The ocean is a giant carbon sponge: Carbon dioxide from the air dissolves into seawater. The ocean surface is a two-way street: CO₂ goes in and comes back out all the time.
The points to remember
- CO₂ from the air dissolves into seawater at the surface: it becomes part of the water.
- More CO₂ in the air above means more CO₂ dissolves.
- Wind and waves mix the surface, so fresh water keeps meeting the air.
- Dissolved CO₂ stays in the water with no living things needed: the physical pathway.
- The oceans hold about 38 000 billion tonnes of carbon, about 45 times the atmosphere.
Remember it as: In from the air, into the water: like the fizz in a sealed drink.
Think of a fizzy drink: the CO₂ in it is invisible because it is dissolved, like sugar in tea. Seawater holds CO₂ in the same way, not as bubbles.
Real example: In the North Atlantic, strong winter winds mix the cold surface water, which then sinks and carries dissolved CO₂ into the deep ocean. This small part of the ocean holds about a quarter of all the carbon from human activity that the oceans have taken up.
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CO₂ does not stay dissolved forever. It comes out of solution and escapes as a gas when the water warms or when deep water rises in an upwelling.
The points to remember
- Cold water holds more dissolved CO₂; warm water holds less.
- When water warms, CO₂ comes out of solution and escapes as a gas.
- Upwelling brings deep, CO₂-rich water to the surface, where the gas escapes.
- Respiration and decomposition in the sea release CO₂ into the water.
- So cold polar seas mostly absorb CO₂; warm tropical seas often release it.
Reading the relationship: Say both halves: 'as temperature decreases, dissolved CO₂ increases', or 'as temperature increases, dissolved CO₂ decreases'. It is the same reason a warm fizzy drink goes flat faster.
Real example: off the coast of Peru, winds push the surface water away from the shore and cold, deep water rises in its place. This upwelled water is rich in dissolved carbon, so the sea there releases CO₂ to the air.
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Living things move carbon too. Tiny phytoplankton take in dissolved CO₂, just as trees do on land.
The points to remember
- Phytoplankton use dissolved CO₂ in photosynthesis: the biological pathway.
- The carbon becomes part of their bodies, then passes along the food chain.
- Dead plankton, faeces and shells sink to the deep ocean: the biological pump.
- In the deep ocean and sea-floor mud, carbon can stay for thousands of years.
- Some is buried in sediments and may become rock over millions of years.
Physical pathway
- CO₂ simply dissolves
- No living things needed
- Cold water carries it down as it sinks
Biological pathway
- Phytoplankton photosynthesise
- Carbon joins the food chain
- Remains, faeces and shells sink to the deep
Remember it as: Dissolve it, eat it, sink it.
Real example: each spring the North Atlantic turns green with a huge phytoplankton bloom, big enough to see from space. When the bloom dies, much of its carbon sinks to the deep ocean.
Net gain of a store: Net gain = (all flows in) − (all flows out)
The ocean as a sink
- A carbon sink takes in more carbon than it gives out.
- Net gain = (flows in) − (flows out), using every arrow into and out of the store.
- Advantage: oceans are large, absorb a lot of CO₂ and so slow global warming.
- Advantage: dissolved CO₂ supports photosynthesis by phytoplankton (primary productivity).
- Disadvantage: more dissolved CO₂ causes ocean acidification, harming coral reefs and shellfish.
Worked example (Figure above): Flows in: ocean uptake 92.5 + rivers 0.6 = 93.1. Flows out: ocean loss 90.0 + burial 0.2 = 90.2.
Net gain = 93.1 − 90.2 = 2.9 billion tonnes of carbon a year, so the ocean is a sink.
A positive answer means more carbon goes in than comes out: the ocean is a sink. A negative answer would make it a source.
Two traps: Count every flow: forgetting rivers or burial gives the wrong answer. And for an advantage, say what the sink does (absorbs CO₂, slows warming), not just 'it is a carbon sink'.
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The ocean is a sink, but it cannot keep up. Fossil fuels release carbon that was stored for millions of years in a few decades, and a warming ocean may absorb less in future.
The points to remember
- Burning fossil fuels releases carbon faster than the ocean can absorb it.
- The ocean takes up only about a quarter of the CO₂ we release; much of the rest stays in the air.
- Warmer oceans hold less CO₂, and may release some.
- The more CO₂ dissolves, the closer surface water gets to saturation, so uptake slows.
- Warm surface layers mix less with deep water (stratification), so fewer nutrients rise.
- Acidification can reduce phytoplankton, so the biological pump weakens.
Two words to use: saturation (the water cannot take up much more) and stratification (a warm lid on the ocean that stops mixing).
Could weaken the sink
- Warmer water holds less CO₂
- Stratification cuts nutrients
- Acidification harms plankton
- Rising seas destroy coastal marshes and mangroves
Could strengthen it
- More CO₂ in the air pushes more in
- Warmth may speed photosynthesis in cold seas
Real example: Since 1750 the oceans have absorbed roughly a quarter of the CO₂ released by people. Without them, the air would hold far more CO₂ than today's 420 ppm.
How this comes up: Paper 2, Section A: a carbon-cycle figure, then calculate the ocean's net gain [1] and describe one advantage and one disadvantage of the oceans as a carbon sink [2].
The Southern Ocean around Antarctica absorbs a large share of the carbon dioxide released by people each year.
Describe one advantage and one disadvantage of the oceans as a carbon sink.
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