We have not invented any new part of the carbon cycle. What we have done is take flows that ran slowly for millions of years and speed them up, and take stores that were quietly filling and turn them into sources. This page is the story of that reversal.
📚 What you need to know
Fossil fuels are carbon stores with extremely long residence times, formed when past ecosystems acted as sinks.
Burning them turns those stores into sources, releasing carbon dioxide into the atmosphere.
Agricultural systems can be sinks or sources depending on how they are managed.
Regenerative practices such as crop rotation, cover cropping and no-till farming build soil organic matter, making soil a sink.
Draining wetlands, monoculture and intensive tillage release soil carbon, making soil a source.
Oceans are a major carbon sink, but we are emitting faster than they can absorb, and warmer water absorbs less.
Dissolved carbon dioxide lowers ocean pH — ocean acidification — which weakens shells and coral skeletons.
Fossil fuels: the reversal
Coal, oil and gas formed when ancient organisms died in conditions where decomposers could not reach them. Their carbon was buried, compressed and heated over millions of years. For all that time, those ecosystems were acting as carbon sinks: taking carbon out of circulation and locking it away.
We found that store and started burning it. Every tank of fuel and every tonne of coal moves carbon that had been out of the cycle for hundreds of millions of years back into the atmosphere within seconds.
This mismatch of timescales is the heart of the problem. Natural sinks cannot absorb carbon anywhere near as fast as we are releasing it.
Farming: sink or source, depending on how it is done
Soil holds a great deal of carbon as organic matter. Whether a farm adds to that store or drains it comes down to management.
Practice
What it does to soil carbon
Role
Crop rotation and cover cropping
Keeps roots and plant residues in the ground, so organic matter builds up
Sink
No-till farming
Leaves soil structure intact, so stored carbon is not exposed and broken down
Sink
Intensive tillage
Ploughing exposes soil organic matter to oxygen, so decomposition speeds up
Source
Draining wetlands
Lets oxygen into waterlogged peat, so long-preserved carbon decomposes
Source
Monoculture
Less varied root systems and residues, so less organic matter returns to the soil
Source
There is a pattern behind that table. Anything that keeps soil covered and undisturbed stores carbon. Anything that exposes soil to air releases it. If you meet an unfamiliar farming practice in an exam, ask which of those two it does.
Timber and forestry
Forests managed sustainably for timber can be significant carbon sinks: trees sequester carbon dioxide as they grow and store it in woody biomass and soil organic matter. The balance flips when harvesting outpaces regrowth. If wood is cut and burned faster than new trees can take carbon back in, release exceeds uptake and the forest becomes a source.
The ocean: an overloaded sink
Carbon dioxide dissolves in sea water, and the ocean is one of the largest carbon sinks on the planet. Dissolved carbon dioxide can also come back out as a gas when conditions change — particularly when the water warms.
Two problems are stacking up. First, we are emitting carbon dioxide faster than the oceans can absorb it, so atmospheric levels keep rising. Second, human-driven warming is heating the oceans, and warmer water holds less dissolved gas — so the sink itself is weakening exactly when we need it most.
Ocean acidification
Notice this is a separate problem from warming. Even if the climate were unaffected, dissolving that much carbon dioxide would still acidify the oceans.
The examiner’s framing: human activities cause particular systems — fossil fuel reserves, farm soils, oceans, forests — to switch between acting as sinks and acting as sources. Until the industrial revolution, fossil fuels were a store. We reversed that.
Worked examples
WORKED EXAMPLE
Explain how burning fossil fuels changes their role in the carbon cycle.
Step 1: their original role
Fossil fuels were formed when ancient ecosystems acted as sinks, trapping organic carbon underground with a residence time of hundreds of millions of years.
Step 2: what combustion does
Burning combines that locked carbon with oxygen, releasing carbon dioxide into the atmosphere.
Step 3: the change of role
Output now vastly exceeds input, because no new fossil fuel is forming on that timescale.
A long-term store has become a major carbon sourceInclude the residence time. It is what makes this different from burning a log.
WORKED EXAMPLE
A farmer switches from intensive ploughing to no-till farming with cover crops. Explain the effect on soil carbon.
Step 1: what ploughing was doing
Tillage exposed soil organic matter to oxygen, speeding up decomposition and releasing carbon dioxide, so the soil acted as a source.
Step 2: what changes
Leaving soil undisturbed slows decomposition, and cover crops add roots and plant residues that build organic matter.
Step 3: the new balance
Inputs of organic carbon now exceed losses.
The soil shifts from source to sinkTwo practices, two separate effects. Explain both if the question names both.
WORKED EXAMPLE
Explain why warming oceans make the problem of rising atmospheric carbon dioxide worse.
Step 1: the normal role
Oceans act as a major sink, absorbing carbon dioxide by dissolving it in sea water.
Step 2: the effect of warming
Warmer water holds less dissolved gas, so absorption falls and dissolved carbon dioxide can come back out of solution.
Step 3: the feedback
More carbon dioxide stays in the atmosphere, which drives more warming, which weakens the sink further.
A positive feedback loop that reduces the ocean’s ability to absorb carbonNaming this as positive feedback links straight back to Topic 2.1 and usually scores.
💡 Exam tip
Frame every answer as a change of role: store to source, source to sink. That is the language of this sub-topic.
For farming questions, decide whether the practice disturbs soil or protects it, then explain the consequence.
Keep ocean acidification and ocean warming as separate effects. Questions often ask about only one.
Use the rate argument for fossil fuels: emitting faster than natural sinks can absorb.
Learn the acidification chain in four steps. It converts a hard three-mark question into a list.
⚠ Common mix-up
Saying the ocean becomes an acid. It becomes less alkaline — the pH falls, but sea water stays above pH 7.
Confusing acidification with warming. One is a chemistry change from dissolved carbon dioxide; the other is a temperature change.
Treating all agriculture as a carbon source. Managed well, soil is a sink.
Saying trees always solve the problem. Only growing forests are sinks, and harvested wood that is burned releases the carbon again.
Forgetting that no new fossil fuel is forming on any useful timescale, which is why this store cannot refill.
Blaming only carbon dioxide. Burning coal also releases sulfur dioxide, which causes acid rain and lowers soil pH.
Up next: Cutting Our Impact on the Carbon Cycle — the eight measures on the syllabus, and how to evaluate them rather than just list them.
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