After the short wavelength (referred to as shortwave) solar energy is absorbed at Earth's surface, a number of processes take place. Radiative exchange can be very complicated and its discussion here is greatly simplified.

2.1 Stored and Reradiated Energy

As Earth's surface absorbs incident shortwave solar energy, it heats up. However, because most common surface cover types are relatively poor conductors, heat absorbed at the surface does not reach to any significant depth. The absorbed energy instead is reradiated (emitted) as long wavelength (longwave) radiant energy. This emission of longwave radiant energy from land and water surfaces happens continually, but is most prominent at night. Radiant energy directly released into the atmosphere, flowing from a surface to the air due to a temperature difference between them, is known as sensible heat.

A portion of the solar energy absorbed at the surface is used to evaporate water, either from oceans and lakes or by means of the release of water vapor via vegetation photosynthesis (a process known as transpiration or evapotranspiration, discussed more in later chapters). Energy or heat in a system which is involved or transferred during phase changes is known as latent heat, or the heat of transformation. Latent heat is released or absorbed when water changes state during the processes of evaporation, evapotranspiration, melting, freezing, condensation, and sublimation (see Figure 2.06).

Since only small amounts of insolation at Earth's surface are converted into a form which can be stored more permanently (for example, fossil fuels), a balance must exist between incoming solar energy and Earth system reradiation or else the surface of Earth would permanently heat up. In general, the longwave radiation emitted by Earth into the atmosphere gets absorbed there. However, when skies are cloudless, some of this radiation can escape from the atmosphere into space since the atmosphere is "transparent" to longwave radiation in the 8.0-11.0 µm spectral wavelengths (except for a narrow ozone absorption band). Transparent means that radiation in specific wavelengths can travel through the atmosphere without being absorbed. The wavelengths of transparency are referred to as an "atmospheric window." This atmospheric window can be partially blocked by clouds or pollution.

Energy absorbed by the atmosphere, either directly from incoming solar energy or from longwave energy emitted by Earth's surface, gets reemitted as longwave radiation in all directions and is reabsorbed, both by the atmosphere and Earth's surface. This process of radiative exchange is complicated. The net result is that radiative losses from the atmosphere are overall balanced by the energy reemitted as heat from Earth's surface. This balance is maintained by transfer of sensible and latent heat from the surface into the atmosphere, with the latent heat flux being several times greater than that of sensible heat. If the surface heat emission and atmospheric loss or deficit were not balanced, the Earth-Atmosphere system would exhibit trends of consistently changing overall temperatures.

Measurements helping scientists to study the global radiation budget were taken by the ERBE (Earth Radiation Budget Experiment) satellite system. Figure 2.07 gives a global view of longwave radiation observed for January 1986. Figure 2.07a shows total scene (cloud effects included) longwave radiation and Figure 2.07b shows clear sky (cloud effects removed) longwave radiation for the same month. From this figure you can see that areas of high longwave radiation values correspond to warmer areas.

2.2 The Global Greenhouse

Though some longwave radiation emitted at Earth's surface escapes from the atmosphere into space, much of the longwave energy gets trapped inside the atmosphere. Water vapor, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and liquid water droplets absorb longwave energy emitted by the surface of Earth, warming the atmosphere in a process referred to as the "greenhouse effect."

For an example of the extremes of this effect, consider the air temperature during the night when the sky is clear as opposed to overcast. In a given season, on clear nights with dry air the temperature is colder than humid nights with thick cloud cover. This is because more longwave energy escapes to space on clear, dry nights, whereas on overcast nights energy cannot pass entirely through the clouds and gets absorbed within the atmosphere. The absorbed energy gets reradiated back to the surface and the cycle continues.

By comparison, the two nearest planets in our solar system have very different greenhouse conditions. Venus is closer to the sun and has larger amounts of greenhouse gases (mostly CO2) in its atmosphere than Earth. Little if any incoming solar energy escapes back to space, so the Venutian atmosphere and surface are hot. Mars, on the other hand, has a thin atmosphere with low absolute amounts of greenhouse gases such that the solar insolation that gets reemitted from the surface cannot be trapped, and it travels back into space.

On average over time, as long as the loss of energy to space balances the energy retained within the Earth-Atmosphere system, the greenhouse effect acts to keep our planet at temperatures conducive to many life forms. If the atmospheric composition was different, as it has been in the geologic past or as a result of current or future dynamics, the radiation budget might not be balanced, and the Earth-Atmosphere system could change its overall temperature characteristics.

Increases in the amounts of greenhouse gases and liquid water could amplify the greenhouse effect such that energy absorption in the atmosphere would increase and cause a persistent rise in global temperatures. This process is known as global warming. Scientists are uncertain about some aspects of global warming, especially prediction of all of its consequences, however the subject is a matter of great concern since atmospheric composition and global climate affect the ability of the planet's surface to sustain life in its present forms.

2.3 Redistribution of Energy Around the Globe

What happens to the heat in the surficial Earth-Atmosphere system? Though overall the system is in radiative balance, various geographic areas and surface types are heated differentially. That is, some areas absorb and emit more or less heat than other areas, giving rise to localized energy surpluses and deficits. On a broad global view, at latitudes higher than 37 degrees more radiant energy is lost to space each year than is received, while at lower latitudes, more energy is gained from the sun than is returned to space as heat (Figure 2.08). The general circulation of the atmosphere and oceans redistributes heat from areas of surplus to areas of deficit.

2.3.1 Atmospheric circulation -- Because so much more insolation is absorbed in tropical than in polar regions, the heat differential between these areas drives atmospheric circulation, which moves a tremendous amount of heat energy and water vapor around the globe. Heat transfer in the atmosphere involves both sensible and latent heat, with most of the exchange being in the form of latent heat. Water in the atmosphere is continually changing between gaseous, liquid, and solid states. With these changes energy is absorbed or released (Figure 2.06). Sometimes the release of energy comes in the form of violent weather events such as thunderstorms, tornados, and hurricanes.

A simplified description of atmospheric circulation is as follows: (1) the atmosphere is heated more around the equator, (2) the warming air is less dense and rises, and (3) the rising tropical air gets replaced by cooler, denser air moving down from the poles by a process known as convection. If Earth did not rotate there would simply be two circulation cells, one in the northern and one in the southern hemisphere. Due to the rotation of Earth and the resulting Coriolis force, a single circulation pattern cannot be maintained and in actuality each hemisphere has three circulation cells (Figure 2.09).

2.3.2 The impact of the oceans -- Ocean circulation transfers massive amounts of energy around the world. Given the tremendous volume of water in the oceans and the ability of water to hold heat, the oceans store more energy than the atmosphere and release it more slowly. The thermal capacity of the oceans imparts a moderating effect on global temperatures. Land surfaces cannot hold heat as well as water and experience greater extremes of temperatures.

The exchange of energy and water between the oceans and the atmosphere exceeds that between land surfaces and the atmosphere. Circulation of the atmosphere and the oceans is to some extent coupled. Changes in either atmospheric or oceanic circulation affect the circulation of the other. A dramatic example of this coupling is the El Niño-Southern Oscillation (ENSO) that occurs periodically in the southern Pacific Ocean: the moisture budget in the atmosphere-ocean system changes substantially and sets into action a series of events. Surface winds are altered and the upwelling of cold nutrient-rich water along the west coast of South America is precluded. Warm nutrient-poor surface waters diminish the fish harvest and increase evaporation to the atmosphere. Though ENSO originates in the tropical Pacific, the associated redistribution of energy disrupts climate patterns around the globe.