. They combine ions of calcium and carbonate (the $C{O}_3^{2-}$ in Figure 1) to form calcium carbonate. This mineral gives their shells and skeletons strength.

Calcium carbonate begins to break down in acidic conditions. With ocean acidification occurring, calcifiers must use more energy to build and maintain their shells. This weakens their health. Also, the gradual corrosion of corals, oyster beds, and starfish skeletons makes them fragile and shortens their lifespans. The damage to these biological structures is especially noticeable in pteropods (Figure 2)—tiny, delicate sea snails, also known as sea butterflies. Pteropods play a crucial role as the first food source for many marine animals in the ocean’s food web.

The impacts of ocean acidification are not limited to the calcifiers. Just like humans, all marine life depends on maintaining specific internal conditions to stay healthy. These conditions are influenced by the surrounding environment. In acidic waters, some marine organisms lose their ability to navigate or find prey effectively. Others must use extra energy to regulate their internal pH. This need for extra energy can impair vital processes such as muscle development and growth. Also, many marine species, like fish and sea urchins, have delicate larval stages that are particularly sensitive to low pH. Even if adults adapt and survive, their offspring may struggle to grow properly, leading to a decline of their populations that could ultimately affect other marine organisms.

On the other hand, some organisms may benefit from higher ocean CO₂ concentrations. Seagrasses and algae, for example, use sunlight and CO₂ to produce energy. Elevated CO₂ levels can significantly boost the growth rates of some of these organisms [2, 3]. However, this rapid growth can also increase the production of toxins in the case of some algae, harming other marine animals and even humans [4].

The effects of ocean acidification extend beyond individual species— they affect the entire marine ecosystem. Changing the number and diversity of ocean organisms can trigger changes, particularly within food webs. The decline of one species may reduce the food supply for others. This creates a “ripple effect” that spreads throughout the ecosystem. Over time, these changes can reshape the composition of marine life and eventually impact humans. With fewer animals in the sea, there is less seafood. To avoid a greater impact on marine life and humans, CO₂ emissions must stop, and changes in seawater pH must be monitored. Marine scientists around the world are monitoring ocean conditions and searching for solutions to ocean acidification.

Measuring and Monitoring of Ocean Acidification

To correct ocean acidification, we must first measure it. Nowadays, CO₂ levels are monitored in the atmosphere in many parts of the world. The longest daily record of CO₂ concentration has been collected since 1958, at station Mauna Loa in Hawaii. This record indicates a consistent, significant rise in CO₂ in the atmosphere. It also shows that, like a sponge, our ocean is increasingly absorbing this CO₂ from the atmosphere. Using measurements from seawater samples and chemical equations, scientists can calculate how much the dissolved CO₂ has decreased seawater pH [5]. Furthermore, scientists have determined that surface ocean pH has decreased about 0.1 pH units since the mid-1700s based on measurements and numerical models. This value seems small to us, but not for marine organisms living under specific environmental conditions.

Besides direct measurements of CO₂, scientists also use satellite data to indirectly map CO₂ in Earth’s atmosphere and the ocean’s surface. Satellites measure how much sunlight is reflected from Earth’s surface in the form of infrared radiation. Because CO₂ partly absorbs infrared radiation, satellites can capture the changes in this radiation and use it to estimate how much CO₂ is present in the atmosphere. Scientists also apply mathematics and chemical equations to determine pH levels in the surface ocean using satellite data. Satellites and other instruments help scientists measure CO₂ worldwide and improve our understanding of not only ocean acidification and its impact, but also the carbon cycle as a whole.

Marine scientists are making significant improvements in measuring ocean acidification. They use a collection of instruments, both on ships and released at sea (Figure 3). These instruments measure the ocean’s pH, carbon concentration, temperature, and other water characteristics, helping scientists understand the ocean’s chemistry. Observations over weeks and years are essential for tracking and distinguishing changes in ocean chemistry caused naturally or by humans.

Programs to monitor ocean conditions are important for improving our understanding of the ocean. The better scientists understand the ocean, the better governments and societies can act to prevent damage to marine life. As a community, we should also take action. The public can help scientists and governments by doing simple, everyday actions to decrease CO₂ emissions, like reducing waste, reusing items, and recycling as much as possible. We can also purchase from local markets, choose renewable energy sources, and use public transportation. Getting to know our marine environment and protecting it is a valuable way to fight against ocean acidification.

Ocean acidification results from human actions. However, sustainable human actions can also reverse the impacts of low pH and help protect marine life. This is a big challenge that scientists, the government, and society should face together. Together, we can do this!

Glossary

Fossil Fuels: ↑ Carbon compounds that formed in the Earth’s crust from the remains of prehistoric organisms. Common fossil fuels include petroleum, coal, and natural gas.

Deforestation: ↑ Clearing of trees from large areas by humans. Since trees capture CO₂ to grow, deforestation decreases CO₂ capture and may also release CO₂ into the soil when trees decompose.

Numerical Models: ↑ Computer programmes that use mathematical and physical relations to solve complex calculations that represent the Earth’s actions and environmental structures.

Calcifiers: ↑ Organisms that build hard structures, such as shells or skeletons, by combining calcium and carbonate ions dissolved in water.

Food Web: ↑ The natural interconnections among food chains, meaning what eats what. Each predator-prey relationship is a food chain.

Carbon Cycle: ↑ A series of pathways through which carbon moves from the atmosphere to the vegetation, soil, and water, and after many years returns to the atmosphere.

Renewable Energy: ↑ All energy sources (sun, wind, water, and internal Earth heat) that are naturally available and replenished, hence, they are considered endless.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to thank Umberto (16 years old) for his thoughtful and insightful feedback on this article. His questions and comments helped us explain our research more clearly and ensured that this article is truly understandable for young readers. We are grateful for the time and curiosity he brought to the review process. JMLA acknowledges CALMAR Project (CNPq/MCTI 23/2023) and CNPq (Postdoc Nr. 385549/2024-5). ACOC and MP acknowledge the MARES Project (CNPq/MCTI-FNDCT No. 61/2022 - Process No. 409417/2022-0). This publication is part of the Brazilian Ocean Acidification Network (BrOA, www.broa.furg.br) efforts to community engagement and ocean communication.

AI Tool Statement

The author(s) declared that Generative AI was used in the creation of this manuscript. The authors declared that the generative AI tool ChatGPT (Version: July 2025; Model: GPT-4o; OpenAI; https://chatgpt.com) was used to refine figures. DALL-E 3 (via ChatGPT, Version: July 2025; OpenAI; https://chatgpt.com) and the web-based version of Canva (Canva Pty Ltd, July 2025 version; https://www.canva.com) were utilized to generate or complement Figures 1, 2, and 3. The author(s) curated, reviewed, and edited all AI-generated content to ensure accuracy and scientific integrity.

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