The Venus Syndrome

[Louisiana1976]

It's that most wonderful time of the year! Christmas decorations are going up all over America. Those colorful little strings of twinkling lights racing down eaves and winding artfully around tree trunks can do more than paint pretty holiday vistas. We can use them as an intuitive probe to look at our nearest neighbor in space and speculate on earth's distant, or not so distant, future.

A typical Christmas tree light puts out about one watt of heat and light energy. That's not a lot but it does the job. On that same scale, the incident solar radiation, or insolation, received by earth when all wavelengths are taken into account works to about 250 watts per square meter1 (usually referred to as just "watts") when averaged over the entire surface, light and dark, from poles to equator. Some of it is reflected back, the rest is absorbed.

If you take an introductory planetary astronomy course you'll hear the usual spiel that the earth is in the solar system's Goldilocks zone, not too hot like our sister planet Venus, not too cold like our smaller cousin Mars. It's actually more complicated than that. At 250 watts, the earth is a little too far away, a little too cold, for our liking. By the laws of simple thermodynamics our lovely blue-green planet should boast an average temperature well below the freezing point of water. And the sun is very slowly heating up, roughly 5 to 10 percent per billion years, so ancient insolation was even less long ago than it is now. The earth should have started out frozen solid right down to the deep ocean trenches and it would be a brilliant iceball hanging in space like a snow-white ornament to this day. What's kept that grim fate at bay for billions of years are greenhouses gases (GHGs).

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http://www.dailykos.com/storyonly/2009/11/29/808593/-The-Venus-Syndrome