This is an interactive version of a radiative transfer calculator call MODTRAN (for “moderate resolution radiative transfer”). For each wavelength of infrared light, it calculates the intensity (brightness) of that radiation reaching outer space. The surface radiates infrared energy.
The smooth red line shows the radiation emitted by the surface, which has the characteristic shape of a “blackbody.” The jagged black line shows the radiation above the atmosphere, the part that's emitted to outer space.
In some wavelengths, thermal radiation passes straight through the atmosphere. These are called “atmospheric windows” because the atmosphere is transparent to radiation emitted form the surface at those wavelengths. You can see this on the graph because radiation reaching outer space (jagged black line) is close to the amount radiated by the Earth's surface (smooth red line).
In other wavelengths, greenhouse gases absorb the upwelling radiation and re-emit it both upward and downward at temperatures lower than the surface temperature. At these wavelengths the jagged black line is below the smooth red line. The lighter colored lines show the radiation that would be emitted by blackbodies at a range of temperatures. These provide a good estimate of how cold the atmosphere is where the radiation is emitted that reaches space at a given wavelength.
The text labels on the graph show the surface temperature (in degrees Kelvin) and the total amount of Outgoing Longwave Radiation (OLR) reaching outer space, in Watts per square meter. The OLR is the integral of the jagged black curve.
Move the sliders at left to change the amount of CO2 and CH4 in the atmosphere, and watch what happens to the jagged black line. This shows the effect of changing the absorbing properties of the air when the temperature stays the same.
Try sliding the CO2 control all the way to the left. See how the black jagged line moves way up, especially at wavelengths around 15 microns? This means the atmosphere is emitting to space at temperatures close to the surface temperature.
Now slide the CO2 control to the right, a little at a time. A big “bite” appears in the radiation spectrum of the Earth, out in space. The dip in the black jagged line shows that adding even a little CO2 pretty much eliminates all the radiation from the surface at wavelengths near 15 microns. As you add more CO2, the bite stops getting deeper and gets wider instead. Once the outgoing radiation is coming from the coldest part of the atmosphere (around 220 Kelvin), adding more CO2 can't reduce it any more. Instead some of the weaker absorbtion features at longer and shorter wavelengths take over.
Notice that as you change the CO2 and CH4, the surface temperature stays the same, but the total outgoing radiation (OLR) changes. If the OLR above the atmosphere is a lot more or less than the incoming sunlight, there will be a net imbalance at the top of the atmosphere. This is indicated on the graph by red text if the Earth is absorbing more radiation than it's emitting, and by blue text if the Earth is emitting more radiation than it's absorbing.
Try sliding the surface temperature control to the right or left to compensate for the net imbalance in radiation at the top of the atmosphere. If there's a net gain of radiation by the Earth (red indicator of net downward radiation), slide the temperature to the right to warm the surface so it emits more. If there's a net loss (blue indicator of net energy loss), slide the temperature to the left to cool the surface so it emits less. This is how the real climate adjusts to changes in the net heat (in minus out) that enters and leaves the planet from outer space.