The Greenhouse Effect

All planets are warmed by the incoming radiation from their parent stars. For Earth, which orbits the sun (named Sol, if you didn't know) at an average distance of 150,000,000 km, you can determine the surface temperature by treating the planet as a blackbody, which is a theoretical object that perfectly absorbs all radiation. As the Earth absorbs radiation, it heats up (like a potato placed in a microwave) and begins to emit its own radiation in the form of heat. The Earth emits back in the form of heat all of the radiation it absorbs from the sun.

The intensity or strength of the radiation from a blackbody can be described using Plank's law, which gives the radiative intensity from an object at some temperature. When calculated this way, the intensity of radiation (the hotness) can be plotted against the wavelength (the color) of the radiation, which results in a characteristic plot which is shown below. If "color" and "hotness" don't seem familiar to you, remember that the yellow part of a candle flame is much, much hotter than the red part at the top.

                           

Using this information, we can calculate the amount of radiation from the sun, which is called the flux, the wavelength of the brightest sunshine (~530 nm, or in the yellow-green region, thus the color of the sun), and the temperature of the sun's surface (roughly 5800 K). When the solar emission spectrum is measured in space, without interference from the atmosphere, this theoretical approximate is very close to what we see, as shown below:

But, you can see from the lowest line, there are big gaps in the measured spectrum when measured at sea level! This is because there are molecules in the atmosphere that absorb at certain parts of the spectrum, making the atmosphere opaque at those wavelengths. (Ozone is one of the most important players in absorption of harmful UV radiation, which is why the ozone hole is such cause for concern.)

Now, back to what happens here on Earth. The earth absorbs the incoming radiation from the sun and then re-emits it, acting itself as a blackbody. In order for the sun-earth system to be in equilibrium, the earth must emit the same amount of energy it absorbs (though not necessarily at the same wavelength). This is indeed what we see, when we look at the earth's emission spectrum from space:



Here again, notice the dips in the spectrum where the observations don't match the theory. Once again, this is due to components of the atmosphere that absorb the outgoing radiation, this time in the infrared region (IR). The major components that absorb in this region are water, nitrous oxide, methane, ozone, and, of course, the major target of the BEACON project, carbon dioxide.

If absorption of IR radiation is normal for the atmosphere, one might ask why we are concerned about adding more absorbers. To illustrate this, let's imagine a case where there are no greenhouse gases in the atmosphere.

With no greenhouse gases, the incoming sunlight will hit the earth's atmosphere, where some of it will be reflected back into space, and the rest will travel to the earth's surface. Some of the radiation will be reflected from there, and the remainder will be absorbed and then re-emitted back into space at a longer wavelength.

 

This scenario results in an earth surface temperature of 255 K (which is 0º F and -18º C), which is much lower than is observed. This is where greenhouse gases come in.

When there are greenhouse gases present in the atmosphere, some of the radiation emitted by the earth is absorbed again before it escapes to space. This radiation absorbed by the atmosphere can then pass back down to the surface, warming it further. The radiation emitted from the greenhouse gases back into space is equal to the radiation from the sun (it must be, to maintain equilibrium), but near the surface there is a larger flux of energy, resulting in a warmer surface.



Another way to describe this effect is to say that, until a certain height, the atmosphere is opaque (you can't see through it) to IR radiation. Radiation can't make it out into space until it reaches a height where the atmosphere is transparent. As you go higher into the atmosphere, the temperature decreases at a rate of 9.8 ºC/km (this is called the atmospheric lapse rate), and the atmosphere becomes transparent at an altitude where the temperature is 255 K (the calculated temperature of the earth without greenhouse gases!). This is the region where the concentration gases thins out enough for IR radiation to pass into space.

This should make sense, given what we have talked about so far. Now, let's consider what happens when you add additional greenhouse gases to the atmosphere. When there are more greenhouse gases, the thickness of the opaque layer increases (that is, IR radiation is emitted from high up in the atmosphere). Because the IR radiation is being emitted from a higher altitude, where it is colder, the equilibrium of the radiation system is off balance.


In order to restore equilibrium, the atmosphere must heat up, so that the emission to space is again equal to that of a 255 K blackbody. As the atmosphere heats up, so to does the earth's surface, resulting in global warming.

In order for the sun-earth system to be in equilibrium, the earth must emit the same amount of energy it absorbs