Fossil fuels at Higher Level: The same ideas as SL, with different examples: Alberta's oil sands, Scotland's peat compared with oil, and global fossil emissions. How each fuel formed is taught in more detail later in this topic.
Practise this as you read
- Compare residence times across stores, with numbers.
- Explain how extraction and combustion turn a store into a source.
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Ancient sunlight, locked underground: Fossil fuels are the carbon of ancient plants and plankton, stored underground for millions of years. They were made by sinks, and they become sources when we burn them.
The points to remember
- Coal, oil and natural gas are fossil fuels: organic stores of carbon underground.
- They formed when past ecosystems were carbon sinks: plants and plankton took in more CO₂ than was released.
- Their remains were buried before they could decompose, then changed by heat and pressure over millions of years.
- Coal came mostly from swamp forests; oil and gas mostly from plankton in ancient seas.
Remember it as: Made by old sinks, stored for ages, a source when burned.
Real example: the oil sands of Alberta, Canada, hold oil made from tiny sea organisms buried more than 100 million years ago.
Keep the formation short: You need the idea (past sinks, buried, not decomposed, millions of years), not the detailed chemistry of each fuel.
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Every carbon store keeps its carbon for a different length of time: its residence time.
The points to remember
- Residence time = the average time a carbon atom stays in a store.
- A leaf holds carbon for months; a tree trunk for decades to centuries; peat for thousands of years.
- Fossil fuels hold carbon for hundreds of millions of years.
- No natural process brings it back up quickly, so without mining the residence time is unlimited.
Real example: carbon in the peat of Scotland's Flow Country has been stored for up to 9,000 years. Carbon in Alberta's oil sands had stayed underground about 10,000 times longer, until it was dug up.
'Unlimited' has a condition: Fossil fuels have unlimited residence times only without human interference. Mining and burning end the storage at once: say so.
People now dig up fossil fuels far faster than any natural process could, and burn them for energy.
The points to remember
- Extraction (mining, drilling) moves ancient carbon from underground stores to the surface.
- Combustion oxidises it to CO₂ in seconds: the store becomes a source.
- We release in one year carbon that took millions of years to lock away.
- Sinks (plants, oceans) absorb only about half; the rest accumulates in the atmosphere.
- More CO₂, a greenhouse gas, means global warming; more CO₂ dissolving acidifies the oceans.
Remember it as: Dug up in a day, burned in a second, stored for ages.
Real example: in 2023 burning fossil fuels released about 37 billion tonnes of CO₂. In 2024 the UK closed Ratcliffe-on-Soar, its last coal power station, ending 142 years of burning coal for British electricity.
Reading a rising CO₂ graph: Describe the trend with numbers from the graph (first and last values), then link it to burning fossil fuels: ancient stored carbon released faster than sinks can absorb it.
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How this comes up: Paper 2, Section B (b): explain why fossil fuels are long-term carbon stores and how burning them changes the carbon cycle [7].
Since 1960, the amount of carbon dioxide released by burning fossil fuels each year has roughly quadrupled.
Explain why fossil fuels are described as carbon stores with unlimited residence times, and how their use has changed the carbon cycle.
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