The greenhouse effect is a naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the Earth's surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18° Celsius, rather than the present 15° Celsius.

As energy from the sun passes through the atmosphere a number of things take place (see Figure 1). A portion of the energy (26% globally) is reflected or scattered back to space by clouds and other atmospheric particles. About 19% of the energy available is absorbed by clouds, gases (like ozone), and particles in the atmosphere. Of the remaining 55% of the solar energy passing through the Earth's atmosphere, 4% is reflected from the surface back to space. On average, about 51% of the sun's radiation reaches the surface. This energy is then used in a number of processes, including the heating of the ground surface; the melting of ice and snow and the evaporation of water; and plant photosynthesis.

The heating of the ground by sunlight causes the Earth's surface to become a radiator of energy in the longwave band (sometimes called infrared radiation). This emission of energy is generally directed to space (see Figure 2). However, only a small portion of this energy actually makes it back to space. The majority of the outgoing infrared radiation is absorbed by the greenhouse gases (see Figure 3).

Absorption of longwave radiation by the atmosphere causes additional heat energy to be added to the Earth's atmospheric system. The now warmer atmospheric greenhouse gas molecules begin radiating longwave energy in all directions. Over 90% of this emission of longwave energy is directed back to the Earth's surface where it once again is absorbed by the surface. The heating of the ground by the longwave radiation causes the ground surface to once again radiate, repeating the cycle described above, again and again, until no more longwave is available for absorption.

The amount of heat energy added to the atmosphere by the greenhouse effect is controlled by the concentration of greenhouse gases in the Earth's atmosphere. All of the major greenhouse gases have increased in concentration since the beginning of the Industrial Revolution (about 1700 AD). As a result of these higher concentrations, scientists predict that the greenhouse effect will be enhanced and the Earth's climate will become warmer. Predicting the amount of warming is accomplished by computer modeling. Computer models suggest that a doubling of the concentration of the main greenhouse gas, carbon dioxide, may raise the average global temperature between 1 and 3° Celsius. However, the numeric equations of computer models do not accurately simulate the effects of a number of possible negative feedbacks. For example, many of the models cannot properly simulate the negative effects that increased cloud cover would have on the radiation balance of a warmer Earth. Increasing the Earth's temperature would cause the oceans to evaporate greater amounts of water, causing the atmosphere to become cloudier. These extra clouds would then reflect a greater proportion of the sun's energy back to space reducing the amount of solar radiation absorbed by the atmosphere and the Earth's surface. With less solar energy being absorbed at the surface, the effects of an enhanced greenhouse effect may be counteracted.

A number of gases are involved in the human-caused enhancement of the greenhouse effect (see Table 1). These gases include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFXClX); and tropospheric ozone (O3). Of these gases, the single most important gas is carbon dioxide which accounts for about 63% of the change in the intensity of the Earth's greenhouse effect as measured in 2005 (Foster et al., 2007). The contributions of the other gases are 18% for methane, 10% for chlorofluorocarbons, and 6% for nitrous oxide. Ozone's contribution to the enhancement of greenhouse effect is still yet to be quantified.

Note that the greenhouse effect does not put the Earth's energy budget "out of balance." The heat is held temporarily in the atmosphere, and then eventually is released from the atmosphere as long wavelength infrared. In effect, the greenhouse effect causes the planet to raise its surface temperature until the amount of heat radiated from the top of the absorbing layer of the atmosphere is equal to the solar radiation at the top of the atmosphere. The effective temperature (-18°C) is reached at the top of the absorbing layer, while down at the surface of the Earth it is much warmer.

In summary, the greenhouse effect causes the atmosphere to trap more heat energy at the Earth's surface and within the atmosphere by absorbing and re-emitting longwave energy. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. The greenhouse effect and other components of the Earth's energy balance is central to climate change science. Human activity such as land use change and the combustion of fossil fuels affect the albedo of the planet, release greenhouse gases, and other wise perturb the climate system. Understanding the important details of the complex system is one of the great challenges facing the world's scientific community. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that the Earth's average global temperature has already increased by 0.3 to 0.6° Celsius, since the beginning of this century, because of this enhancement. Predictions of future climates indicate that by the middle of the next century the Earth's global temperature may be 1 to 3° Celsius higher than today.

Sources

• Pidwirny, Michael (Lead Author); Howard Hanson (Topic Editor). 2008. Greenhouse effect.  In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth October 18, 2006; Last revised September 17, 2008; Retrieved June 2, 2009].