Escola Secundária de Loulé
Greenhouse effect receives its name after the similar effect that occurs in greenhouses because the glass is opaque to infrared radiation, which produces a rise in temperature. This temperature increase happens because all of the other radiation can cross the glass both ways, but infrared cannot. Inside the greenhouse most of the energy emitted by the bodies and the soil is infrared, and since glass is opaque to this radiation a big part of it will be absorbed and reemitted to the inside of the greenhouse. Since infrared emission is the major emission coming from the inside, but visible emission is the major emission from the outside, the energy that comes in is bigger than the energy that gets out. This produces an energy increase inside the greenhouse.
The temperature of a planet is a balance between the absorbed energy and the emitted energy.
Earth only absorbs 70% of the energy that arrives from the Sun. This means that Earth’s albedo is 0.3 (30% of the energy is reflected back to space).
In the most simple approach to the radiation balance between Earth and its surroundings Earth’s atmosphere is considered completely transparent, and this would imply that the 70% of the solar energy would arrive to the ground and all energy emitted by Earth would be transmitted to outer space.
If scientists considered only the radiation balance between Earth and its surroundings their calculations of Earth’s mean temperature would be 33 K (33 ºC) lower than the real observed temperature of about 288K (ca. 15 ºC). This implies that Earth's energy balance is much more complex (Figure 1).
But Earth’s atmosphere is not transparent, because there is some absorption by gases in the atmosphere. The most abundant gases (oxygen and nitrogen) are in fact transparent to infrared radiation. But some minor gases like water vapour, carbon dioxide, ozone and methane are very opaque to infrared radiation and this is what induces greenhouse effect (Figure 2). One may think that the atmosphere has so little amount of matter that it seems impossible that it can induce so big changes in climate with such small amounts of greenhouse gases.
But Earth’s atmosphere is, in fact, inducing major changes in global temperature. Though water vapour presents big greenhouse effect, its contribution remains mostly constant. Carbon dioxide, on the other hand, has had a consistent concentration rise over the years and this has produced a clear global warming over the last decades (Figure 3).
Though it represents only 0,04% of the gaseous composition of the atmosphere, CO2 emissions have been continuously growing and this is responsible for global warming.
Considering its albedo of 76%, the expected temperature, from a simple energy balance without greenhouse effect would be 232 K. Nonetheless, because its atmosphere has about 96% of CO2, its real surface temperature is about 700 K. Greenhouse effect makes the temperature of the dark side of the planet to be the same as the temperature of the bright side.
Since most CO2, emitted is produced in fossile fuel combustions to produce energy, this is also a major concern on the quest for new energy sources.
Bibliography
• Cabrera,M.E, Hoyos,C., Ledesma,J.L., Nieto,J.M., Revuelta,J.L., Romero,T., Salamanca,C., Torres,M.D., Velasco,J.M. (1996) Ciencias de la Tierra y del Medio Ambiente, Madrid: Editorial Editex.
• MacCracken, M.C., Luther, F.M, (eds), 1985. Projecting the climatic effects of increasing carbon dioxide. DOE/ER-0237, United States Department of Energy, Washington D.C., pp. 381.
• Mitchell, J.F.B. 1989. The "greenhouse" effect and climate change. Reviews of Geophysics 27:115.
• Sturman, A.P. and N.J. Tapper. (1996) The Weather and Climate of Australia and New Zealand, Melbourne: Oxford University Press.
The temperature of a planet is a balance between the absorbed energy and the emitted energy.
Earth only absorbs 70% of the energy that arrives from the Sun. This means that Earth’s albedo is 0.3 (30% of the energy is reflected back to space).
In the most simple approach to the radiation balance between Earth and its surroundings Earth’s atmosphere is considered completely transparent, and this would imply that the 70% of the solar energy would arrive to the ground and all energy emitted by Earth would be transmitted to outer space.
If scientists considered only the radiation balance between Earth and its surroundings their calculations of Earth’s mean temperature would be 33 K (33 ºC) lower than the real observed temperature of about 288K (ca. 15 ºC). This implies that Earth's energy balance is much more complex (Figure 1).
Figure 1. Earth’s energy balance. Adapted from MacCracken & Luther, 1985, de Mitchell, 1989, Sturman and Tapper,1996 e Cabrera et al.,1996. The percentages are relative to the radiation that reaches Earth from the Sun per square meter.
But Earth’s atmosphere is not transparent, because there is some absorption by gases in the atmosphere. The most abundant gases (oxygen and nitrogen) are in fact transparent to infrared radiation. But some minor gases like water vapour, carbon dioxide, ozone and methane are very opaque to infrared radiation and this is what induces greenhouse effect (Figure 2). One may think that the atmosphere has so little amount of matter that it seems impossible that it can induce so big changes in climate with such small amounts of greenhouse gases.
Figure 2. Absorption spectra of the atmosphere and of its two major greenhouse gases (water vapour and carbon dioxide). Adapted from Cabrera et al., 1996.
But Earth’s atmosphere is, in fact, inducing major changes in global temperature. Though water vapour presents big greenhouse effect, its contribution remains mostly constant. Carbon dioxide, on the other hand, has had a consistent concentration rise over the years and this has produced a clear global warming over the last decades (Figure 3).
Figure 3. Global warming over time (Credit: Climatic Research Unit (http://www.cru.uea.ac.uk/)).
Though it represents only 0,04% of the gaseous composition of the atmosphere, CO2 emissions have been continuously growing and this is responsible for global warming.
Some people are very sceptical about global warming. We recommend them to study Venus (Figure 4).
Considering its albedo of 76%, the expected temperature, from a simple energy balance without greenhouse effect would be 232 K. Nonetheless, because its atmosphere has about 96% of CO2, its real surface temperature is about 700 K. Greenhouse effect makes the temperature of the dark side of the planet to be the same as the temperature of the bright side.
Since most CO2, emitted is produced in fossile fuel combustions to produce energy, this is also a major concern on the quest for new energy sources.
Bibliography
• Cabrera,M.E, Hoyos,C., Ledesma,J.L., Nieto,J.M., Revuelta,J.L., Romero,T., Salamanca,C., Torres,M.D., Velasco,J.M. (1996) Ciencias de la Tierra y del Medio Ambiente, Madrid: Editorial Editex.
• MacCracken, M.C., Luther, F.M, (eds), 1985. Projecting the climatic effects of increasing carbon dioxide. DOE/ER-0237, United States Department of Energy, Washington D.C., pp. 381.
• Mitchell, J.F.B. 1989. The "greenhouse" effect and climate change. Reviews of Geophysics 27:115.
• Sturman, A.P. and N.J. Tapper. (1996) The Weather and Climate of Australia and New Zealand, Melbourne: Oxford University Press.
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