It all begins with a craving for electrons
To understand the atmosphere, you first have to read the periodic table differently: as a map of the elements' chemical moods.
In the top right sit the oxidizers: those that want to gain electrons. Chlorine, for instance, naturally forms the Cl⁻ ion in water. In the bottom left stand the reducers, which prefer to lose their electrons — like sodium, which becomes Na⁺.
Between them, a few versatile elements like carbon play both sides. Naturally, strong oxidizers are drawn to strong reducers: oxygen readily bonds to hydrogen to form water.
Oxidizers — top right
Cl → Cl⁻They gain electrons. Oxygen is the champion of our story.
Reducers — bottom left
Na → Na⁺They lose electrons. Hydrogen, very light, is one of them.
The heavy sinks, the light rises
As the Earth forms, gravity sorts matter.
The heaviest elements sink toward the centre: the planet's interior is therefore reducing, and volcanism spews it back to the surface. Conversely, the atmosphere keeps only the light elements — hydrogen, carbon, nitrogen, oxygen — assembled into molecules: dioxygen O₂, carbon dioxide CO₂, dinitrogen N₂, water H₂O, methane CH₄, dihydrogen H₂.
Dihydrogen and helium are so light they escape into space: that is why they are missing. As for lithium, beryllium and boron, they are simply extremely rare in the Universe.
Carbon, torn between fire and sky
Versatile carbon exists both in oxidized form (CO₂) and reduced form (methane). The balance between them is arbitrated by two opposing forces: one rises from the depths, the other descends from the Sun.
The Sun oxidizes
Its radiation drives light hydrogen — a reducer — out into space. So the atmosphere oxidizes from above. And as the Sun's core contracts, fusion intensifies: this oxidation grows over time.
Volcanism reduces
It spews reducing material, favouring methane. But it is fuelled by radioactivity (uranium…), which decays: the Earth cools and volcanism slowly dies down.
Oxidation rising, reduction fading: the atmosphere naturally slides from a reducing to an oxidizing state. This slow tipping is the thread running through the whole story that follows.
Life speeds up the tipping
Life is built on long chains of carbon and hydrogen — that is, on a deeply reducing medium.
To build these molecules, life must shed its excess oxygen: it releases it as dioxygen O₂, which oxidizes the atmosphere even further. Life thus becomes a formidable accelerator of the natural tipping.
But making life means swimming against the current: the dioxygen produced would immediately attack organic molecules again. So a powerful energy source is needed to resist the natural direction of the reactions.
The main one is solar radiation, the engine of photosynthesis. But life can also tap the Earth's internal heat, released by volcanism — which lets it exist far from any light, in the abyss.
Four and a half billion years, at a glance
Time is counted from the Earth's formation (0 = 4.5 billion years ago). Twice, the planet tips into a “Snowball Earth”.
A world of lava at 2000 degrees
The energy of collisions and gravitational contraction keeps the ground as lava.
Besides CO₂, methane and dinitrogen, water itself is entirely gaseous. The temperature reaches 2000 °C and the pressure exceeds 100 bars. At this stage, the Earth would look more like Venus than the blue world we know.
When the rain quenched the furnace
In 300 million years, gravitational settling drops the temperature to 230 °C.
Under gigantic pressures, water can finally turn liquid: these are the first oceans. And liquid supercharges chemical reactions, where gases are too dilute and solids react only at their surface.
A major consequence: CO₂ dissolves massively in the water, forming carbonates that settle to the bottom… which frees up room to dissolve even more CO₂. The carbon-capture machine is running.
After 800 million years, the pressure has already dropped sharply. The greenhouse effect eases, temperatures fall. And in the intense chemistry of liquid water, life appears and begins to produce oxygen.
But at first, this O₂ is immediately destroyed by the reducing material spewed by volcanoes. Oxygen cannot accumulate yet.
The day oxygen devoured methane
As volcanism weakens, O₂ finally wins out. Around 1.6 billion years, it attacks atmospheric methane, turning it into CO₂.
It is a climate catastrophe: methane is a greenhouse gas thirty times more potent than CO₂. Its disappearance, while the Sun still shines 20% weaker than today, triggers a brutal temperature crash.
It is the first glaciation in Earth's history, and life's first great crisis. The collapse of bacterial populations eventually stabilizes the climate, 150 million years later.
DNA, nucleus, eukaryotes
Facing an increasingly hostile environment, life invents itself some armour.
To better guard against mutations, RNA doubles into a helix of DNA, which then takes refuge in a nucleus: these are the first eukaryotes. A new explosion of life follows, drawing down CO₂ massively and releasing O₂ massively.
This is the start of the Great Oxidation.
The collapse of the greenhouse effect triggers a glaciation even more brutal than the last. The temperature plunges to around −25 °C and the Earth is entirely covered in ice: the first Snowball Earth. Life is decimated — which, once again, slowly brings the system back to balance.
After 300 million years of crisis, the temperature edges just above its present value, and the CO₂ pressure drops below that of dinitrogen.
Respiration, or the art of burning oxygen
Life then learns to use dioxygen as an energy source: this is respiration.
It exactly counterbalances photosynthesis. From now on, the CO₂ balance is no longer set by volcanoes and oceans, but by the production and destruction of carbon by life itself. By steering CO₂, life also steers the planet's temperature.
The carbon drawn down by photosynthesis settles on the ocean floor as carbonates (CaCO₃)… before one day being sent back to the sky by volcanism. A great carbon cycle sets in, a loop that regulates the climate over hundreds of millions of years.
Ozone, then the second great freeze
The atmosphere is now so oxidized that an even more oxidizing species appears: ozone O₃.
By blocking the ultraviolet rays that shattered life's molecules, ozone lets life rise back toward the surface of the oceans to capture more light. A new explosive phase, a new surge of photosynthesis — and so a new collapse of the greenhouse effect.
For the second and last time, the Earth is entirely covered in ice — for 85 million years, punctuated by partial thaws. A final reset that brings temperatures back to reasonable values at last.
The world is ready for complex life
The ice retreats, the ozone layer shields the surface: everything is in place.
The first multicellular organisms develop and get ready to conquer the continents. In broad strokes, the atmosphere has just reached its present composition.
What follows will be nothing but smaller fluctuations — yet decisive for the emergence of humankind.
The signature of a cooling world
A single chart sums up 4.5 billion years: the pressure collapses as CO₂ is trapped, while the temperature drops in brutal steps — each step a glaciation.