Enhanced Version: Oceans in the Carbon Cycle and Impacts of Acidification
- Oceans are not passive backdrops to the planet’s ecosystems, they are active participants, absorbing, storing, and cycling carbon dioxide (CO₂).
- This process helps stabilize global temperatures but is increasingly under threat from human activities.
- 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
- The oceans act as Earth’s largest carbon sink, absorbing over 25% of the CO₂ emitted into the atmosphere annually.
Oceans as Carbon Sinks
- 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.
- A portion of it dissolves into the ocean’s surface waters.
- This exchange is driven by a natural equilibrium: as atmospheric CO₂ levels rise, the ocean absorbs more CO₂ to restore balance.
- Once dissolved, CO₂ undergoes a series of chemical reactions.
- It reacts with water to form carbonic acid (H₂CO₃), which then dissociates into bicarbonate ions (HCO3−) and hydrogen ions (H+).
- These reactions allow the ocean to store carbon in stable forms over long periods.
Colder waters absorb CO₂ more effectively than warmer waters. This makes polar regions critical in regulating global carbon levels.
- Marine organisms also play a vital role.
- 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.
For example, phytoplankton, microscopic plants in the ocean, use photosynthesis to convert dissolved CO₂ into organic matter.
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:
- Upwelling: In regions where deep, nutrient-rich waters rise to the surface, stored CO₂ can escape into the atmosphere.
For instance, upwelling along the coast of Peru releases carbon-rich water, influencing both local and global carbon cycles.
- 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.
An example of this is during periods of intense volcanic activity or changes in ocean currents, when oceans may release more CO₂ than they absorb, temporarily acting as carbon sources.
NoteAlthough the oceans currently absorb more CO₂ than they release, this balance could shift if global warming disrupts thermohaline circulation or biological processes.
Ocean Acidification: A Consequence of Absorbing CO₂
- The ocean’s ability to absorb CO₂ has helped mitigate climate change, but it comes with a cost: ocean acidification.
- 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
- When CO₂ dissolves in seawater, it forms carbonic acid.
- This acid dissociates, releasing hydrogen ions (H+) that lower the pH.
- Over the past century, the pH of surface oceans has dropped by approximately 0.1 units, a 30% increase in acidity.
- While this change may seem minor, it has profound consequences for marine life.
It’s a common misconception that ocean acidification involves the direct addition of acid to seawater. In reality, it’s the increase in hydrogen ions from CO₂ reactions that drives the pH change.
