The energy that reaches the Earth's surface, that which we feel, is converted to heat energy, (infrared radiation) which then escapes upward and out from the Earth. If this heat did not escape, we would soon all cook to death from the accumulated heat of the sun.
However, not all of this heat escapes. Some of it is trapped within the atmosphere by molecules of gas, called "greenhouse gases". Most notably, these are water vapor, (H2O), methane (CH4) and carbon dioxide (CO2). Interestingly, if all the heat escaped, we would soon freeze to death. So we need the atmosphere to retain some of the heat, not all of it, just a nice comfortable amount. We humans are rather fragile in that regard. Too much heat is bad, too little is bad. So the Earth has somehow worked out a nice balance. More accurately, humans have evolved and adapted to the way things are.
So what is the big deal about global warming? What are humans supposedly doing to upset this nice comfortable balance of energy in....energy out. Well, for the last 150 years, or so, we have been burning coal, for heat, power, and now mostly to generate electricity; and then since about 1920, mainly because of automobiles, we have been burning an ever-increasing amount of oil. By doing this we have been adding a lot of carbon dioxide to the atmosphere. Let's look at the dynamics of where carbon dioxide fits into the picture and try to understand it's importance.
Energy (E) to heat the Earth comes from incident radiation from the sun. E is inversely proportional to the wavelength of light so more energetic waves are found at shorter wavelengths. In Figure 4, h is Plank's constant and c is the speed of light. Scanning from left to right on the electromagnetic spectrum shown in Figure 4, we move from higher energy (shorter wavelength) for ultraviolet radiation (UV) to lower energy (longer wavelength) for infrared radiation.
Energy reradiated from the Earth is not absorbed uniformly in the atmosphere. Certain molecules play a larger role than others. In first year chemistry, the quantization of the energy levels for a hydrogen atom were described. Only energies having a specific wavelength could be absorbed or emitted in moving an electron to different energy levels. Quantization of energy also is important when looking at the change in molecular vibrational and rotational energies. Atmospheric heating can take place by the absorbtion of infrared radiation. Figure 6 shows the vibrational potential energy curve for an oxygen molecule that has an average interatomic distance of ro. If the atoms were joined by a spring, the potential well would be symmetrical, as for a simple harmonic oscillator. However, an electrical bond can be considered to be like a very weak spring that breaks easily if enough energy is added. The energy needed to break the interatomic bond is the dissociation energy. O2 dissociation does occur in the Mesosphere and Thermosphere where high energy photons from the sun get absorbed. If some energy is added, but not enough to break the interatomic bond, then the molecular vibrational potential increases discretely, rather than uniformly. This reflects the quantum nature of the vibrational potential energy for a molecule. A similar quantization occurs for rotational energy. Different compounds absorb energy in different quantum steps. Since energy is related to wavelength the absorbtion spectrum for specific compounds is unique.
Water vapour and carbon dioxide molecules are common in the atmosphere, and their vibrational and rotational absorbtion bands overlap strongly with outgoing infrared radiation from the earth, so they affect the global radiation budget. Water vapour absorbs in both the infrared and and at higher energy. Below is a diagram showing how energy from the sun is absorbed by molecules in the atmosphere at specific wavelengths.
The annual incoming radiation from the sun, averaged over the Earth's surface, is 343 W/m2. The majority of this radiation is in the visible region of the EM spectrum. 103 W/m2 of the incoming radiation is scattered back to space by the atmosphere (89 W/m2) and the surface of the Earth (14 W/m2). These two terms determine the albedo of the Earth. Albedo is the ratio of the reflected radiation to incoming radiation. The albedo of a highly reflecting surface is close to 1. Fresh snow can have an albedo of 0.9 for example. Radiation coming in from the sun that is not scattered back to space is absorbed by the Earth and atmosphere.
The Earth and atmosphere then re-radiate 240 W/m2 in the infrared region of the EM spectrum.
The figure below shows the balance of radiation emitted from the Earth and the atmosphere. The balance between incoming and outgoing radiation is what determines the average temperature of the Earth.