The carbon cycle is Earth's natural way of moving carbon around, which is super important for all life on our planet.

The carbon cycle has two main speeds: a fast one and a slow one. The fast cycle is also called the biological carbon cycle. It happens pretty quickly, sometimes in just a few years! Carbon moves from the air to plants and animals, and then back to the air. Think of plants breathing in carbon dioxide and animals breathing it out. The slow cycle, also called the geological carbon cycle, is much, much slower. It can take millions of years! This cycle moves carbon deep into Earth's crust, through rocks, soil, oceans, and the air. It's like carbon taking a very long nap inside rocks.

For many centuries, people have been changing this natural carbon cycle. We do this by changing how we use land and by digging up and burning ancient carbon, like coal, oil, and gas (these are called fossil fuels). Because of this, the amount of carbon dioxide in the air has gone up a lot, making our planet warmer. This extra carbon dioxide also makes the ocean a little more acidic, which can be bad for sea creatures. But remember, carbon dioxide is also super important for plants to grow through a process called photosynthesis!

“The slow cycle, also called the geological carbon cycle, is much, much slower. It can take millions of years!”

Carbon doesn't just float around randomly; it lives in different 'homes' or storage places called carbon reservoirs. Scientists usually divide these homes into five main areas. First, there's the air, which we call the atmosphere. Then there's the terrestrial biosphere, which means all the living things on land, like plants and animals. The ocean is another huge home for carbon, including all the sea creatures and dissolved carbon. Next are the sediments, which include things like fossil fuels and dead stuff at the bottom of lakes. And finally, there's Earth's inside, like the mantle and crust, which hold a lot of carbon too. Carbon moves between these homes through different natural processes like chemical reactions, physical changes, and living things.

The ocean actually holds the biggest amount of active carbon near the Earth's surface! Normally, the amount of carbon moving between the air, ocean, land, and sediments is pretty balanced. This means that without people changing things, the carbon levels would stay about the same.

In the air, carbon mainly exists as two gases: carbon dioxide and methane. Both of these gases are like a blanket, trapping heat and making Earth warm enough to live on. This is called the greenhouse effect. Methane is better at trapping heat than carbon dioxide, but there's much less of it, and it doesn't stay in the air as long. So, carbon dioxide is the main gas that causes the greenhouse effect.

Plants take carbon dioxide out of the air through photosynthesis. Carbon dioxide also dissolves into water, like oceans and lakes, and even in raindrops! When it dissolves in water, it can make the water a little acidic. Over the last 200 years, human activities have increased the amount of carbon in the air by almost 50%! This is mostly from burning fossil fuels and making things like cement.

Way, way in the future (like billions of years from now!), the Sun will get brighter. This will speed up how rocks absorb carbon dioxide, and eventually, most of the carbon dioxide will be trapped in Earth's crust. This means plants won't be able to do photosynthesis anymore. Even further in the future, when Earth's oceans dry up, plate tectonics (how Earth's big pieces move) might stop. This would mean no more volcanoes releasing carbon dioxide, and the carbon cycle would end.

“Over the last 200 years, human activities have increased the amount of carbon in the air by almost 50%!”

The terrestrial biosphere includes all the carbon in living and dead plants and animals on land, plus the carbon stored in the soil. There's a huge amount of carbon stored in plants above ground, and even more in the soil! Most of this carbon is organic carbon, which means it comes from living things. Plants take carbon dioxide from the air and turn it into organic carbon. Animals get carbon by eating plants or other animals.

Because plants take in carbon, the amount of carbon in the air changes a little bit with the seasons and even during the day. When plants grow a lot, like in spring and summer, they take in more carbon. Carbon leaves the land in different ways. When things burn or decay, carbon goes back into the air. It can also wash into the ocean or stay in the soil for thousands of years. Scientists have noticed that carbon released from soil has been increasing, probably because warmer temperatures make things decay faster.

The ocean is like a giant sponge for carbon! It has a top layer that mixes with the air often, and a deep layer where water stays for hundreds of years. The deep ocean holds much more carbon than the air, about 50 times more! Carbon gets into the ocean mainly by carbon dioxide from the air dissolving into it. It can also come from rivers. Tiny ocean plants use carbon to grow, and then it moves up the food chain. When sea creatures die, their bodies and shells sink to the bottom, storing carbon there.

The ocean is normally a bit basic (the opposite of acidic). But with more carbon dioxide in the air, the ocean is absorbing more of it, making it more acidic. This is called ocean acidification, and it can harm sea creatures, especially those with shells. The ocean's ability to soak up carbon is very important, but if it gets too acidic, it might not be able to absorb as much carbon dioxide in the future.

The geosphere is all about the carbon stored in Earth's rocks and deep inside the planet. This part of the carbon cycle moves very slowly, but it's super important for controlling how much carbon is in the air and, therefore, how warm Earth is. Most of Earth's carbon is stored deep inside the planet, especially in rocks like limestone. Limestone forms from the shells of tiny sea creatures that died long ago. Some carbon is also stored as kerogens, which are like the building blocks of fossil fuels.

Carbon can leave the geosphere in a few ways. When rocks deep inside Earth melt and change, they can release carbon dioxide. This carbon dioxide can then come out through volcanoes. Also, humans dig up kerogens (fossil fuels) from the geosphere and burn them, releasing huge amounts of carbon into the air.