Solar radiation interacts with the surface of the earth in several ways. Some portion of this energy is reflected back into space by the earth's atmosphere, another portion is dispersed and scattered by the molecules in the atmosphere and a large portion penetrates through the earth's atmosphere to reach the surface of the earth. The radiation reaching the earth's surface is largely absorbed resulting in surface warming (Figure 1).
Figure 1. The greenhouse effect. Select image for larger version (77KB, opens in new window).
Much of this absorbed energy is eventually re-radiated in longer infrared wavelengths. As it leaves the earth, it once again interacts with the atmosphere. Some of this re-radiated energy escapes to space, but much of this re-radiated energy is reflected back to the earth's surface by molecules in the earth's atmosphere. This phenomenon is similar to the warming that occurs in an automobile parked outside on a sunny day (Figure 2).
Figure 2. A parked car in the sun creates a localized greenhouse effect. Select image for larger version (81KB, opens in new window).
The molecules responsible for this phenomenon are called greenhouse gases, i.e. water (H2O), nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) because they act like the glass in a greenhouse, trapping re-radiated energy. Without these gases most life on earth would not be possible, as the surface temperature of the earth would likely be about 60°F colder.
In essence, greenhouse gases act like an insulator or blanket above the earth, keeping the heat in. Increasing the concentration of these gases in the atmosphere increases the atmosphere's ability to block the escape of infrared radiation. In other words, the earth's insulator gets thicker. Therefore too great a concentration of greenhouse gases can have dramatic effects on climate and significant repercussions upon the world around us. Climates suitable for human existence do not exist simply above some minimum threshold level of greenhouse gas concentration, rather they exist within a finite window - a limited range of greenhouse gas concentrations that makes life as we know it possible.