Carbon Sequestration & Ocean Acidification (HL) (DP IB Environmental Systems & Societies (ESS))

Carbon Sequestration & Ocean Acidification

The role of oceans in carbon sequestration

• The oceans act as a carbon sink, absorbing large amounts of carbon dioxide (CO₂) from the atmosphere

  • This helps slow the increase of CO₂ in the atmosphere, which has been rising due to human activities like the burning of fossil fuels

  • By absorbing CO₂, the oceans have been moderating the pace of climate change

  • Without this natural process, the effects of global warming would be even more severe

• Carbon sequestration in the ocean occurs when CO₂ dissolves in seawater

• It is stored either as:

  • Dissolved carbon

  • In marine organisms

  • In sediments on the ocean floor

• This process of CO₂ dissolving into the water at the ocean's surface is responsible for much of the carbon stored in oceans

• Over the long term, carbon is also sequestered through biological processes, such as when marine organisms absorb CO₂ to build their shells or tissues

• While the oceans play a vital role in sequestering carbon, they may eventually reach a saturation point

  • This would occur when they can no longer absorb CO₂ as efficiently

  • If this happens, CO₂ levels in the atmosphere could rise even faster, accelerating climate change

Ocean acidification

• As CO₂ is absorbed by the ocean, it reacts with seawater to form carbonic acid

• This lowers the pH of the water and causes ocean acidification

  • Ocean acidification harms marine life, especially species with calcium carbonate shells or skeletons, e.g. corals, molluscs, and some plankton

  • Their ability to form and maintain their shells is reduced, affecting marine food webs

  • The continued acidification of the oceans could have significant ecological impacts, including the collapse of certain marine ecosystems, such as coral reefs

• Short-term sequestration of carbon in the ocean mainly occurs as dissolved CO₂, which contributes to this acidification process

Long-term carbon sequestration

• Seabed sediments contain both organic and inorganic carbon

  • Organic carbon is stored in the remains of dead marine organisms, e.g. plankton

    • This organic matter sinks to the ocean floor after these organisms die

    • Over time, some of this organic matter becomes buried in sediments and is not fully decomposed, thus trapping carbon.

  • Inorganic carbonates are formed when dissolved CO₂ in seawater reacts with minerals like calcium to form compounds such as calcium carbonate (CaCO₃)

    • Marine organisms use these compounds to build their skeletons and shells

    • When these organisms die, their shells and skeletons accumulate on the seabed, becoming part of the sediments

    • This process sequesters carbon in a more stable form, often for millions of years

• Over geological time, these seabed sediments may undergo further changes due to pressure and temperature

  • This eventually transforms them into fossil fuels such as coal, oil, and natural gas

  • This long-term carbon storage process helps regulate the global carbon cycle by preventing large amounts of CO₂ from re-entering the atmosphere

    • However, the extraction and burning of fossil fuels by humans to generate energy is rapidly releasing all this previously stored carbon into the atmosphere