There’s a simple story of the greenhouse effect: A blanket of carbon dioxide envelops the planet, letting sunlight in but trapping its heat. As a result, Earth warms.
But how does this actually work? Carbon dioxide amounts to only a tiny smattering of gas molecules — 0.042%, or roughly 420 parts per million — in our thick atmosphere. And yet, we know that doubling carbon dioxide levels can change the character of life on Earth.
The answer is quantum mechanics, which determines whether a molecule can interact with the right type of radiation.
Part 1: Maintaining Energy Balance
But first, we need a basic understanding of how radiation, such as sunlight, interacts with objects, such as planets.
Everything in the universe radiates, pumping out heat. A light bulb radiates heat; so does a rock sitting on the ground. Same with your phone, your body and Earth itself.
The radiation given off by an object takes the form of light, or electromagnetic waves. These magnetic and electric fields undulate as they move through space, carrying energy with them.
Hotter objects give off more heat; their waves are more energetic, oscillating with a shorter wavelength. Objects on Earth tend to be cool (generally under 30 degrees Celsius) and radiate light with relatively long wavelengths, known as infrared radiation. The sun is much hotter, about 5,000 degrees Celsius, so it radiates visible radiation with shorter wavelengths.
A radiating object will cool off unless there’s a source of heat replenishing it. For example, Earth releases heat, but it doesn’t cool down. That’s because all the heat that it loses gets replenished by the sun’s radiation. As long as Earth absorbs the same amount of heat from the sun as the amount it gives off, it will stay the same temperature — in equilibrium.
Imagine Earth with no atmosphere. Its surface butts up right against the cold vacuum of space, with no barrier in between. Even if the Earth’s surface was extremely cold, about minus 18 degrees Celsius, it would be warm enough to radiate all the heat it’s taking in. At that low temperature, the Earth and sun would already be in a happy equilibrium.
Now add an atmosphere — a thicket of gas molecules bound to the Earth by its gravitational pull. Say some of these molecules are greenhouse gases that interact with the outgoing radiation. Some of Earth’s radiation is now redirected back to its surface. Instantly, the amount of heat escaping the planet drops. But the same amount of heat is entering from the sun as before. We are out of equilibrium.
With more heat entering than leaving, the planet’s temperature begins to rise. But remember, the hotter an object, the more it radiates. So as Earth warms up, it begins pumping out more heat. This trend continues until the same amount of heat is escaping as is entering. Balance is restored at this new, hotter equilibrium.