The natural carbon cycle

Now that we understand what carbon is, why it matters, and where it is stored, it's time to look at how it moves.

Carbon is not stationary: it circulates throughout the Earth in very different ways — some slow and stable, others quick and intense.

These routes form what we call the natural carbon cycle, which consists of two major movements:

• The geological cycle, slow, deep, occurring over millions of years;

• And the biological cycle, fast, powered by living organisms.

The geological carbon cycle

The geological cycle has been functioning since the planet formed, about 4.5 billion years ago.

It begins with emissions of carbon dioxide (CO₂) from volcanoes, one of the main natural sources of atmospheric CO₂ in Earth's early history.

This CO₂ mixes with water from clouds, rain, and aquifers, forming carbonic acid (H₂CO₃).

This acid infiltrates rocks — mainly igneous ones — and triggers a process called chemical weathering, which releases calcium (Ca²⁺) and magnesium ions (Mg²⁺).

These ions react with bicarbonates and form calcium carbonate (CaCO₃), which precipitates and, over millions of years, turns into carbonate rocks — like those seen in mountain formations.

This process is essentially the

transformation of an atmospheric gas into a solid mineral

A path of

millions of years

This cycle has been — and still is — fundamental for regulating atmospheric CO₂ throughout the planet's geological history.

The biological carbon cycle

The biological cycle, on the other hand, occurs on a shorter time scale, like the pace of human life — days, months, years.

It involves plants, animals, microorganisms, and the exchange of carbon between the biosphere and atmosphere.

Two central processes make this cycle happen:

1. Photosynthesis

Photosynthesis is one of the most powerful chemical reactions on Earth.

With sunlight, plants convert:

CO₂ + H₂O + sunlight → glucose (C₆H₁₂O₆) + O₂

This glucose is the foundation for the formation of proteins, lipids, and nucleic acids — in other words, the structure of all living beings.

It is also the main path for removing CO₂ from the atmosphere.

2. Cellular respiration

The opposite process is cellular respiration, performed by both plants and animals:

Glucose + O₂ → CO₂ + H₂O + energy (ATP)

This is how organisms release carbon back into the atmosphere to generate energy.

It is not lung respiration — it is a biochemical reaction that occurs inside cells.

A cycle with balance

What makes the biological cycle special is that it has a positive balance:

• Plants capture more CO₂ than they emit over the year;

• The ocean also removes dissolved CO₂ from the atmosphere, especially via phytoplankton.

Together, these two systems — terrestrial and marine — help to counterbalance part of human emissions. But they are not enough to offset everything, as we will see in future posts.