Cycles on Earth

In Grade 6 you saw that water and nutrients move in cycles. Carbon does too. Carbon is a key element in every living thing, and it is endlessly recycled around the planet.

Carbon is stored in several places — the air, living things, the oceans, soil and rocks — and processes move it between them:

• Photosynthesis takes carbon dioxide out of the air, locking carbon into plants.
• Respiration in plants and animals returns carbon dioxide to the air.
• Feeding passes carbon from plants into animals.
• Decay of dead matter by decomposers releases carbon back to the air and soil.
• Combustion (burning) of wood and fossil fuels releases stored carbon to the air.

For thousands of years these processes were roughly balanced — about as much carbon went into the air as came out. That balance is the key to the rest of this topic.

Why does carbon dioxide in the air matter so much? Because it is a greenhouse gas — and greenhouse gases control the temperature of the planet.

Here is how the greenhouse effect works, step by step:

• Sunlight passes easily through the atmosphere and warms the Earth's surface.
• The warmed Earth gives off energy as heat (infrared radiation).
• Greenhouse gases — carbon dioxide, methane and water vapour — absorb some of that heat instead of letting it escape to space.
• This trapped heat keeps the lower atmosphere warmer than it would otherwise be.

It is just like glass in a greenhouse, which lets light in but keeps warmth from escaping.

The crucial point: this is a natural and necessary process. Without any greenhouse effect, the Earth's average temperature would be roughly −18 °C — far too cold for life as we know it. The greenhouse effect is not the problem. The problem comes when we make it too strong.

For thousands of years the amount of carbon dioxide in the air stayed fairly steady, because the carbon cycle was balanced. Over the last two hundred years that balance has been broken.

Humans now add extra carbon dioxide faster than natural processes can remove it. The main causes are:

• Burning fossil fuels — coal, oil and gas burned in power stations, vehicles and factories release carbon that had been locked underground for millions of years.
• Deforestation — cutting down forests removes trees that would have taken carbon dioxide out of the air by photosynthesis. Burning that wood releases carbon too.
• Some farming — livestock and rice fields release methane, another powerful greenhouse gas.

More greenhouse gases means more heat trapped. This strengthens the greenhouse effect, raising the Earth's average temperature. This long-term warming, and the changes that come with it, is called climate change (or global warming when we focus on the temperature rise).

The evidence is wide-ranging: rising average temperatures, melting ice sheets and glaciers, rising sea levels, and more extreme weather events. Scientists are confident the main cause is the extra greenhouse gases produced by human activity.

People often mix up weather and climate, but they mean different things — and the difference is time.

• Weather is the day-to-day state of the atmosphere in one place — temperature, rain, wind, cloud. It can change from hour to hour. "It is raining this afternoon" describes weather.
• Climate is the average pattern of weather in a place over a long time, usually decades. "This region has hot, dry summers" describes climate.

A simple way to remember it: weather is your mood today; climate is your personality over years.

This difference matters when we talk about climate change. One cold day does not disprove climate change — that is just weather. Climate change is about the long-term average shifting. To see it, scientists study patterns over many decades, not single days.

Because human activity is the main cause of climate change, human choices can also reduce it. Every choice that changes how much carbon dioxide we release affects the climate.

Choices that add more carbon dioxide:

• Generating electricity by burning coal, oil or gas.
• Travelling in vehicles that burn petrol or diesel.
• Cutting down forests for farmland or timber.

Choices that reduce carbon dioxide:

• Generating electricity from renewable sources — sunlight, wind and flowing water.
• Walking, cycling or using public transport instead of a car.
• Using energy efficiently — insulating buildings, switching off devices.
• Planting and protecting forests, which absorb carbon dioxide.

These choices are made at every level — by individuals, by businesses and by whole governments. No single action solves climate change, but together, many smaller choices add up to a real difference.

The science you are learning is exactly what these decisions should be based on. Understanding the carbon cycle and the greenhouse effect lets you judge for yourself which choices help and which make the problem worse.

Understanding the carbon cycle and climate change connects to a lot of later science:

• Energy resources — the choice between fossil fuels and renewables is a central physics topic.
• Ecosystems — climate change affects habitats, food chains and biodiversity worldwide.
• Chemical reactions — combustion, photosynthesis and respiration are reactions you will study in more detail.
• Planet Earth — sustainable use of resources is the other half of the same big environmental picture.

Climate change is one of the most important issues of your lifetime. Whenever you read about energy, the environment or extreme weather in the news, the carbon cycle and greenhouse effect from this guide are usually behind it. Take your time here — these ideas matter far beyond the classroom.