Greenhouse Effect
Global Warming

The effect of infrared re-radiation being absorbed in the atmosphere is called the "Greenhouse Effect" since it mimics what happens in a real greenhouse.  There, the radiation is trapped by glass window panes, which are optically opaque in the infrared region of the spectrum.  Since the infrared radiation does not pass through the glass, it remains in the greenhouse and keeps the inside temperature warmer than the outside temperature (the same effect keeps the inside of your car warm even on a cold sunny day).

The science behind this effect in the atmosphere is fairly well understood.  Certain gases, such as water vapor, carbon dioxide, and methane, are able to absorb infrared radiation very well.  The Earth re-radiates absorbed sunlight back into outer space mostly in the infrared range of the electromagnetic spectrum.  When these gases are present in the atmosphere, they will absorb this energy before it gets back into space, and thereby heat the atmosphere.  We see this in action everyday, both on our planet and on others in the Solar System.  For example, Venus has an atmosphere that contains almost a million times the concentration of carbon dioxide (a greenhouse gas) as our atmosphere.  If Venus were to have no atmosphere, its average temperature would be about 230 K (-45 ^oF); because of this carbon dioxide concentration (plus a small amount of other greenhouse gases), it has a temperature of 740 K (about 900 ^of).  A similar, but smaller, effect is seen on Mars and other planets that contain greenhouse gases.  Without the greenhouse gases that we have on Earth, it is estimated that our average daily temperature would be about -10 ^of, instead of the 60 ^of that it is.

While water vapor has the greatest contribution to atmospheric heating due to the greenhouse effect here on Earth, most of the attention in this area lately has been focused on carbon dioxide.  The reason for this is that the levels of carbon dioxide have increased from about 292 ppm (parts per million) to over 360 PPM over the last 100 years.  This increase in concentrations has corresponded to the same time period over which we have seen the average tropospheric temperature increase about 1 ^oC.  The correlation between these two events, plus our knowledge of how greenhouse gases work, has led many to hypothesize that the Earth will continue to get warmer as we release more and more greenhouse gases into the atmosphere.