Greenhouse Gases
Most gases in our atmosphere are invisible to the naked eye--we would be hard-pressed to describe what nitrogen gas or oxygen gas looks like (even though, respectively, they make up 70 and 21 percent of the atmosphere). But there’s one atmospheric gas we can see--can you guess what it is? Here’s a hint: it’s big, wet, white and fluffy.
That’s right: clouds. Clouds are water in a gaseous state, and clouds–or water vapor–are one of the most powerful greenhouse gases (GHGs) on the planet. A GHG (the culprits behind the greenhouse effect) is any gas that absorbs the sun’s infrared radiation, or heat, and reemits it, sometimes back towards Earth. GHGs like carbon dioxide, methane, and water vapor all absorb and reemit the sun’s infrared heat, causing warming.
Remember: when water is exposed to heat, it boils, and then evaporates. A hotter planet, then, equals more water vapor. More water vapor equals more warming, which, in turn, equals--you guessed it--more water vapor. This loop is a “positive feedback” system, and it can cause a runaway greenhouse effect, which, like a rolling snowball growing bigger and bigger, can get out of hand, making things hotter and hotter.
But Earth is clearly not in a super-greenhouse state. If it were, we wouldn’t be here. because our planet would be something like Venus, whose atmosphere–made of 97 percent carbon dioxide–results in surface temperatures in excess of 750 degrees Fahrenheit (hot enough to melt lead).