A schematic representation of the exchanges of energy between outer space, the Earth's atmosphere, and the Earth's surface. The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect.

The greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation.^[1] Thus, greenhouse gases trap heat within the surface-troposphere system.^[2] This mechanism is fundamentally different from that of an actual greenhouse, which works by isolating warm air inside the structure so that heat is not lost by convection. The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896.^[3]

In the absence of the greenhouse effect and an atmosphere, the Earth's average surface temperature^[4] of 14 °C (57 °F) could be as low as −18 °C (−0.4 °F), the black body temperature of the Earth.^[5][6][7] Global warming, a recent warming of the Earth's surface and lower atmosphere,^[8] is believed to be the result of an "enhanced greenhouse effect" mostly (more than 50%) due to human-produced increases in atmospheric greenhouse gases.^[9] This human induced part is refered to as anthropogenic global warming (AGW).

Basic mechanism

The Earth receives energy from the Sun mostly in the form of visible light and nearby wavelengths. About 50% of the sun's energy is absorbed at the Earth's surface. Like all bodies with a temperature above absolute zero the Earth's surface radiates energy in the infrared range. Greenhouse gases in the atmosphere absorb most of the infrared radiation emitted by the surface and pass the absorbed heat to other atmospheric gases through molecular collisions. The greenhouse gases also radiate in the infrared range. Radiation is emitted both upward, with part escaping to space, and downward toward Earth's surface. The surface and lower atmosphere are warmed by the part of the energy that is radiated downward, making our life on earth possible.^[5]

Greenhouse gases

In order, Earth's most abundant greenhouse gases are:

• water vapor
• carbon dioxide
• methane
• nitrous oxide
• ozone
• CFCs

Real Climate ranks by their contribution to the greenhouse effect:^[10]

• water vapor, which contributes 36–70%
• carbon dioxide, which contributes 9–26%
• methane, which contributes 4–9%
• ozone, which contributes 3–7%

The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the atmosphere.^[11]

Anthropogenic greenhouse effect

Carbon dioxide is the human-produced greenhouse gas that contributes most of radiative forcing from human activity. CO₂ is produced by fossil fuel burning and other human activities such as cement production and tropical deforestation.^[12] Measurements of CO₂ from the Mauna Loa observatory show that concentrations have increased from about 313 ppm ^[13] in 1960 to about 383 ppm in 2009. The current observed amount of CO₂ exceeds the geological record maxima (~300 ppm) from ice core data.^[14] The effect of combustion-produced carbon dioxide on the global climate, a special case of the greenhouse effect first described in 1896 by Svante Arrhenius, has also been called the Callendar effect.

Because it is a greenhouse gas, elevated CO₂ levels will contribute to additional absorption and emission of thermal infrared in the atmosphere, which could contribute to net warming. In fact, according to Assessment Reports from the Intergovernmental Panel on Climate Change, "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations".^[15]

Over the past 800,000 years,^[16] ice core data shows unambiguously that carbon dioxide has varied from values as low as 180 parts per million (ppm) to the pre-industrial level of 270ppm.^[17] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.^[18]

The distinction between the greenhouse effect and real greenhouses

A modern Greenhouse in RHS Wisley

The term "greenhouse effect" can be a source of confusion as actual greenhouses do not function in the same way as the atmospheric greenhouse effect does.

Various materials at times imply incorrectly that they do, or do not make a distinction between the warming effect and the mechanisms involved. Many sources make the "heat trapping" analogy of how a greenhouse limits convection to how the atmosphere performs a similar function through the different mechanism of infrared absorbing gases.

A greenhouse is usually built of glass, plastic, or a plastic-type material. It heats up mainly because the sun warms the ground inside it, which then warms the air in the greenhouse. The air continues to heat because it is confined within the greenhouse, unlike the environment outside the greenhouse where warm air near the surface rises and mixes with cooler air aloft. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (Wood, 1909) that a "greenhouse" with a cover of rock salt heats up an enclosure similarly to one with a glass cover.^[19] Thus greenhouses work by a different mechanism, primarily by preventing convective cooling.^[20][21]

Bodies other than Earth

In our solar system, Mars, Venus, and the moon Titan also exhibit greenhouse effects. Titan has an anti-greenhouse effect, in that its atmosphere absorbs solar radiation but is relatively transparent to infrared radiation. Pluto also exhibits behavior similar to the anti-greenhouse effect.^[22][23][24]

A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.^[25] A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.^[26]

See also

Wikibooks has a book on the topic of Climate Change

Wikiversity has learning materials about Topic:Climate change

• Anti-greenhouse effect
• Climate change
• Climate forcing
• Earth's energy budget
• Earth's radiation balance
• Global dimming
• Global warming
• Idealized greenhouse model

Literature

• Earth Radiation Budget, http://marine.rutgers.edu/mrs/education/class/yuri/erb.html
• Fleagle, RG and Businger, JA: An introduction to atmospheric physics, 2nd edition, 1980
• IPCC assessment reports, see http://www.ipcc.ch/
• Ann Henderson-Sellers and McGuffie, K: A climate modelling primer (quote: Greenhouse effect: the effect of the atmosphere in re-readiating longwave radiation back to the surface of the Earth. It has nothing to do with glasshouses, which trap warm air at the surface).
• Idso, S.B.: "Carbon Dioxide: friend or foe," 1982 (quote: ...the phraseology is somewhat in appropriate, since CO₂ does not warm the planet in a manner analogous to the way in which a greenhouse keeps its interior warm).
• Kiehl, J.T., and Trenberth, K. (1997). "Earth's annual mean global energy budget," Bulletin of the American Meteorological Society '78' (2), 197–208.

References