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The greenhouse effect is a natural process in which some radiative energy leaving a planet’s surface is absorbed by atmospheric gases. On Earth those gases are: water vapor, carbon dioxide, methane, and ozone. While this is a naturally occurring process, many people believe that humans have caused this effect to increase by releasing too many gases into the atmosphere, thus contributing to global warming.
The most basic explanation of the greenhouse effect is this: the Earth receives energy from the Sun in the form of visible light. This light is absorbed at the Earth’s surface and then it is radiated as thermal radiation. Some of this thermal radiation is absorbed by the atmosphere which bounces it upwards and downwards. What is radiated downwards is absorbed by the Earth’s surface. Energy is also reflected away from bright areas, like snow. This is known as the albedo effect.
Human activities have strengthened the effect. This is known as the enhanced(anthropogenic) greenhouse effect. This increase is mainly due to increased atmospheric carbon dioxide levels. Carbon dioxide is produced by fossil fuel burning, cement production and tropical deforestation. Measurements from the Mauna Loa observatory show that concentrations have increased from about 313 ppm in 1960 to about 389 ppm in 2010. The current observed amounts exceed the geological record maximum taken from ice core data. The higher levels contribute to additional absorption and emission of thermal energy by the atmosphere resulting in a net warming effect on the Earth.
The greenhouse effect is not limited to our planet. In our solar system Mars, Venus, and Saturn’s moon Titan show signs of the effect. Titan; however, has an anti-greenhouse effect. Its atmosphere absorbs solar radiation but is nearly transparent to infrared radiation. On a small scale, Pluto behaves similarly. In extreme cases, a runaway greenhouse effect occurs if conditions are right that lead to the evaporation of all greenhouse gases into the atmosphere. It is thought that a runaway effect involving water vapor and carbon dioxide occurred on Venus.
Global warming is becoming a concern mainly because of the rapid increase in carbon dioxide and the growing industrial complexes in underdeveloped countries. A big question is: will this abundance of carbon dioxide breakdown and alleviate after we have depleted our fossil fuels?
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.