How does CO₂ warm the Earth?

It is not the most powerful greenhouse gas of all. Nor is it the most abundant gas in the atmosphere. And yet carbon dioxide (better known as CO₂) plays a crucial role in determining how our planet retains and redistributes the energy it receives from the Sun.

To understand why, we need to set aside, at least for a moment, the idea of CO₂ as a substance that “produces heat” or “traps heat beneath a blanket”. Neither image truly describes what happens in the atmosphere.

CO₂ does not generate energy, nor does it imprison it permanently. Instead, it interacts with the infrared radiation emitted by the Earth, changing the path that this energy must take before eventually returning to space. It is a process involving the physics of molecules which, although invisible to our eyes, plays a fundamental role in determining Earth's climate.

Energy: from the Sun to the Earth

Almost all the energy that powers the climate system comes from the Sun.

Solar radiation reaches the Earth in the form of electromagnetic waves with different wavelengths. The wavelengths most important to our planet are mainly those of visible light and near-infrared radiation. The atmosphere is almost completely transparent to these wavelengths, which is why sunlight can pass through the air without being significantly absorbed and reach the Earth's surface.

Oceans, forests, farmland, deserts and cities absorb this energy and warm up. At this stage, CO₂ plays only a marginal role.

But every warm object also emits energy. This is true of a radiator, of our own bodies and of the Earth itself. There is, however, one fundamental difference between the Sun and our planet: their temperature.

The surface of the Sun is around 5,500°C, while the Earth's average surface temperature is around 15°C. As a result, the Sun emits mainly visible light, whereas the Earth releases the energy it receives primarily in the form of infrared radiation, which has longer wavelengths and is invisible to the human eye.

And it is precisely this radiation that CO₂ interacts with.

Like countless tiny springs

Nitrogen (N₂) and oxygen (O₂), which together make up around 99% of the atmosphere excluding water vapour, allow much of the infrared radiation emitted by the Earth to pass through almost undisturbed.

CO₂ behaves differently, and the reason lies in its molecular structure.

A carbon dioxide molecule consists of one carbon atom bonded to two oxygen atoms (O=C=O). Like all molecules, CO₂ molecules are not motionless: their atoms can stretch, bend and vibrate in very specific ways, known as vibrational modes.

Quantum physics tells us that these vibrations can absorb only specific amounts of energy. One of these corresponds particularly closely to infrared radiation emitted by the Earth's surface, especially at wavelengths around 15 micrometres.

In other words, when an infrared photon carrying the right amount of energy encounters a CO₂ molecule, it can be absorbed. This does not happen because CO₂ “attracts” heat, but because its molecular structure is compatible with that specific frequency of radiation.

Imagine a tiny spring. When compressed, it stores elastic energy. When released, it gives that energy back as it returns to its original position.

CO₂ behaves in a broadly similar way, although the actual process is quantum mechanical.

When it absorbs an infrared photon, the molecule moves into an excited vibrational state. Its atoms vibrate with greater energy: the energy carried by the photon has not disappeared, but has been temporarily stored in the molecule's vibration.

This state lasts only for an extremely short time, on the order of microseconds or even nanoseconds. Yet that is enough for the next step to take place — the one that really matters for the Earth's climate.

Why energy remains in the atmosphere for longer

The atmosphere is an extremely dynamic environment. Molecules are constantly colliding with one another. When a CO₂ molecule is in an excited state, it will often collide with molecules of nitrogen or oxygen before spontaneously re-emitting the photon it absorbed.

During these collisions, some of the vibrational energy is transferred to the surrounding molecules. As a result, nitrogen and oxygen molecules move slightly faster.

From a physical point of view, this is what we mean by temperature: the average kinetic energy of the molecules that make up a substance.

The energy initially carried by the infrared photon can therefore become energy associated with the motion of molecules in the air.

Through subsequent interactions, other CO₂ molecules can absorb energy and emit infrared photons again in random directions: towards space, sideways, or back towards the Earth's surface.

This continuous sequence of absorption, collisions and re-emission makes the journey of energy far longer and more complex than it would be in an atmosphere without greenhouse gases.

More CO₂ means more warming — without creating new energy

But if CO₂ re-emits the energy it absorbs, why does the temperature rise?

The answer is that the total amount of energy does not change.

What changes is the amount of time that energy remains within the climate system before eventually escaping into space.

Imagine following the journey of a single infrared photon. In an atmosphere containing little CO₂, it would have a greater chance of escaping the planet relatively quickly.

