Changes

absorb and re-emit longwave [[infrared rays]] emitted from the earth's surface into the atmosphere (see [[radiational cooling]]).  

absorb and re-emit longwave [[infrared rays]] emitted from the earth's surface into the atmosphere (see [[radiational cooling]]).  

Sunlight falling on the surface of a planet is primarily in the visible part of the spectrum. The surface absorbs some of the incident sunlight, and some is reflected, depending on the [[albedo]] of the surface. The reflected light mostly passes through the atmosphere back to space. The absorbed light warms the surface.  

Sunlight falling on the surface of a planet is primarily in the visible part of the spectrum. The surface absorbs some of the incident sunlight, and some is reflected, depending on the reflectivity of the surface (see [[albedo]]). The reflected light mostly passes through the atmosphere back to space. The absorbed light warms the surface.  

Any body with a temperature radiates according to Plank's law, and the Earth is no exception. Due to its relatively low temperature, the emission from the ground is in the infrared (IR)  part of the electromagnetic spectrum. Greenhouse gases, by definition, are good absorbers at these wavelengths. The gas absorbs the IR radiation, and emits IR radiation, both up and down. Because these gases are above the surface, they are at a lower temperature than the surface, so they emit at a different frequency. The net effect is that energy is retained by the Earth system, warming it.<ref>D. Hartmann, Global Physical Climatology</ref> This is the natural greenhouse effect, which allows the Earth to be habitable. When the atmospheric composition is changed by human activity resulting in increases in greenhouse gases, the warming associated is called the anthropogenic greenhouse effect.

Any body with a temperature radiates according to Plank's law, and the Earth is no exception. Due to its relatively low temperature, the emission from the ground is in the infrared (IR)  part of the electromagnetic spectrum. Greenhouse gases, by definition, are good absorbers at these wavelengths. The gas absorbs the IR radiation, and emits IR radiation, both up and down. Because these gases are above the surface, they are at a lower temperature than the surface, so they emit at a different frequency. The net effect is that energy is retained by the Earth system, warming it.<ref>D. Hartmann, Global Physical Climatology</ref> This is the natural greenhouse effect, which allows the Earth to be habitable. When the atmospheric composition is changed by human activity resulting in increases in greenhouse gases, the warming associated is called the anthropogenic greenhouse effect.