The role of oceans in the global carbon cycle

Oceans play a pivotal role in the global carbon cycle, acting as both a major store and a dynamic regulator of carbon dioxide (CO₂). They are critical in maintaining the balance of carbon across Earth's systems, influencing climate and supporting marine life through various processes.

Importance of oceans in carbon regulation

• Largest carbon sink - Oceans hold over 90% of the world's carbon, making them the most significant reservoir on the planet, far surpassing other stores like the atmosphere or fossil fuels.
• Photosynthesis by plankton - Tiny marine organisms, known as plankton, convert CO₂ into organic material through photosynthesis. This process helps remove carbon from the atmosphere and incorporates it into the marine food chain.
• Carbon sequestration in deep ocean - Over time, organic carbon from plankton and other marine life sinks to the ocean floor, where it can be stored as sediments for millennia, effectively locking away carbon.
• Potential as a carbon source - Through thermohaline circulation, a global ocean current system, carbon stored on the ocean floor can be brought back to the surface. Under certain conditions, this could release CO₂ back into the atmosphere, turning oceans into a source rather than a sink.
• Long-term carbon transfer - The movement of carbon between the ocean and atmosphere operates on extended timescales, often spanning thousands of years, highlighting the slow but significant impact of oceanic processes on global carbon levels.

Carbon storage and fluxes involving oceans

Oceans interact with other carbon stores through a series of fluxes, or transfers, that dictate how carbon moves between different parts of the Earth system. These interactions are quantified in gigatonnes of carbon (GtC) for storage and GtC per year for fluxes, providing a clear picture of the ocean's role in the carbon cycle.

Key carbon stores and their magnitudes

Major carbon fluxes involving oceans

• Atmosphere to surface ocean - Approximately 95 GtC per year moves from the atmosphere into the surface ocean as CO₂ dissolves into the water.
• Surface ocean to atmosphere - Around 105 GtC per year is released back into the atmosphere from the surface ocean, showing a near balance with absorption.
• Surface ocean to deep ocean - About 90 GtC per year transfers from the surface to the deep ocean, often through sinking organic material or currents.
• Deep ocean to surface ocean - Roughly 85 GtC per year returns to the surface from the deep ocean via upwelling currents like thermohaline circulation.
• Surface ocean to marine biota - Approximately 45 GtC per year is taken up by marine organisms through photosynthesis and feeding.
• Marine biota to surface ocean - About 35 GtC per year returns to the surface ocean as organic material decomposes or is excreted.
• Marine biota to deep ocean - Around 6 GtC per year sinks to the deep ocean as dead organisms or waste settle to the seafloor.
• Deep ocean to sediments - A small but significant 0.3 GtC per year is incorporated into ocean floor sediments, representing long-term storage.

The process and impact of ocean acidification

Ocean acidification is a direct consequence of increased carbon dioxide levels in the atmosphere, primarily due to human activities. This process alters the chemical balance of seawater, with wide-ranging implications for marine environments.

Steps in ocean acidification

1. Dissolution of atmospheric CO₂ - Carbon dioxide from the atmosphere dissolves into the ocean surface, combining with water (H₂O) to initiate the acidification process.
2. Formation of carbonic acid - The dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃), a weak acid that quickly dissociates in seawater.
3. Release of ions - Carbonic acid breaks down into hydrogen ions (H⁺), bicarbonate ions (HCO₃^-), and carbonate ions (CO₃^2-). The increase in hydrogen ions raises the acidity of the water, lowering its pH.
4. Disruption of marine chemistry - The higher acidity and reduced availability of carbonate ions affect marine organisms that rely on these ions for building shells and skeletons.

Anthropogenic causes of acidification

• Industrial emissions - Carbon emissions from factories, power plants, and other industrial sources contribute significantly to the increased CO₂ in the atmosphere.
• Transport emissions - Vehicles and aeroplanes release substantial amounts of CO₂, adding to the atmospheric load that eventually dissolves into oceans.
• Rate of absorption - Oceans currently absorb around 1.2 million tonnes of CO₂ every hour, leading to a 25% increase in acidity compared to pre-industrial levels.

Effects of acidification on marine ecosystems

The increasing acidity of oceans poses severe threats to marine life, particularly to organisms that depend on calcium carbonate for their structural integrity. This environmental change disrupts ecosystems and food chains, with broader implications for global biodiversity.

Impacts on marine organisms and habitats

• Coral reefs - Acidification reduces calcification in corals, slowing their growth rates and weakening their skeletal structures. For instance, on the Great Barrier Reef, growth rates of certain coral species have dropped by 12% since 1995 due to combined acidification and thermal stress.
• Shellfish beds - Organisms like oysters and clams struggle to build and maintain their shells in more acidic waters, leading to higher mortality rates and reduced populations.
• Fish stocks - The decline in coral reefs and shellfish impacts fish that rely on these habitats for food and shelter, threatening commercial and subsistence fisheries.
• Ecosystem-wide effects - The loss of key species due to acidification disrupts marine food webs, affecting biodiversity and the balance of oceanic ecosystems.