The carbon cycle is Earth's way of recycling carbon, moving it between living things, the air, oceans, and rocks, which is essential for all life.

Key idea: Carbon is stored in major reservoirs like the atmosphere, land, oceans, and rocks, and it constantly moves between them.

To understand the carbon cycle, it helps to think of Earth as having several big storage areas, or 'reservoirs,' for carbon. These are like different rooms in a house where carbon hangs out.

The main carbon reservoirs are: the atmosphere (the air around us), the terrestrial biosphere (all living things on land, like plants and animals, and carbon in soil), the ocean (carbon dissolved in water and in marine life), and sediments and rocks (like fossil fuels and limestone, which store carbon for very long times). Even the Earth's deep interior, its mantle and crust, holds carbon.

Carbon moves between these reservoirs through various natural processes. For example, plants take carbon from the air, animals eat plants, and when plants and animals die, their carbon can go into the soil or eventually form rocks. The ocean is a huge carbon storage area, holding much more carbon than the atmosphere.

Key idea: Atmospheric carbon, mainly carbon dioxide, traps heat and is essential for photosynthesis, but human activities have significantly increased its levels, intensifying the greenhouse effect.

Let us look at carbon in the air. In the atmosphere, carbon mainly exists as two gases: carbon dioxide (CO2) and methane. Both of these gases are like a warm blanket for Earth. They trap heat, which is a natural process called the greenhouse effect that keeps our planet warm enough to live on.

Methane is very good at trapping heat, but there is much less of it, and it does not stay in the air as long as carbon dioxide. So, carbon dioxide is the main gas that contributes to the greenhouse effect.

Plants are amazing at removing carbon dioxide from the air through a process called photosynthesis. They use sunlight to turn carbon dioxide and water into food for themselves, releasing oxygen. Carbon dioxide also dissolves directly into oceans and lakes, and it can even fall to Earth with rain. When it dissolves in water, it forms a weak acid, which is why increased carbon dioxide in the air can make the oceans more acidic.

Over the last 200 years, human activities have increased the amount of carbon dioxide in the atmosphere by almost 50%. This is mostly from burning fossil fuels like coal, oil, and gas, and from changing how we use land. This extra carbon dioxide is making Earth's blanket thicker, leading to global warming.

“Carbon dioxide is the main gas that contributes to the greenhouse effect.”

Key idea: The terrestrial biosphere stores carbon in living organisms and soil, with plants taking carbon from the air and releasing it through respiration or decomposition.

The land is another vital part of the carbon cycle. The terrestrial biosphere includes all the carbon found in living things on land, both alive and dead, and in the soil.

Plants take carbon dioxide from the air and convert it into organic carbon, which is what their bodies are made of. Animals then get this carbon by eating plants or other animals. When plants and animals die, their carbon goes into the soil. Soil can store a huge amount of carbon, much more than all the plants on Earth.

Carbon leaves the land in several ways. When living things breathe, they release carbon dioxide back into the air. When plants or trees burn, they also release carbon dioxide quickly. Carbon in the soil can stay there for thousands of years, but it can also be released back into the atmosphere by tiny organisms in the soil, or washed into rivers and eventually the ocean.

Because plants grow more in warmer months and less in colder months, the amount of carbon dioxide in the atmosphere goes up and down each year. This seasonal change is very clear in measurements, especially in the Northern Hemisphere where there is more land and more plants.

Key idea: The ocean is a vast carbon reservoir, absorbing atmospheric carbon dioxide and moving it to deep waters through biological processes, but this absorption causes ocean acidification.

The ocean is a massive carbon sink, meaning it absorbs and stores a lot of carbon. It holds about 50 times more carbon than the atmosphere!

Carbon enters the ocean mainly by dissolving directly from the atmosphere. Once in the water, some of it turns into carbonic acid, which is why more carbon dioxide in the air makes the ocean more acidic. Marine plants, like tiny phytoplankton, also use carbon dioxide for photosynthesis, just like land plants. This carbon then moves up the food chain.

