Energy Balance
Sunlight reaching the Earth can heat the land, ocean, and atmosphere. Some of that sunlight is reflected back to space by the surface, clouds, or ice. Much of the sunlight that reaches Earth is absorbed and warms the planet.
This diagram represents the energy balance of the planet and is based on various observations. For our purposes, the exact numbers are not to be memorized, but only serve as a reference.
Incoming Solar Radiation
Units used to quantify incoming solar radiation are watts per square meter (Wm-2). Globally averaged, over all latitudes and including both day and night sides of the globe, the earth intercepts 342 Wm-2.
Reflected Solar Radiation
Of the incoming 342 Wm-2 intercepted by the planet, about 107 Wm-2 is reflected back to space and is not used to warm the planet. 107 is about 31% of 342 so the albedo of the planet is 31%.
Albedo refers to the amount of energy reflected by a surface and is usually expressed as a percentage.
* Dark colors have a low albedo.
* Light colors have an albedo close to 100%.
Absorbed by the surface
On average, the surface of the earth absorbs about 168 Wm-2, an energy gain for the surface.
Absorbed by the atmosphere
Clouds, aerosols and gases in the atmosphere absorb about 67 Wm-2 of the solar energy. So, the planet absorbs about 70% of what is incident at the top of the atmosphere.
Aerosols are airborne particulates (like smoke from forest fires, pollution from factories, or volcanic ash) that have a complex effect on Earth's energy balance: they cause cooling by reflecting incoming sunlight back out to space, and warming by absorbing solar energy and releasing heat. However, the net effect of aerosols is atmospheric cooling.
Longwave to surface
The Earth's surface also gains energy as a result of atmospheric emission of longwave energy. Objects emit radiation depending on their temperature and emittance. The atmosphere emits 324 W m-2 down to the surface.
Longwave from atmosphere
At the same time that the atmosphere emits radiant energy downward to the surface, it is also emitting longwave energy out to space.
Longwave from surface
When the surface is absorbing longwave radiation from the atmosphere, it loses its own longwave energy by emission upward.
Outgoing Longwave Radiation
So on average, the Earth is emitting 235 Wm-2 out to space.
Radiative Balance
When summed together, the surface's gains equal the atmosphere's losses. This means that the Earth and its atmosphere, when viewed as one system, are in radiative balance.
Sensible and Latent Heat
Energy is transferred from the surface to the atmosphere through ways other than radiation: sensible and latent heat transfers.
Sensible heating represents the combined processes of conduction and convection.
Evaporation from oceans and lakes and sublimation from glaciers requires energy from the Sun and causes cooling. As water vapor rises in the atmosphere, this energy is lying in wait -latent- ready to be released again when vapor is condensed into liquid. When water vapor condenses to form clouds and precipitation, latent heat is released and warming occurs.
