Carbon is everywhere
Carbon is in your body, the air you breathe out, the food you eat, the wood of trees, the shells of sea creatures and the gasoline in cars. Carbon atoms don’t disappear; they move from one place to another over and over. This journey is called the carbon cycle.
Where carbon is stored
| Store | Form of carbon |
|---|---|
| Atmosphere | Carbon dioxide (CO₂) and methane gas |
| Living things | Sugars, proteins, fats, wood |
| Oceans | Dissolved CO₂; shells of sea creatures |
| Soil | Dead plants, animals and wastes |
| Rocks | Limestone and chalk (made from ancient shells) |
| Underground | Fossil fuels: coal, oil and natural gas |
The largest stores are rocks and the deep ocean. The atmosphere holds a relatively small amount, but changes there have a big effect on climate.
How carbon moves
Photosynthesis: air to plants
Plants and algae take in carbon dioxide from the air or water. Using sunlight, they turn it into sugar in a process called photosynthesis.
carbon dioxide + water + sunlight → sugar + oxygen
The carbon becomes part of the plant’s leaves, stems and roots.
Eating: plants to animals
When animals eat plants (or eat animals that ate plants), carbon moves into their bodies.
Respiration: living things to air
All living things release energy from food through respiration. This gives off carbon dioxide.
sugar + oxygen → carbon dioxide + water + energy
You breathe out carbon dioxide with every breath.
Decomposition: dead things to soil and air
When plants and animals die, decomposers like fungi and bacteria break them down. This decomposition releases carbon dioxide into the air and adds carbon to the soil.
Oceans: air to water and back
The ocean absorbs carbon dioxide from the air, and releases some back. Sea creatures use dissolved carbon to build shells. When they die, their shells sink and can slowly form limestone rock over millions of years.
Fossil fuels: the slow path
Millions of years ago, some dead plants and sea creatures were buried before they could decompose. Over time, heat and pressure turned them into coal, oil and natural gas. This locked away carbon for millions of years.
Volcanoes
Volcanoes release carbon dioxide from deep inside Earth, slowly returning carbon from rocks to the air.
Fast and slow cycles
| Fast carbon cycle | Slow carbon cycle |
|---|---|
| Days to centuries | Millions of years |
| Photosynthesis, respiration, decomposition, ocean exchange | Rock formation, fossil fuel formation, volcanoes |
Humans and the carbon cycle
Humans have changed the carbon cycle in two main ways:
- Burning fossil fuels for cars, electricity and heating releases carbon that was locked away for millions of years, in just a few centuries.
- Cutting down forests releases carbon stored in trees and removes plants that would absorb CO₂.
As a result, the amount of carbon dioxide in the air has risen from about 280 parts per million before the Industrial Revolution to more than 420 today. Carbon dioxide traps heat, so this increase is warming Earth’s climate.
The extra CO₂ also dissolves in the ocean, making it slightly more acidic. This makes it harder for corals, clams and oysters to build their shells.
Carbon sinks
A carbon sink absorbs more carbon than it releases.
| Carbon sink | How it stores carbon |
|---|---|
| Forests | Trees store carbon in wood, leaves and roots |
| Oceans | Absorb about a quarter of human CO₂ emissions |
| Peatlands and wetlands | Waterlogged soil slows decomposition |
| Soils | Store carbon from dead plants |
Canada’s boreal forest and peatlands store huge amounts of carbon, making them important to protect.
What can we do?
- Use less energy and choose renewable sources like wind, solar and hydro.
- Walk, bike or use public transit.
- Plant trees and protect forests and wetlands.
- Reduce waste; landfills release methane, a powerful greenhouse gas.
Common mistakes
- Thinking plants only take in carbon dioxide. Plants also respire and release some CO₂.
- Thinking carbon gets used up. Carbon atoms are recycled; they just move between stores.
- Thinking all CO₂ is bad. It’s essential for plants; the problem is too much, too fast.
Try it yourself
- Draw the carbon cycle with arrows showing photosynthesis, respiration, decomposition, combustion and ocean exchange.
- Trace the journey of one carbon atom from a car’s exhaust to your lunch.
- Put a sprig of pondweed in water with a few drops of indicator (with an adult). Leave one jar in light and one in the dark. How does the colour change?
Quick recap
- Carbon moves among the air, living things, oceans, soil and rocks.
- Photosynthesis removes CO₂ from the air; respiration and decomposition return it.
- Fossil fuels and limestone store carbon for millions of years.
- Burning fossil fuels and clearing forests add extra CO₂ to the air.
- Forests, oceans and wetlands act as carbon sinks.