Oceans & Carbon Dioxide (DP IB Geography): Revision Note

Oceans as a Carbon Dioxide Store

• Oceans contain large amounts of dissolved carbon 

• They are carbon sinks or reservoirs for carbon storage 

• Oceans capture and absorb carbon dioxide from the atmosphere:

  • This is the process of carbon sequestration 

• Carbon moves from surface ocean layers to deeper ocean layers through:

  • Biological Carbon Pump

    • Tiny phytoplankton living in the upper layers need to photosynthesise. They absorb carbon dioxide during this process

    • Phytoplankton act as a source of food for other marine life. Carbon moves through the marine food chain into the different layers of the ocean

    • Species like plankton also sequester carbon dioxide and use it for their skeletons or shells

  • Carbonate Pump

    • After organisms die, skeletons or shells can dissolve into the water, enriching deep ocean currents with carbon 

    • Decaying organisms can also release carbon dioxide

    • As animals breathe, carbon is also released into oceans

    • Dead organisms can build up and eventually compress on the seafloor and turn into limestone sediment

  • Ocean Circulation (physical pump) increases the amount of carbon that can be stored in oceans:

    • Thermohaline circulation moves surface and deep ocean currents around the world in a cyclical pattern

    • Dissolved carbon moves around oceans through this circulation

    • Water density brings carbon-rich waters deep down into ocean stores in a process called downwelling

Diagram illustrating carbon cycling at a 'sere' (lithosere) level

• Oceans are important for the future and for climate change, as they absorb vast amounts of carbon dioxide

• However, warmer waters cannot absorb as much carbon dioxide as colder waters. As ocean temperatures rise, this could worsen climate change by reducing the efficiency of the oceans as a carbon sink

Oceans as a Source of Carbon Dioxide

• Oceans are not only a store but also a source of carbon dioxide

• The process of upwelling can bring carbon-rich waters to the surface; this carbon can then make its way back into the atmosphere

• Disruption to the thermohaline circulation would cause large amounts of carbon to move upwards from deep ocean layers to the surface:

  • The ocean would become a greater source of carbon dioxide than a carbon sink 

  • This could be catastrophic for the climate 

• As climate change worsens, oceans may become a larger source of carbon dioxide:

  • As sea ice melts, ocean waters can mix more freely, bringing up those carbon-rich waters from the depths of the ocean

  • Gasses are released more easily when ocean waters are warmer

Impacts of Ocean Acidification on Coral Reefs

• As a result of burning fossil fuels, more carbon dioxide enters the atmosphere 

• Oceans also absorb lots more carbon dioxide 

• This increase in carbon dioxide increases the acidity of ocean waters 

• This is the process of ocean acidification  

  • Coral reefs need to produce calcium carbonate to grow 

  • Each coral organism (polyp) secretes a skeleton of calcium carbonate; these form the reef

  • As the ocean acidifies, coral reefs struggle to produce this calcium carbonate

  • This means that coral reef skeletons cannot grow as quickly; they become much weaker and are more likely to break

• Coral reefs are a vital food source and shelter for marine life

  • As coral reefs start to degrade, this threatens the marine wildlife that is dependent on coral reefs for survival

  • Coral reefs are also important for humans as they:

    • Are useful for protecting coastlines from erosion and storms

    • Are a hotspot for fishing industries 

    • Bring tourism to local areas

    • Produce extracts that are used in medicine

Image showing the process and effects of ocean acidification