But as the concentration of carbon dioxide increases, so does the probability that the photon will be absorbed, that its energy will be transferred to other molecules, that it will be emitted again, and that the process will begin once more.

Each individual step lasts only a tiny fraction of a second. But billions upon billions of these events take place continuously throughout the atmosphere, altering the Earth's overall energy balance.

To restore the balance between the energy received from the Sun and the energy emitted into space, the Earth's surface must reach a higher average temperature. Only then can it emit enough infrared radiation to compensate for the energy coming in.

This is the physical foundation of the greenhouse effect.

Why CO₂ is different from other greenhouse gases

The atmosphere contains several gases capable of absorbing infrared radiation. These include water vapour, methane (CH₄), nitrous oxide (N₂O), ozone and CO₂ itself.

Water vapour is the most important contributor to the natural greenhouse effect. Without it, the Earth would be much colder and probably inhospitable to most forms of life.

So why does scientific attention focus so strongly on CO₂?

The difference is that the amount of water vapour in the atmosphere depends largely on temperature. As air warms, it can hold more moisture; as it cools, water vapour rapidly condenses and returns to the surface as rain or snow. Its average residence time in the atmosphere is around ten days.

CO₂ follows a completely different dynamic.

Its concentration is not primarily determined by atmospheric temperature over short timescales, but by the balance between emissions and absorption by oceans, soils and vegetation. When human activities release large quantities of carbon dioxide into the atmosphere, natural systems cannot absorb it at the same rate.

This is why CO₂ acts as a climate forcing: it alters the planet's energy balance and triggers warming which, in turn, allows the atmosphere to hold more water vapour. Water vapour then amplifies the warming already initiated by CO₂, but it does not initiate it.

Methane represents yet another case. Molecule for molecule, it is far more effective than CO₂ at absorbing infrared radiation, but it remains in the atmosphere for around twelve years on average. CO₂, by contrast, is less efficient on a molecule-by-molecule basis, but continues to accumulate over time.

It is this combination of abundance, persistence and ability to alter the global energy balance that makes CO₂ the leading indicator of human-caused climate change.

How long does CO₂ remain in the atmosphere?

One of the most important characteristics of CO₂ is its longevity. There is no single answer to the question “how long does a CO₂ molecule live?”, because carbon dioxide is constantly exchanged between the atmosphere, oceans, vegetation and soils.

• Some of it is absorbed within a few years by plants through photosynthesis or by the surface ocean.
• Another portion takes decades or centuries to be transferred into the deep ocean.
• A significant fraction remains in the atmosphere for centuries, while a small proportion can continue to influence the Earth's climate for thousands of years, until extremely slow geological processes eventually remove it.

This means that emissions produced today will continue to affect future generations.

This is one of the characteristics that distinguishes CO₂ from most other air pollutants: its effects do not end when emissions stop, but persist over time because the climate system takes a long time to reach a new equilibrium.

Conclusioni

Understanding the role of CO₂ means understanding how the Earth's climate itself works.

Carbon dioxide does not produce energy, generate heat out of nowhere or permanently prevent radiation from leaving the planet. It interacts with the infrared radiation emitted by the Earth's surface, transfers energy to molecules in the atmosphere and makes the path through which that energy eventually returns to space longer and more complex.

On a microscopic scale, this involves the absorption of a photon by a single molecule.

On a planetary scale, billions upon billions of identical interactions alter the Earth's energy balance and contribute to an increase in global average temperature.

It is within this process — seemingly tiny, yet repeated continuously throughout the atmosphere — that we find one of the keys to understanding contemporary climate change.

And because removing CO₂ from the atmosphere is one of the great challenges of the present and the future, here is Treedom's special bundle for planting trees that absorb more CO₂.

Bibliography and recommended reading

• IPCC (2021). Climate Change 2021: The Physical Science Basis. Working Group I Contribution to the Sixth Assessment Report.
• IPCC (2023). AR6 Synthesis Report.
• NASA Earth Observatory. The Atmosphere: Getting a Handle on Carbon Dioxide.
• UCAR Center for Science Education. Carbon Dioxide Absorbs and Re-emits Infrared Radiation.
• Royal Society. Climate Change: Evidence and Causes.
• Archer, D. (2011). Global Warming: Understanding the Forecast. Wiley.
• Pierrehumbert, R. T. (2010). Principles of Planetary Climate. Cambridge University Press.
• Pacchioni, G. (2021). W la CO₂! Possiamo crescere felici senza distruggere il pianeta. Il Mulino.