Climate Change Lesson 2 : Earth's Energy Balance

The Earth receives energy from the sun in relatively high energy shortwave radiation. This reaches the Earth primarily in the form of visible light, although it includes some infrared energy (heat), ultraviolet energy, and other wavelengths of the electromagnetic spectrum. Taking into account night and day and the seasons, on average about 340 Watts of energy enter every square meter of the Earth system. This is slightly less than the energy that six 60 Watt light bulbs would produce - for every square meter of the Earth.

This energy warms the earth, which emits relatively low energy longwave radiation. This is lost to space or reflected back by the atmosphere. It is an oversimplification to say that energy comes to the earth as light and is radiated back from the earth as heat, but that may be a useful simplification for some students.

As it reaches the Earth system, some of the sunlight is reflected back to space by clouds and the atmosphere (particularly dust particles or aerosols in the atmosphere). A little more sunlight is reflected to space from the Earth surface, particularly from bright regions such as snow- and ice-covered areas. In total, about 30% of sunlight is reflected directly back to space. About 70% of the sunlight is absorbed by the Earth system (atmosphere and surface) and heats it up.

The Sun doesn’t heat the Earth evenly. The elements of the Earth system (surface, atmosphere, clouds) emit infrared radiation according to their temperature. Cold objects emit less energy; warm objects emit more. This infrared radiation is emitted in all directions. The amount of sunlight the Earth absorbs depends on the reflectiveness of the atmosphere and the Earth’s surface. The term albedo is used to define the percentage of radiation reflected back by a surface with “one” being virtually all radiation reflected and “zero” being no reflectivity. Snow and ice have a high albedo where most of the sunlight hitting the surface bounces back towards space. Water is much more absorbent and less reflective. Dark colored land surfaces are strongly absorptive (low albedo) and contribute to warming.

Because of the shape and orientation of the Earth, the sun heats equatorial regions more than Polar Regions. The atmosphere and ocean work nonstop to even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds, and ocean circulation. This coupled atmosphere and ocean circulation is known as Earth’s heat engine.

One net effect of all the infrared emission is that an amount of heat energy equivalent to ~70% of the incoming sunlight leaves the Earth system and goes back into space. This is because the Earth system constantly tends toward equilibrium between the energy that reaches the Earth from the Sun and the energy that is emitted to space. Scientists refer to this process as Earth’s “radiation budget”, and it happens because the system tends toward equilibrium.

Another net effect of the infrared emission is that about 340 Watts of infrared energy is directed back to the surface from the atmosphere. This is called the greenhouse effect, and is due mainly to water vapor in the atmosphere. Carbon dioxide, methane and other infrared-absorbing gases enhance this effect (more about the Greenhouse Effect in Lesson 3). Without an atmosphere, the Earth would have an average temperature of -18 °C, too cold for life as we know it.

At the surface, two additional heat transfer mechanisms operate to balance the system, in addition to the radiation transfer: 1) convection and conduction in the form of thermals (which create weather), and 2) a change of state of water through evapotranspiration (which also feeds weather).

The Earth’s heat engine must not only redistribute solar heat from the equator toward the poles, but also from the Earth’s surface and lower atmosphere back to space. Otherwise, the Earth would endlessly heat up. The Earth’s temperature doesn’t infinitely rise because the surface and the atmosphere are simultaneously radiating heat to space. This net flow of energy into and out of the Earth system is Earth’s energy budget. The balance between the amount of solar radiation absorbed, and the amount of longwave radiation emitted by the earth is known as radiative equilibrium. Radiative forcing is the change in the balance between solar radiation entering the atmosphere and the Earth’s radiation going out. This concept will be further developed in Lesson 5.