Cycles on Earth

In Grade 6 you learned that materials are recycled in cycles. One of the most important is the carbon cycle.

Carbon is an element. It is in the gas carbon dioxide in the air, it is in every living thing, and it is locked inside rocks and fuels underground. The carbon cycle is the never-ending journey carbon takes between these stores.

In Grade 6 the water cycle had four stages: evaporation, condensation, precipitation and collection. That is correct — but the real cycle has more pathways.

Transpiration. Plants draw water up from the soil through their roots and lose it as vapour from their leaves. Across a whole forest, transpiration adds a huge amount of water vapour to the air — almost as much as a lake.

Groundwater. Not all rain runs straight into rivers. Much of it soaks into the ground and slowly moves underground through gaps in soil and rock. This hidden water — called groundwater — feeds springs and wells, and may take years to reach the sea.

Surface run-off. Rain that does not soak in flows over the surface into streams and rivers.

So the journey of a raindrop is not a single straight loop. It might evaporate from the sea, fall on a forest, be drunk up by a tree and transpired, fall again, soak into the ground, travel underground for years, and only then reach a river.

The big Stage 8 idea: a cycle can have many branching routes, not just one path. Water still always returns — but it can take many different journeys to get there.

People often muddle these two words, but they mean different things.

Weather is what the air is doing right now, or over the next few days — sunny, rainy, windy, hot or cold. Weather changes from hour to hour and place to place.

Climate is the average pattern of weather for a place, measured over many years — usually 30 years or more. Climate is what you expect; weather is what you get.

For example, a single cold day in a normally hot country is unusual weather. It does not change that country's climate, which is still 'hot'. To talk about climate you need years of data, not one day.

Earth's cycles can run in balance for millions of years. But human activity can unbalance them.

The carbon cycle. For most of history, the carbon dioxide removed by photosynthesis roughly matched the carbon dioxide returned by respiration and decay. Two human activities now add extra carbon dioxide:

• Burning fossil fuels — coal, oil and gas release carbon that was locked away for millions of years.
• Cutting down forests (deforestation) — fewer trees means less photosynthesis to remove carbon dioxide.

The result is more carbon dioxide entering the air than leaving it. Carbon dioxide traps heat, so the extra gas is warming the planet — a change in climate.

The water cycle. Humans affect this cycle too:

• Covering land with concrete and tar means rain cannot soak in, so less groundwater forms and floods are more likely.
• Taking too much groundwater for farming and cities can drain underground stores faster than rain refills them.

The key Stage 8 idea: cycles depend on balance. When humans add to one part or remove from another, the whole cycle shifts — and that can change weather, climate and water supplies.

Understanding cycles helps with many later topics:

• Ecosystems and food chains — the carbon cycle runs through every food chain, since carbon passes from plants to animals.
• Photosynthesis and respiration — these two processes are the engine that drives the carbon cycle.
• Energy resources — fossil fuels store ancient carbon; renewable energy avoids adding extra carbon dioxide.
• Climate change — the unbalanced carbon cycle is the science behind global warming, one of the biggest issues of your lifetime.

Whenever a topic mentions carbon dioxide, forests, fuels or a warming planet, the carbon cycle from this guide is behind it. Take your time — these ideas matter far beyond the classroom.