2012: Advent Atmosphere: The Greenhouse Effect

by on December 2, 2012

Venus, Earth and Mars are all similar in size, in similar orbits, and condensed out of the same gas cloud, so they ought to have about the same makeup of elements. But Mars is freezing cold with almost no atmosphere, Earth is nicely liveable, and Venus is boiling hot with horrific storms. So how did they end up so different?

To explain that, I need to first explain the greenhouse effect.

An atmosphere doesn’t just give you air to breathe and something for birds to fly through. It has an effect on the planet as a whole. An atmosphere acts like a blanket, keeping the surface of the planet warm. How good the blanket is and how warm it gets underneath depends on how thick it is and what gases make it up.

Most of the energy the Earth gets from the Sun is in the visible spectrum. Our atmosphere is mostly transparent to visible light, so it shines through and hits the surface, where most of is is absorbed. Things that absorb light get warmer, and then re-emit that energy as infra-red.

So energy comes from the Sun, reaches us at wavelengths that can pass through the atmosphere, and then the surface gives that energy back as infra-red. But the atmosphere is nearly opaque to infrared. For that energy to reach space and let the Earth cool down again, it’s got to be absorbed and re-emitted thousands of times by molecules on the atmosphere. The net effect is to slow down how much heat can escape, bouncing it back down towards the surface.

It’s called the greenhouse effect because this is exactly how a greenhouse works. Visible light shines in through the glass and is absorbed by the things inside, which get warmer, and emit the energy as infrared light, which the glass reflects. The heat gets trapped inside.

Leave a Reply