Greenhouse Effect

The greenhouse effect is the retention by the Earth's atmosphere in the form of heat some of the energy that arrives from the Sun as light. Certain gases, including carbon dioxide (CO₂) and methane (CH₄), are transparent to most of the wavelengths of light arriving from the Sun but are relatively opaque to infrared or heat radiation; thus, energy passes through the Earth's atmosphere on arrival, is converted to heat by absorption at the surface and in the atmosphere, and is not easily re-radiated into space. The same process is used to heat a solar greenhouse, only with glass, rather than gas, as the heat-trapping material. The greenhouse effects happens to maintain the Earth's surface temperature within a range comfortable for living things; without it, the Earth's surface would be much colder.

The greenhouse effect is mostly a natural phenomenon, but its intensity, according to a majority of climatologists, may be increasing because of increasing atmospheric concentrations of CO₂ and other greenhouse gases. These increased concentrations are occurring because of human activities, especially the burning of fossil fuels and the clearing of forests (which remove CO₂ from the atmosphere and store its carbon in cellulose, [C₆H₁₀O₅]_n). A probable consequence of an intensification of Earth's greenhouse effect will be a significant warming of the atmosphere. This in turn would result in important secondary changes, such as a rise in sea level (already occurring), variations in the patterns of precipitation. These, in turn, might accelerate the rate at which species are already being to extinction by human activity, and impose profound adjustments on human society.

The energy budget

To understand the greenhouse effect, Earth's energy budget must be known. An energy budget is an account of all of the energy coming into and leaving a system and of any energy that is stored in (or produced by) the system itself. Almost all of the energy coming to Earth from space has been radiated by the closest star, the Sun. The Sun emits electromagnetic energy at a rate and spectral quality determined by its surface temperature. In this it resembles all bodies having a temperature greater than absolute zero (i.e., -459°F or -273°C). Fusion reactions occurring in the core the Sun give it a high surface temperature, about 10,800°F (6,000°C). As a consequence, about one-half of the Sun's emitted energy is visible radiation with wavelengths between 0.4 and 0.7 æm, so called because this is the range of electromagnetic wavelengths that the human eye can perceive. Most of the remainder is in the near-infrared wavelength range, between about 0.7 and 2.0 æm. The Sun also emits radiation in other parts of the electromagnetic spectrum, such as ultraviolet and x rays; however, these wavelengths convey relatively insignificant amounts of energy away from the Sun.

At the average distance of Earth from the Sun, the rate of input of solar energy to the Earth's surface is about 2 calories per minute per square centimeter, a value termed the solar constant. There is a nearly perfect energetic balance between this quantity of energy incoming to Earth and the amount that is eventually dissipated to outer space. The myriad ways in which the incoming energy is reflected, dispersed, transformed, and stored make up Earth's energy budget.

REFLECTION. On average, one-third of incident solar radiation is reflected back to space by the Earth's atmosphere or its surface. Earth's local reflectivity (albedo) is strongly dependent on cloud cover, the density of tiny particulates in the atmosphere, and the nature of the surface, especially vegetation and ice and snow.

Atmospheric absorption and radiation

Another one-third of incoming solar radiation is absorbed by certain gases and vapors in Earth's atmosphere, especially water vapor and carbon dioxide. Upon absorption, the solar electromagnetic energy is transformed into thermal kinetic energy (i.e., heat or energy of molecular vibration). The warmed atmosphere then reradiates energy in all directions as longer-wavelength (7–14 æm) infrared radiation. Much of this reradiated energy escapes to outer space.

ABSORPTION AND RADIATION AT THE SURFACE. Much of the solar radiation that penetrates to Earth's surface is absorbed by living and nonliving materials. This results in a transformation to thermal energy, which increases the temperature of the absorbing surfaces and of air in contact with those surfaces. Over the medium term (days) and longer term (years) there is little net storage of energy as heat; almost all of the thermal energy is re-radiated by the surface as electromagnetic radiation of a longer wavelength than that of the original, incident radiation. The wavelength spectrum of typical, reradiated electromagnetic energy from Earth's surface peaks is within the long-wave infrared range.

EVAPORATION AND MELTING OF WATER. Some of the electromagnetic energy that penetrates to Earth's surface is absorbed and transformed to heat. Much of this thermal energy subsequently causes water to evaporate from plant and open-water surfaces, or melts ice and snow.

WINDS, WAVES, AND CURRENTS. A small amount (less than 1%) of the absorbed solar radiation causes mass-transport processes to occur in the oceans and lower atmosphere, which disperses of some of Earth's unevenly distributed thermal energy. The most important of these physical processes are winds and storms, water currents, and waves on the surface of the oceans and lakes.

PHOTOSYNTHESIS. Although small, an ecologically critical quantity of solar energy, averaging less than 1% of the total, is absorbed by plant pigments, especially chlorophyll. This absorbed energy is used to drive photosynthesis, the energetic result of which is a temporary storage of energy in the interatomic bonds of certain biochemical compounds. This energy is released when plant material is digested or burned.

Now we are ready to explain the greenhouse effect. If the atmosphere was transparent to the long-wave infrared energy that is reradiated by Earth's atmosphere and surface, then that energy would travel unobstructed to outer space. However, so-called radiatively active gases (or RAGs; also known as "greenhouse gases") in the atmosphere are efficient absorbers within this range of infrared wavelengths, and these substances thereby slow the radiative cooling of the planet. When these atmospheric gases absorb infrared radiation, they develop a larger content of thermal energy, which is then dissipated by a reradiation (again, of a longer wavelength than the electromagnetic energy that was absorbed). Some of the secondarily reradiated energy is directed back to Earth's surface, so the net effect of the RAGs is to slow the rate of cooling of the planet.

This process has been called the "greenhouse effect" because its mechanism is analogous to that by which a glass-enclosed space is heated by solar energy. That is, a greenhouse's glass and humid atmosphere are transparent to incoming solar radiation, but absorb much of the re-radiated, long-wave infrared energy, slowing down the rate of cooling of the structure.

Water vapor (H₂O) and CO₂ are the most important radiatively active constituents of Earth's atmosphere. Methane (CH₄), nitrous oxide (N₂O), ozone (O₃), and chlorofluorocarbons (CFCs) play lesser roles. On a per-molecule basis, all these gases differ in their ability to absorb infrared wavelengths. Compared with CO₂, methane is 11–25 times more effective at absorbing infrared, nitrous oxide is 200–270 times, ozone 2,000 times, and CFCs 3,000–15,000 times.

Other than water vapor, the atmospheric concentrations of all of these gases have increased in the past century because of human activities. Prior to 1850, the concentration of CO₂ in the atmosphere was about 280 parts per million (ppm), while 2002 it was over 360 ppm. During the same period, CH₄ increased from 0.7 ppm to 1.7 ppm, N₂O from 0.285 ppm to 0.304 ppm, and CFCs from nothing to 0.7 parts per billion. These increased concentrations are believed by climatologists to contribute to a significant increase in the greenhouse effect. Overall, CO₂ is estimated to account for about 60% of this enhancement of the greenhouse effect, CH₄ for 15%, N₂O for 5%, O₃ for 8%, and CFCs for 12%.