Annual Global Mean Radiation Budget
The globally averaged, total amount of solar energy incident at the top of the atmosphere is 342 W/m2 (see above figure). This is an energy gain for the planet. Units of W/m2 (Watts per square meter) represent a heat flux density, the amount of energy crossing an area over a certain time interval. The annual average albedo of the planet is 30%. Thus, of the incident solar energy at the top of the atmosphere, approximately 30% or 103 W/m2 are sent back out to space and 70% (239 W/m2) are absorbed by the atmosphere and surface. The solar energy reflected back to space is considered a loss of energy. Of the 239 W/m2 gained by absorption, the atmosphere absorbs approximately 82 W/m2 (24% of the incoming solar energy) and the surface 157 W/m2 (46% of the incoming solar energy at the top of atmosphere). This absorption, and thus energy gain, by the atmosphere is primarily accomplished by water vapor, clouds, aerosols and ozone. Solar radiation is an energy supply for the atmosphere and the surface.
Earth's surface gains energy due to atmospheric emission of longwave energy but also loses radiant energy by emission. The losses exceed the gains and the surface loses 51 W/m2 as longwave radiation. Some of the surface-emitted energy escapes to space and the rest is absorbed by the atmosphere. The atmosphere emits radiant energy out to space and towards the surface, these losses being larger than gains from the surface (188 W/m2). The net terrestrial radiation to space is 239 W/m2, and so the radiation energy gains and losses are balanced at the top of the atmosphere. When we add the net solar energy gains to the net longwave energy losses we see that the surface has a net gain of radiant energy (157 W/m2 - 51 W/m2 = 106 W/m2) and the atmosphere experiences a net loss of radiant energy (82 W/m2-188 W/m2 = -106 W/m2).
A loss of 106 W/m2 by the atmosphere is equivalent to the atmosphere cooling more than 200°C over the course of a year! We do not observe this large cooling because energy is transferred from the surface to the atmosphere. The transfer of 106 W/m2 is accomplished by sensible and latent heat transfers. Sensible heat transfer represents the combined processes of conduction and convection, and amounts to a total of 21 W/m2. Latent heating transfers 85 W/m2 from the surface to the atmosphere. Evaporation from oceans and lakes and sublimation from glaciers also results in a cooling of the surface. Some of the water that is evaporated into the atmosphere condenses to form clouds and precipitation, releasing latent heat.
When globally averaged over a year, net energy gains are balanced by energy losses, or nearly so. This is not the case when the radiation gains and losses are averaged as a function of latitude.