THE FIRST 4 BILLION YEARS

In its long history, Earth has seen extremes of fire and ice that would have made human life impossible. The planet was born in a cauldron of volcanic eruptions and million-megaton impacts from outer space. Any hydrogen or helium in the early atmosphere soon escaped into space. In time, only heavier molecules remained, including nitrogen and the greenhouses gases: water vapor, carbon dioxide, methane, and ammonia. No free oxygen was present.

A gas is called a greenhouse gas (GHG) because it lets sunlight through but absorbs outgoing heat radiation from the surface of the Earth. The absorbed heat energy is reradiated in all directions. Because some of it returns to the surface, the GHG boosts the planet's temperature. If no GHG were present in the atmosphere, the world's current average temperature would be about 0°F rather than the actual level of 58°F (14.5°C).

About half a billion years after its formation as a planet, Earth had cooled enough for rain to fall and the oceans to form. Life emerged soon – in geological terms – thereafter. The Sun was weaker back then, emitting only about 70% of the radiation of today. The Earth would have frozen solid were it not for the high concentrations of GHG in the atmosphere. The Sun was less radiant than today largely because it had less helium. As with any star, nuclear fusion in the core of the Sun converts hydrogen into helium.