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] Greenhouse gases, which include water vapor, carbon dioxide and methane, are almost transparent to solar radiation but strongly absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system.[2][3][4][5] 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.[6]
In the absence of the greenhouse effect and an atmosphere, the Earth's average surface temperature[7] of 14 °C (57 °F) could be as low as −18 °C (−0.4 °F), the black body temperature of the Earth.[8][9][10] Anthropogenic global warming (AGW), a recent warming of the Earth's lower atmosphere as evidenced by the global mean temperature anomaly trend,[11] is believed to be the result of an "enhanced greenhouse effect" mainly due to human-produced increased concentrations of greenhouse gases in the atmosphere.[12]
Basic mechanism
The Earth receives energy from the Sun mostly in the form of visible light. The atmosphere is almost transparent to visible light, so that about 50% of the sun's energy reaches the Earth and is absorbed by the surface. Like all bodies with a temperature above absolute zero the Earth's surface radiates energy in the infrared range. Greenhouse gases are not transparent to infrared radiation so they absorb it. Infrared radiation is absorbed from all directions and is passed as heat to all gases in the atmosphere. The atmosphere also radiates in the infrared range (because of its temperature, in the same way the Earth's surface does) and does so in all directions. The surface and lower atmosphere are warmed because of the greenhouse gases, making our life on earth possible.[8]
Detailed explanation
The Earth receives energy from the Sun in the form of radiation. Most of the energy is in visible wavelengths and in infrared wavelengths that are near the visible range (often called "near infrared"). The Earth reflects about 30% of the incoming solar radiation. The remaining 70% is absorbed, warming the land, atmosphere and ocean.
For the Earth's temperature to be in steady state so that the Earth does not rapidly heat or cool, this absorbed solar radiation must be very closely balanced by energy radiated back to space in the infrared wavelengths. Since the intensity of infrared radiation increases with increasing temperature, one can think of the Earth's temperature as being determined by the infrared flux needed to balance the absorbed solar flux. The visible solar radiation mostly heats the surface, not the atmosphere, whereas most of the infrared radiation escaping to space is emitted from the upper atmosphere, not the surface. The infrared photons emitted by the surface are mostly absorbed in the atmosphere by greenhouse gases and clouds and do not escape directly to space.
The reason this warms the surface is most easily understood by starting with a simplified model of a purely radiative greenhouse effect that ignores energy transfer in the atmosphere by convection (sensible heat transport, Sensible heat flux) and by the evaporation and condensation of water vapor (latent heat transport, Latent heat flux). In this purely radiative case, one can think of the atmosphere as emitting infrared radiation both upwards and downwards. The upward infrared flux emitted by the surface must balance not only the absorbed solar flux but also this downward infrared flux emitted by the atmosphere. The surface temperature will rise until it generates thermal radiation equivalent to the sum of the incoming solar and infrared radiation.
A more realistic picture taking into account the convective and latent heat fluxes is somewhat more complex. But the following simple model captures the essence. The starting point is to note that the opacity of the atmosphere to infrared radiation determines the height in the atmosphere from which most of the photons are emitted into space. If the atmosphere is more opaque, the typical photon escaping to space will be emitted from higher in the atmosphere, because one then has to go to higher altitudes to see out to space in the infrared. Since the emission of infrared radiation is a function of temperature, it is the temperature of the atmosphere at this emission level that is effectively determined by the requirement that the emitted flux balance the absorbed solar flux.
But the temperature of the atmosphere generally decreases with height above the surface, at a rate of roughly 6.5 °C per kilometer on average, until one reaches the stratosphere 10–15 km above the surface. (Most infrared photons escaping to space are emitted by the troposphere, the region bounded by the surface and the stratosphere, so we can ignore the stratosphere in this simple picture.) A very simple model, but one that proves to be remarkably useful, involves the assumption that this temperature profile is simply fixed, by the non-radiative energy fluxes. Given the temperature at the emission level of the infrared flux escaping to space, one then computes the surface temperature by increasing temperature at the rate of 6.5 °C per kilometer, the environmental lapse rate, until one reaches the surface. The more opaque the atmosphere, and the higher the emission level of the escaping infrared radiation, the warmer the surface, since one then needs to follow this lapse rate over a larger distance in the vertical. While less intuitive than the purely radiative greenhouse effect, this less familiar radiative-convective picture is the starting point for most discussions of the greenhouse effect in the climate modeling literature.
Greenhouse gases
In order, Earth's most abundant greenhouse gases are:
When these gases are ranked by their contribution to the greenhouse effect, the most important are:
- 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 greenhouse gases.[13][14]
Runaway greenhouse effect
A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.[15] A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.[16] It is not possible that such an event will occur on Earth as a result of anthropogenic perturbations, but other potential runaway climate change effects involving Arctic methane release from permafrost have been suggested.[17] Clathrates have also been proposed as a major potential methane source capable of inducing a 'runaway' effect.[18]
Anthropogenic greenhouse effect
Of the human-produced greenhouse gases, the one that contributes the bulk in terms of radiative forcing is carbon dioxide. CO2 production from increased industrial activity (fossil fuel burning) and other human activities such as cement production and tropical deforestation[19] has increased the concentrations in the atmosphere. Measurements of CO2 from the Mauna Loa observatory show that concentrations have increased from about 313 ppm (mole fraction in dry air[20]) in 1960 to about 375 ppm in 2005. The current observed amount of CO2 exceeds the geological record maxima (~300 ppm) from ice core data.[21]
The effect of combustion-produced carbon dioxide on the global climate, a special case of the greenhouse effect first demonstrated in the 1930s, may be 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".[22]
Over the past 800,000 years,[23] 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.[24] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.[25]
Responses to anthropogenic global warming fall into three categories:
- Adaptation - dealing with the effects of global warming, such as by building flood defences
- Mitigation - reducing carbon emissions, such as by using renewable energy and energy efficiency measures.
- Geoengineering - directly intervening in the climate using techniques such as solar radiation management
Real greenhouses
The term "greenhouse effect" can be a source of confusion as actual greenhouses do not function by the same mechanism the atmosphere does. Various materials at times imply incorrectly that they do, or do not make the distinction between the processes of radiation and convection[26].
The term 'greenhouse effect' originally came from the greenhouses used for gardening, but as mentioned the mechanism for greenhouses operates differently.[27] 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.[28]
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.[29] Greenhouses thus work primarily by preventing convection; the atmospheric greenhouse effect however reduces radiation loss, not convection.[30][27]
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.[31][32][33]
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