We have not invented any new carbon. What we have done is change which parts of the cycle act as sinks and which act as sources. Fossil fuels spent millions of years quietly absorbing carbon; in about two hundred years we turned them into one of the largest sources on the planet.
📘 What you need to know
Fossil fuels are carbon stores with extremely long residence times.
They formed when past ecosystems acted as carbon sinks, trapping organic carbon underground.
Burning them turns them into carbon sources, releasing carbon dioxide into the atmosphere.
Agriculture can be a sink or a source depending on how the land is managed.
Oceans are normally a major sink, but they cannot absorb carbon dioxide as fast as we emit it.
Acidification reduces calcium carbonate deposition, weakening shells and coral skeletons.
Fossil fuels: sinks turned into sources
Coal, oil and natural gas began as ancient plants and marine organisms. Their remains were buried, and heat and pressure slowly converted them. For that whole period the ecosystem was a sink — carbon went in and effectively never came out.
Burning reverses it. The carbon locked in those hydrocarbons combines with oxygen and leaves as carbon dioxide. The store becomes a source, and the switch is entirely our doing.
The point examiners want. Human activity does not create carbon — it changes the rate at which carbon moves between stores. Fossil carbon that would have stayed put for hundreds of millions of years now enters the atmosphere in a single combustion reaction.
Agriculture: it can go either way
Farmland is genuinely interesting here, because the management decisions decide the outcome.
Acting as a sink
Acting as a source
Crop rotation, which keeps soil covered and fed
Draining wetlands, which exposes stored organic matter to oxygen
Cover cropping between main harvests
Intensive tillage, which breaks up soil and speeds decomposition
No-till farming, which leaves soil structure intact
Monoculture, which lowers soil organic matter over time
The mechanism behind the left column is the same in each case: more organic matter stays in the soil. The mechanism behind the right column is also the same: more organic matter is broken down and released.
Timber and forestry
Sustainably managed forests act as significant sinks — trees sequester carbon dioxide through photosynthesis and store it in woody biomass and soil. Clear-cutting flips the same forest into a source, because harvested wood that is burned releases its carbon far quicker than new growth can take it back up.
If a question asks whether an agricultural system is a sink or a source, do not guess from the word “farm”. Look for the clue in the practice described — anything that adds or protects soil organic matter points to a sink.
Oceans: a sink under strain
Carbon dioxide dissolves in seawater, and it can come back out again as a gas when conditions change — particularly when the water warms. Normally the ocean absorbs a large share of atmospheric carbon dioxide, which helps regulate the climate.
Two things are undermining that:
We are releasing carbon dioxide faster than the oceans can absorb it, so atmospheric levels keep rising.
Warmer water holds less dissolved gas, so as the climate warms the ocean’s ability to act as a sink is reduced.
Ocean acidification
The carbon dioxide that does dissolve has a chemical consequence. It lowers the pH of the seawater, and even small drops in pH make it harder for organisms to build structures out of calcium carbonate.
Each box causes the next one. Writing the chain out like this is exactly what an “explain the consequences” question is asking you to do.
The knock-on effects are ecological, not just chemical. Weakened shells leave molluscs more vulnerable to predators, and reduced coral growth produces smaller, less diverse reef structures that support fewer species.
WORKED EXAMPLE
Explain why burning fossil fuels has a much larger effect on atmospheric carbon dioxide than the respiration of living organisms, even though both release carbon dioxide.
Step 1: Think about where the carbon came fromRespired carbon was taken from the air recently by photosynthesisStep 2: Compare with fossil carbonFossil carbon left the cycle millions of years ago and was not going to returnStep 3: State the differencerespiration recycles; combustion addsCombustion moves carbon out of a very long-term store into the atmosphere
WORKED EXAMPLE
A farm switches from intensive tillage to no-till farming with cover crops. Soil carbon rises from 32 to 41 tonnes per hectare over ten years. Calculate the mean annual change and state the farm’s role.
Step 1: Find the total change41 − 32 = 9 tonnes per hectareStep 2: Divide by the number of years9 ÷ 10 = 0.9Step 3: Decide the roleCarbon is accumulating, so input exceeds output0.9 tonnes ha−¹ yr−¹ – the soil is acting as a sink
💡 Exam tip
Frame every answer as a change of role: this system used to be a sink or store, human activity turned it into a source.
Say rate. The problem is not that carbon moves, it is how fast we move it compared with natural processes.
For agriculture, always link the practice to soil organic matter — that is the mechanism mark.
Ocean acidification questions want the full chain: dissolved carbon dioxide, lower pH, less calcium carbonate, weaker shells.
Mention that warming water absorbs less carbon dioxide — it is a positive feedback that many students miss.
⚠ Common mix-up
Saying humans “create” carbon. We move it between stores; the total stays the same.
Ocean acidification confused with warming. They share a cause but they are different problems — one is chemical, one is thermal.
Thinking acidified seawater becomes an acid. The pH falls but stays above 7; it becomes less alkaline.
Assuming all farming is a carbon source. Regenerative practices make soil a sink.
Forgetting the ocean is still absorbing. It has not stopped being a sink — it just cannot keep up.
Up next: Cutting Our Impact on the Carbon Cycle — the two things any strategy can do, and the named examples worth learning.
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