Radiation from the Sun peaks at a wavelength of around 10nm, according to Wien's Law with a temperature of 5780K. The Earth's atmosphere is transparent at this wavelength, so the radiation can pass through, However, the re-emitted radiation from the Earth peaks at a wavelength of about 500nm, due to the lower surface temperature of the Earth (about 287K on average). As can be seen from figure 1, the Earth's atmosphere is opaque at this wavelength. Effectively, the radiation can pass through the atmosphere inwards but most of it cannot pass outwards. The amount of radiation which escapes is dependent on the concentration of carbon dioxide and other greenhouse gases in the atmosphere - carbon dioxide alone accounts for about 26% of the greenhouse effect, and the effects of carbon dioxide are specifically known as the Callendar effect. The wavelengths of radiation which are absorbed by a gas can be worked out using quantum mechanics, and it can be shown that heteronuclear diatomic and triatomic gas molecules absorb at these wavelengths, while homonuclear molecules don't - this is why water and carbon dioxide absorb this radiation but oxygen and nitrogen do not.

Figure 1: Atmospheric absorption of electromagnetic radiation along a 1 NM sea level path.

The process normally referred to as the greenhouse effect is misnamed - greenhouses stay warmer than the air outside because the Sun heats up the ground inside the greenhouse, and the glass panes prevent this warm air from rising and flowing away from the area - convection is prevented. However, the "greenhouse effect" on the Earth prevents radiation loss occurring, not convection.

Nevertheless, the increased concentrations of carbon dioxide, methane, and other greenhouse gases in the atmosphere are at least partly responsible for global warming. The concentration of these gases is currently the highest it has been for 420000 years - 22 billion tonnes of carbon dioxide and other greenhouse gases are produced as a result of human activities per year. As discussed in the basic explanation of the effects of carbon dioxide, global warming could have disastrous consequences for the whole planet. Anything we can do to reduce the concentration of carbon dioxide in the atmosphere, or at least keep it at a constant value, must be done to help prevent these consequences.