IB Environmental systems and societies 4.1C Oceans as a Carbon Sink Notes

Oceans as a carbon sink: the ocean absorbs about a quarter of human CO2 emissions

1. The oceans are the largest active carbon sink, absorbing roughly 25 to 30% of anthropogenic CO2 from fossil fuel combustion, deforestation, and cement production.
2. This uptake moderates atmospheric CO2 and slows the rate of climate change.
3. The long-term removal and storage of this CO2 in the ocean is called carbon sequestration.

Mechanisms of Carbon Absorption

1. Three carbon pumps move surface carbon into the deep ocean where it stays for centuries.
2. Physical (solubility) pump: CO2 dissolves in surface water and, because cold water holds more CO2, sinks with cold dense water at high latitudes (see 4.1B for gas solubility).
   1. Ocean currents then carry this dissolved inorganic carbon to depth.
3. Biological pump: phytoplankton fix dissolved CO2 into organic matter by photosynthesis.
   1. When they die, this organic carbon sinks as marine snow, burying carbon in deep-sea sediments.

1. Carbonate pump: calcifiers such as corals, molluscs, and coccolithophores build CaCO3 shells and skeletons.
   1. Their remains sink to the seabed and form carbonate sediment that can lock carbon away for millions of years.

Limits to Oceanic Carbon Uptake

1. The ocean's capacity to absorb CO2 is not unlimited.
2. As surface waters take up more CO2, they approach a saturation point where further uptake slows.
3. Warming lowers CO2 solubility, so a hotter ocean is a weaker carbon sink.

Ocean Acidification and Long-Term Sequestration

Short-term storage: dissolved carbon and acidification

1. Dissolved CO2 reacts with water to form carbonic acid (H2CO3).
2. H2CO3 dissociates into hydrogen ions (H+) and bicarbonate (HCO3-), raising H+ and lowering pH.
3. Those extra H+ ions consume carbonate ions (CO3 2-), leaving less available for shell-building.

Surface ocean pH has fallen from about 8.2 before industrialisation to roughly 8.1 today, about a 30% rise in H+ concentration.

Ecological impacts of acidification

1. Reduced calcification: lower carbonate ion availability hinders shell and skeleton formation.
2. Coral reef decline: weaker skeletons reduce reef biodiversity and resilience.
3. Food web disruption: harm to planktonic calcifiers such as coccolithophores alters food supply for higher trophic levels and fisheries.

Long-term sequestration: sedimentation into rock

1. Sinking organic matter and CaCO3 shells accumulate on the seabed as sediment.
2. Over millions of years, pressure and heat compact these sediments into limestone or fossil fuels such as coal, oil, and gas.
3. This is the ocean's slowest, most permanent carbon store, holding carbon over geological time.
4. Extracting and burning fossil fuels reverses this, rapidly returning ancient carbon to the atmosphere.

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