The Greenhouse Effect

Visible light from sun Earth radiates in IR

Energy from Sun Energy lost to space Balance when T is constant

BUT some molecules in the troposphere have polar bonds. These absorb photons of IR radiation whose energy corresponds to the gap between the vibrational energy levels of the molecule. This increases the vibrational energy of the molecule and so increases the temperature of the stratosphere as the energy is passed on to other molecules when they collide.

Energy from Sun Energy lost to space With less energy lost to space there is now a net gain of energy by the earth – until a new balance is reached at a higher temperature.

Mars Earth Venus T/ºC Size of greenhouse effects

Mars has a small greenhouse effect as the carbon dioxide atmosphere is very thin. Venus has a massive greenhouse effect (the surface temperature could melt lead) as there is a thick carbon dioxide atmosphere. Earth has an atmosphere mostly composed of non-polar nitrogen and oxygen molecules that do not absorb ir radiation. The low concentrations of water vapour and carbon dioxide cause a small natural greenhouse effect 15ºC. A significant increase in greenhouse gases might be expected to have an appreciable affect on climate – global warming.