Energy Budgets

Physical energy budgets consider a particular, defined system, and then analyze the inputs of energy, its various transformations and storages, and the eventual outputs. This concept can be illustrated by reference to the energy budget of Earth.

The major input of energy to Earth occurs as solar electromagnetic energy. At the outer limits of Earth's atmosphere, the average rate of input of solar radiation is 2.00 calories per cm² per minute (this flux is known as the solar constant). About half of this energy input occurs as visible radiation, and half as near-infrared. As noted previously, Earth also emits its own electromagnetic radiation, again at a rate of 2.00 cal/cm²/min, but with a spectrum that peaks in the longer-wave infrared, at about 10 æm. Because the rate of energy input equals the rate of output, there is no net storage of energy, and no substantial, longer-term change in Earth's surface temperature. Therefore, Earth represents a zero-sum, energy flow-through system. (Actually, over geological time there has been a small storage of energy, occurring as an accumulation of undecomposed biomass that eventually transforms geologically into fossil fuels. There are also minor, longer-term variations of Earth's temperature surface that represent climate change. However, these represent quantitatively trivial exceptions to the preceding statement about Earth as a zero-sum, flow-through system for energy.) Although the amount of energy emitted by Earth eventually equals the amount of solar radiation that is absorbed, there are some ecologically important transformations that occur between these two events. The most important ways by which Earth deals with its incident solar radiations are: (1) An average of about 30% of the incident solar energy is reflected back to outer space by Earth's atmosphere or its surface. This process is related to Earth's albedo, which is strongly influenced by the solar angle, the amounts of cloud cover and atmospheric particulates, and to a lesser degree by the character of Earth's surface, especially the types and amount of water (including ice) and vegetation cover. (2) About 25% of the incident energy is absorbed by atmospheric gases, vapors, and particulates, converted to heat or thermal kinetic energy, and then re-radiated as longer-wavelength infrared radiation. (3) About 45% of the incident radiation is absorbed at Earth's surface by living and non-living materials, and is converted to thermal energy, increasing the temperature of the absorbing surfaces. Over the longer term (that is, years) and even the medium term (that is, days) there is little or no net storage of heat. Virtually all of the absorbed energy is re-radiated by the surface as long-wave infrared energy, with a wavelength peak of about 10 æm. (4) Some of the thermal energy of surfaces causes water to evaporate from plant and non-living surfaces (see entry on evapotranspiration), or it causes ice or snow to melt. (5) Because of the uneven distribution of thermal energy on Earth's surface, some of the absorbed radiation drives mass-transport, distributional processes, such as winds, water currents, and waves on the surface of waterbodies. (6) A very small (averaging less than 0.1%) but ecologically critical portion of the incoming solar energy is absorbed by the chlorophyll of plants, and is used to drive photosynthesis. This photoautotrophic fixation allows some of the solar energy to be "temporarily" stored in the potential energy of biochemicals, and to serve as the energetic basis of life on Earth.

Certain gases in Earth's atmosphere absorb long-wave infrared energy of the type that is radiated by heated matter in dissipation mechanisms 2 and 3 (above). This absorption heats the gases, which then undergo another re-radiation, emitting even longer-wavelength infrared energy in all directions, including back to Earth's surface. The most important of the so-called radiatively active gases in the atmosphere are water and carbon dioxide, but the trace gases methane, nitrous oxide, ozone, and chlorofluorocarbons are also significant. This phenomenon, known as the greenhouse effect, significantly interferes with the rate of radiative cooling of Earth's surface.

If there were no greenhouse effect, and Earth's atmosphere was fully transparent to long-wave infrared radiation, surface temperatures would average about 17.6°F (-8°C), much too cold for biological processes to occur. Because the naturally occurring greenhouse effect maintains Earth's average surface temperature about 60 degrees warmer than this, at about 77°F (25°C), it is an obviously important factor in the habitability of our planet. However, human activities have resulted in increasing atmospheric concentrations of some of the radiatively active gases, and there are concerns that this could cause an intensification of Earth's greenhouse effect. This could lead to global warming, changes in the distributions of rainfall and other climatic effects, and severe ecological and socioeconomic damages.