Combustion and the Carbon Cycle

Introduction

Every carbon atom in your body has been somewhere else before: in the air, in a plant, perhaps in an ancient ocean or a lump of coal. Carbon is constantly recycled between living things, the air, the oceans and the rocks. For thousands of years these flows were roughly balanced. Burning fossil fuels has changed that by releasing, in a couple of centuries, carbon that was locked away for hundreds of millions of years.

Core explanation

The main stores. Carbon is held in several large reservoirs: the atmosphere (mainly as CO₂), living organisms (as carbohydrates, proteins and fats), the oceans (as dissolved CO₂ and hydrogencarbonate ions), soils, carbonate rocks such as limestone, and fossil fuels deep underground.

Processes that remove CO₂ from the air:

- Photosynthesis — plants and algae use light energy to turn carbon dioxide and water into glucose: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ - Dissolving in the oceans — seawater absorbs CO₂, and marine organisms build shells from carbonates.

Processes that return CO₂ to the air:

- Respiration by plants, animals and microorganisms, which is the reverse of photosynthesis overall. - Decomposition of dead matter by decomposers, which respire. - Combustion of wood and fossil fuels. - Volcanic activity, on a much smaller scale.

How fossil fuels fit in. Coal formed from ancient plants buried in swamps; crude oil and natural gas formed mainly from marine plankton buried under sediment. Heat and pressure over millions of years converted them into carbon-rich fuels. This carbon was effectively removed from the fast cycle. Burning fossil fuels, for example C₈H₁₈ in petrol, converts that stored carbon straight back into CO₂.

An imbalance. Natural sinks — oceans, forests and soils — currently absorb roughly half of the CO₂ humans emit. The rest accumulates in the atmosphere. Atmospheric CO₂ was about 280 parts per million (ppm) before the Industrial Revolution and is now over 420 ppm, a level not seen for millions of years.

The enhanced greenhouse effect. The Sun's radiation warms the Earth's surface, which emits infrared radiation. Greenhouse gases absorb some of this infrared and re-emit it in all directions, including back towards the surface, keeping the planet warmer than it would otherwise be. This natural effect is essential for life. Adding more CO₂ strengthens it, raising average global temperatures. Consequences include melting ice, rising sea levels, more extreme weather and ocean acidification, because extra dissolved CO₂ makes seawater slightly more acidic.

Step-by-step reasoning

Tracing one carbon atom from a fossil fuel:

1. Millions of years ago a plant absorbs CO₂ by photosynthesis. 2. The plant dies and is buried before it decays fully. 3. Heat and pressure turn it into coal. 4. The coal is mined and burnt in a power station, forming CO₂. 5. The CO₂ enters the atmosphere, where it may be absorbed by a tree or the ocean, or remain for a long time.

Visual explanation

Draw the atmosphere as a box at the top of a page, with arrows downward labelled "photosynthesis" and "dissolving in oceans", and arrows upward labelled "respiration", "decomposition" and "combustion". Add a box underground labelled "fossil fuels" with a thin arrow into it over millions of years and a thick, recent arrow out of it labelled "burning".

Real-world analogy

The atmosphere is like a bath with the tap running and the plug slightly open. For a long time water flowed in and out at the same rate and the level stayed steady. Burning fossil fuels is like turning the tap up: the drain cannot keep pace, so the level rises.

Real-world example

Measurements of CO₂ taken continuously at Mauna Loa in Hawaii since 1958 show a steady rise year on year, with a small seasonal wiggle. The wiggle occurs because northern-hemisphere plants absorb more CO₂ during their spring and summer growth, and less in winter.

Why?

Why does burning wood count differently from burning coal? The carbon in wood was absorbed from the air only years or decades ago, so burning it returns carbon to the fast cycle it recently left. The carbon in coal has been locked away for millions of years, so burning it adds extra carbon that the fast cycle has not held for a very long time.

Common misconception

"The greenhouse effect is caused by the hole in the ozone layer." These are separate problems. Ozone depletion lets more ultraviolet radiation reach the surface; the greenhouse effect involves gases trapping infrared radiation emitted by the Earth.

Worked example

Question: How much carbon dioxide forms when 12 kg of pure carbon (as coal) burns completely? (C = 12, O = 16)

Reasoning: C + O₂ → CO₂. 12 g of carbon gives 44 g of CO₂, so the mass ratio is 44 : 12. For 12 kg of carbon, the mass of CO₂ = 12 × 44 ÷ 12 = 44 kg.

Answer: 44 kg of carbon dioxide — more than three and a half times the mass of the fuel, because each carbon atom gains two oxygen atoms from the air.

Quick check

1. Name the process by which plants remove carbon dioxide from the air. Answer: Photosynthesis.

Exam focus

Be able to name processes that add CO₂ to the atmosphere (respiration, decomposition, combustion) and remove it (photosynthesis, dissolving in oceans). Explain the greenhouse effect in terms of infrared radiation being absorbed, and distinguish evidence from predictions.

Advanced insight

Different greenhouse gases differ in strength and lifetime. Methane traps far more infrared per molecule than CO₂ but breaks down within about a decade, whereas a portion of emitted CO₂ stays in the atmosphere for centuries. Scientists combine these into a "global warming potential" to compare emissions of different gases.

Summary

Carbon cycles between the atmosphere, living things, oceans, rocks and fossil fuels. Photosynthesis and ocean uptake remove CO₂; respiration, decay and combustion release it. Burning fossil fuels returns long-stored carbon to the air faster than sinks can absorb it, raising CO₂ levels and strengthening the greenhouse effect, which drives climate change.

Practice questions

1. Give two processes that release carbon dioxide into the atmosphere. Answer: For example, respiration and combustion of fuels. 2. Explain how fossil fuels come to contain carbon. Answer: They formed from ancient organisms that took in carbon (directly or through food chains) from atmospheric CO₂; their remains were buried and slowly converted into coal, oil or gas. 3. How do greenhouse gases keep the Earth warm? Answer: They absorb infrared radiation emitted by the Earth's surface and re-emit some of it back towards the surface. 4. Why has atmospheric carbon dioxide risen even though oceans and forests absorb some of it? Answer: Human emissions are larger than the amount the natural sinks can absorb, so the excess accumulates in the atmosphere. 5. State one effect of extra dissolved carbon dioxide in seawater. Answer: The oceans become slightly more acidic, which makes it harder for some organisms to build carbonate shells.