B.1.4 Oceans as Carbon Stores and Sources Notes

Enhanced Version: Oceans in the Carbon Cycle and Impacts of Acidification

1. Oceans are not passive backdrops to the planet’s ecosystems, they are active participants, absorbing, storing, and cycling carbon dioxide (CO₂).
2. This process helps stabilize global temperatures but is increasingly under threat from human activities.
3. One major consequence of this disruption is ocean acidification, a process that poses significant risks to marine ecosystems. In this section, we’ll uncover how oceans contribute to the carbon cycle and examine the cascading effects of acidification.

The Role of Oceans in the Carbon Cycle

1. The oceans act as Earth’s largest carbon sink, absorbing over 25% of the CO₂ emitted into the atmosphere annually.

Oceans as Carbon Sinks

1. When CO₂ is released into the atmosphere whether through natural processes like volcanic eruptions or human activities such as burning fossil fuels, it doesn’t remain confined to the air.
2. A portion of it dissolves into the ocean’s surface waters.
3. This exchange is driven by a natural equilibrium: as atmospheric CO₂ levels rise, the ocean absorbs more CO₂ to restore balance.
4. Once dissolved, CO₂ undergoes a series of chemical reactions.
5. It reacts with water to form carbonic acid (H₂CO₃), which then dissociates into bicarbonate ions (HCO3−) and hydrogen ions (H+).
6. These reactions allow the ocean to store carbon in stable forms over long periods.

1. Marine organisms also play a vital role.
2. Some of this carbon is transferred through the food chain, while the rest sinks to the ocean floor as part of the biological pump, where it can remain sequestered for millennia.

Oceans as Carbon Sources

While oceans are efficient carbon sinks, they can also release CO₂ back into the atmosphere. This occurs through processes such as:

1. Upwelling: In regions where deep, nutrient-rich waters rise to the surface, stored CO₂ can escape into the atmosphere.

1. Thermohaline Circulation: The global "ocean conveyor belt" moves carbon between the surface and the deep ocean. Over geological timescales, shifts in temperature and salinity can bring stored carbon back to the surface, where it reenters the atmosphere.

Ocean Acidification: A Consequence of Absorbing CO₂

1. The ocean’s ability to absorb CO₂ has helped mitigate climate change, but it comes with a cost: ocean acidification.
2. This phenomenon occurs as increased CO₂ levels in the atmosphere, once dissolved in oceans, lead to a drop in the ocean’s pH, making the water more acidic.

The Chemistry of Acidification

1. When CO₂ dissolves in seawater, it forms carbonic acid.
2. This acid dissociates, releasing hydrogen ions (H+) that lower the pH.
3. Over the past century, the pH of surface oceans has dropped by approximately 0.1 units, a 30% increase in acidity.
4. While this change may seem minor, it has profound consequences for marine life.

Impacts on Marine Ecosystems

1. Ocean acidification affects marine ecosystems in complex ways, with some of the most significant impacts on organisms that depend on calcium carbonate (CaCO3) to build their shells and skeletons.

Coral Reefs: A Case Study

1. Coral reefs, often referred to as the "rainforests of the sea," are particularly vulnerable to acidification.

1. Corals rely on calcium carbonate to construct their skeletons, but as acidity increases, the availability of carbonate ions (CO32-) a critical component for calcification where it declines. This results in:

• Reduced Calcification Rates: Corals grow more slowly and produce weaker skeletons, making them more susceptible to damage.
• Coral Bleaching: Acidification often coincides with thermal stress caused by rising sea temperatures.

1. Biodiversity Loss: Coral reefs support over 25% of marine species.

Other Marine Organisms

1. Shellfish: Species like oysters, clams, and mussels struggle to maintain their shells in more acidic waters, threatening fisheries and aquaculture industries.

1. Plankton: Certain plankton species, which form the foundation of the marine food web, are also affected by acidification.

The Bigger Picture: Implications and Challenges

1. Ocean acidification is not just an environmental issue, it has social and economic dimensions as well.
2. As marine ecosystems collapse, human communities face significant challenges.
3. Fisheries, which provide food and livelihoods for millions, are particularly at risk.
4. The degradation of coral reefs reduces coastal protection, leaving communities more vulnerable to storms and erosion.

Mitigating the Impacts

1. To address ocean acidification, we must tackle its root cause: CO₂ emissions.

1. Locally, some communities are experimenting with solutions like adding crushed limestone to coastal waters to buffer acidity, though such methods are not feasible on a global scale.

Reflection and Broader Implications

1. The oceans have long acted as Earth’s guardians, absorbing excess CO₂ and shielding us from the worst impacts of climate change.
2. As acidification accelerates, they also serve as indicators of the damage caused by human activity.
3. Understanding the role of oceans in the carbon cycle and the impacts of acidification is not merely academic, it’s a call to action.

1. By delving into these questions, you’ll gain a deeper appreciation of the interconnectedness of Earth’s systems and the urgency of protecting our planet’s most vital resources.