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The greenhouse effect refers to the phenomena wherein the Earth experiences warming due to the accumulation of atmospheric gases that block the release of re-radiated heat to outer space.
Electromagnetic waves coming from the Sun strike the Earth after passing through the atmosphere. Most of these waves belong to the (relatively) short wavelength end of the spectrum. Part of these waves are absorbed by the planet’s surface, and heat it up. Some of it, on the other hand, are re-radiated in the form of infrared waves. These waves have longer wavelengths than the ones that initially passed through the atmosphere.
Incidentally, the gases in the atmosphere known as greenhouse gases are very good absorbers of infrared waves. Some of the infrared radiation manages to escape into outer space but a good fraction of it is absorbed. The electromagnetic waves absorbed by the Earth’s surface and the atmosphere both contribute to the total warming of the planet.
Needless to say, the more greenhouse gases there are in the atmosphere, the more substances there will be that may absorb the infrared waves. This can subsequently result into a greater amount of warming.
The most prominent greenhouse gases (i.e., those that contribute to the greenhouse effect) are: water vapor, carbon dioxide, methane, nitrous oxide, ozone, and CFC’s. Of those mentioned, it is carbon dioxide that has undergone a substantial build up since the Industrial Revolution. It is for this reason that carbon dioxide is identified as the main cause of the escalation of the greenhouse effect.
Since 1960, the amount of carbon dioxide has increased by about 70 ppm, bringing it up to 383 ppm. This is the highest level of carbon dioxide ever recorded, including those that can be determined from ice core data.
The greenhouse effect is not a bad thing. In fact, we need it so that the Earth’s temperature will not drop to critically low levels, just like what happened during the Ice Age or even the Little Ice Age. However, the Earth’s ideal range of temperatures is a pretty narrow one. So a notch lower or a notch higher can have devastating effects. Right now, we’re suffering from the effects of the latter.
Although the greenhouse effect in this context gets its name from the greenhouse effect of a real greenhouse, there is a difference in the main concept. In the former, temperature rise is brought about by the trapping of infrared radiation. The latter, on the other hand, achieves temperature rise through the trapping of warm air which should have been lost through convection.
The Earth is not the only planet that experiences the greenhouse effect. Read about the greenhouse effect taking place in Venus here in Universe Today. We’ve also got an interesting article that talks about a real greenhouse on the Moon by 2014.
Here’s a simplified explanation of the greenhouse effect on the EPA’s website. There’s also NASA’s Climate Change page.