The Carbon Cycle and Atmospheric Balance

Introduction

The carbon atoms in your body have been almost everywhere. A carbon atom in your breakfast may once have been in a dinosaur, a volcanic gas, a seashell or a lump of coal. Carbon is never used up; it is passed around between the air, living things, the oceans and the rocks. This endless movement is called the carbon cycle . For thousands of years it kept the amount of carbon dioxide in the air roughly steady, and understanding it helps us see why that balance is now changing.

Core explanation

Where carbon is stored. Carbon is found in several large stores:

- the atmosphere , mainly as carbon dioxide (about 0.04% of dry air); - living things , as carbohydrates, proteins and fats; - the oceans , as dissolved carbon dioxide and hydrogencarbonate ions; - rocks , especially limestone and chalk (calcium carbonate); - fossil fuels — coal, oil and natural gas — formed from ancient plants and sea creatures.

Processes that remove carbon dioxide from the air.

- Photosynthesis. Green plants and algae absorb carbon dioxide and use light energy to make glucose: carbon dioxide + water → glucose + oxygen - Dissolving in the oceans. Carbon dioxide dissolves in seawater. Some is used by sea creatures to build shells of calcium carbonate, which may eventually become limestone.

Processes that add carbon dioxide to the air.

- Respiration. Plants, animals and microorganisms release energy from glucose: glucose + oxygen → carbon dioxide + water - Decomposition. When living things die, decomposers such as bacteria and fungi respire as they feed on the remains, releasing carbon dioxide. - Combustion. Burning fuels that contain carbon releases carbon dioxide. For methane, the main gas in natural gas: methane + oxygen → carbon dioxide + water - Volcanoes release carbon dioxide stored deep in the Earth.

Atmospheric balance. Notice that photosynthesis and respiration are the reverse of each other. Over a year, the carbon dioxide taken in by the world's plants was roughly matched by the carbon dioxide released by respiration and decay. Before the Industrial Revolution the carbon dioxide level was about 280 parts per million (ppm) and stayed close to that for thousands of years.

The balance upset. Burning fossil fuels releases carbon that was locked away for millions of years, far faster than photosynthesis and the oceans can absorb it. Cutting down forests also reduces photosynthesis. As a result, carbon dioxide in the air has risen to over 420 ppm today.

Formulae

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O

Step-by-step reasoning

Follow one carbon atom around the cycle:

1. It starts in a carbon dioxide molecule in the air. 2. A leaf absorbs it during photosynthesis, and it becomes part of a glucose molecule. 3. A rabbit eats the leaf; the glucose is digested and used in respiration. 4. The rabbit breathes out the carbon atom as carbon dioxide, and it is back in the air.

Visual explanation

Picture a diagram with the atmosphere at the top and boxes for plants, animals, decomposers, oceans and fossil fuels below. Arrows labelled "photosynthesis" point down from the air into plants and oceans, while arrows labelled "respiration", "decomposition" and "combustion" point back up into the air.

Real-world analogy

The carbon cycle is like a bank account. Photosynthesis and the oceans are withdrawals of carbon dioxide from the air; respiration, decay and burning are deposits. If deposits and withdrawals match, the balance stays steady. Burning fossil fuels is like suddenly paying in an old savings account, so the balance rises.

Real-world example

Measurements at the Mauna Loa observatory in Hawaii since 1958 show carbon dioxide rising every year, with a small zigzag. The level dips each northern summer, when plants photosynthesise strongly, and rises each winter, when respiration and decay dominate.

Why?

Why does burning fossil fuels change the balance when respiration does not? Respiration returns carbon that plants took from the air only recently, so it is part of the short-term loop. Fossil fuels contain carbon removed from the air millions of years ago, so burning them adds extra carbon to the cycle.

Common misconception

"Plants only photosynthesise, and animals only respire." Plants respire all the time, day and night. In daylight they photosynthesise faster than they respire, so overall they take in carbon dioxide.

Worked example

Question: Classify each process as adding or removing carbon dioxide from the air: (a) burning wood, (b) a tree growing, (c) bacteria decomposing dead leaves.

Reasoning: Burning and decomposition (which is respiration by microorganisms) release carbon dioxide. Growth of a tree depends on photosynthesis, which absorbs it.

Answer: (a) adds, (b) removes, (c) adds.

Quick check

1. Name the process that removes carbon dioxide from the air and produces oxygen. Answer: Photosynthesis.

Exam focus

Be ready to draw or label a carbon cycle diagram and to write the word equations for photosynthesis, respiration and combustion. Explanations of rising carbon dioxide should mention both burning fossil fuels and deforestation.

Advanced insight

The oceans have absorbed roughly a quarter of the carbon dioxide released by humans. Dissolved carbon dioxide forms carbonic acid, so seawater has become slightly less alkaline, a change called ocean acidification. This makes it harder for corals and shellfish to build calcium carbonate shells.

Summary

Carbon cycles between the air, living things, oceans and rocks. Photosynthesis and dissolving in the oceans remove carbon dioxide from the air; respiration, decomposition, combustion and volcanoes add it. These processes once kept carbon dioxide close to 280 ppm, but burning fossil fuels and deforestation have raised it to over 420 ppm.

Practice questions

1. Write the word equation for respiration. Answer: Glucose + oxygen → carbon dioxide + water. 2. Name two processes that add carbon dioxide to the atmosphere. Answer: Any two of respiration, decomposition, combustion and volcanic eruptions. 3. Explain how cutting down forests increases carbon dioxide in the air. Answer: Fewer trees means less photosynthesis to remove carbon dioxide, and if the trees are burned or rot, their carbon is released as carbon dioxide. 4. Why is carbon dioxide slightly lower in summer than in winter in the northern hemisphere? Answer: In summer plants photosynthesise strongly and absorb more carbon dioxide; in winter respiration and decay are greater than photosynthesis. 5. Explain why burning fossil fuels upsets the carbon balance. Answer: It releases carbon that was stored underground for millions of years, adding carbon dioxide faster than photosynthesis and the oceans can remove it.