When marine organisms die, their bodies and shells (which are often made of calcium carbonate) sink to the bottom. This process, called the biological pump, moves carbon from the surface waters to the deep ocean, where it can be stored for hundreds or even thousands of years. Eventually, deep ocean currents can bring this carbon back to the surface.

The ocean's ability to absorb carbon is crucial, but it is not endless. As the ocean absorbs more carbon dioxide, it becomes more acidic. This change in ocean chemistry can harm marine life, especially creatures that build shells or skeletons, and might reduce the ocean's ability to absorb even more carbon in the future.

“The ocean holds about 50 times more carbon than the atmosphere!”

Key idea: The geosphere stores vast amounts of carbon in rocks and fossil fuels over millions of years, releasing it slowly through volcanoes or rapidly through human activities.

The slowest part of the carbon cycle involves the Earth's rocks and deep interior, known as the geosphere. This part of the cycle takes millions of years, but it plays a huge role in controlling Earth's climate over very long periods.

Most of Earth's carbon is actually stored in rocks, especially limestone, which forms from the shells of ancient marine organisms. Another significant amount is stored as fossil fuels (coal, oil, and natural gas), which are formed from ancient plants and animals buried under heat and pressure over millions of years.

Carbon can leave the geosphere in several ways. Volcanoes release carbon dioxide into the atmosphere and oceans. This happens when rocks containing carbon are pushed deep into the Earth and melt. Humans also extract fossil fuels from the geosphere and burn them, releasing huge amounts of stored carbon into the atmosphere much faster than natural processes would.

Scientists are even studying how much carbon might be stored deep inside the Earth's mantle and core. This deep carbon cycle is still a bit mysterious, but it is thought to be incredibly important for regulating the planet's carbon balance over geological timescales.

Key idea: The carbon cycle has a fast part (days to years) involving living things and a slow part (millions of years) involving rocks, both of which are being affected by human actions.

We can think of the carbon cycle as having two main speeds: a fast cycle and a slow cycle.

The fast carbon cycle involves the quick movement of carbon between the atmosphere, living things (plants and animals), and the surface ocean. This cycle can happen over days, months, or years. For example, a plant takes in carbon dioxide, an animal eats the plant, and then both release carbon dioxide when they breathe or decompose. This is the biological carbon cycle.

The slow carbon cycle, also called the deep carbon cycle, involves carbon moving through rocks, soil, and the deep ocean. This cycle takes millions of years. It includes processes like the formation of fossil fuels, the weathering of rocks, and volcanic eruptions. This slow cycle is what has kept Earth's climate stable for hundreds of millions of years by slowly balancing carbon levels.

Humans have mainly impacted the fast carbon cycle by adding a lot of extra carbon dioxide to the atmosphere very quickly, but we are also tapping into the slow carbon cycle by burning fossil fuels that took millions of years to form.

“The fast carbon cycle involves the quick movement of carbon between the atmosphere, living things, and the surface ocean.”

Key idea: Human activities, primarily burning fossil fuels and changing land use, are rapidly releasing stored carbon into the atmosphere, disrupting the natural carbon cycle and causing global warming.

Since the Industrial Revolution, human activities have significantly changed the carbon cycle. We are taking carbon that was stored deep underground for millions of years (as fossil fuels) and releasing it into the atmosphere at an unprecedented rate.

Burning fossil fuels for energy is the biggest way we add carbon dioxide to the air. We also release carbon when we make cement. These actions directly transfer carbon from the geosphere to the atmosphere.

Another major impact comes from how we use land. Cutting down forests (deforestation) to create farmland or cities removes many trees that would otherwise absorb carbon dioxide from the air. This not only releases carbon stored in the trees but also reduces Earth's ability to pull carbon out of the atmosphere. While planting new trees (reforestation) can help reverse some of this, the scale of current deforestation is a major concern.

These human changes are making the 'blanket' of greenhouse gases around Earth thicker, leading to global warming and climate change. The oceans and land still absorb some of this extra carbon, but they cannot keep up with the speed at which we are releasing it. This means more carbon stays in the atmosphere, warming our planet.