Greenhouse Effect ...

The greenhouse effect is the rise in temperature that the Earth experiences because certain gases in the atmosphere (water vapor, carbon dioxide, nitrous oxide, and methane, for example) trap energy from the sun. Without these gases, heat would escape back into space and Earth’s average temperature would be about 60ºF colder. Because of how they warm our world, these gases are referred to as greenhouse gases.

Have you ever seen a greenhouse? Most greenhouses look like a small glass house. Greenhouses are used to grow plants, especially in the winter. Greenhouses work by trapping heat from the sun. The glass panels of the greenhouse let in light but keep heat from escaping. This causes the greenhouse to heat up, much like the inside of a car parked in sunlight, and keeps the plants warm enough to live in the winter. The Earth’s atmosphere is all around us. It is the air that we breathe. Greenhouse gases in the atmosphere behave much like the glass panes in a greenhouse. Sunlight enters the Earth's atmosphere, passing through the blanket of greenhouse gases. As it reaches the Earth's surface, land, water, and biosphere absorb the sunlight’s energy. Once absorbed, this energy is sent back into the atmosphere. Some of the energy passes back into space, but much of it remains trapped in the atmosphere by the greenhouse gases, causing our world to heat up. (D) The greenhouse effect is important. Without the greenhouse effect, the Earth would not be warm enough for humans to live. But if the greenhouse effect becomes stronger, it could make the Earth warmer than usual. Even a little extra warming may cause problems for humans, plants, and animals. See an animation of how enhancing the greenhouse effect likely contributes to global warming. (Macromedia Flash Version 5 or higher plug-in required)

n climate modelling, the Simple Biosphere model is a model of the biosphere for use with atmospheric General Circulation Models (GCMs) that attempts to incorporate a biophysical realism in the formulation of the radiation balance and of energy at a level of appropriate sophistication to exist in the models GCMs

An interactive method of optimization was used to define a new group of morphologic and physiologic parameters to the vegetation. Micrometeorological measures including energy flows collected above of the Amazonian Forest’s dossal were used to validate and to calibrate the parameters of the SiB. The new group of parameters generated a better adjustment between the calculated latent and sensible heat flows and the observed ones. The values of the dossal conductance found in this work were smaller than those described in the literature, and it was evidenced that the stomatal response to the water vapor pressure deficit and to the leaf water potential plays an important role in the assessing of daily transpiration of the tropical forest.

The strategy adopted by the researchers in this model was to model the own vegetation allowing that the plants determine by itself its interaction with the atmosphere. The vegetation was separated in two different layers; the first one was composed by trees and bushes (higher plants) and the other one by grass and herbaceous plants (lower plants). The SiB model requires parameters of the morphologic, physic and physiologic plants properties. The model SiB has in its formulation seven prognostic variables of the physical state: two temperatures, two of storage of water in the in the plant, and three layers indicating the humidity of the soil.

Miniature Greenhouse & Grow Racks

What are cold frames? Let’s review; they are used for 3 or the 4 seasons in the following capacities, spring: Before seedlings can be planted outside, they need a transition period of hardening off. This allows them to become acclimated to the outdoors. If plants are moved too abruptly from a warm greenhouse or indoor setting to the exposed garden, they can suffer stunted growth, disease or even death. Cold-frame provides the perfect interim environment while delicate seedlings adjust to the cool weather. It is also possible to start growing cool season crops like radishes or carrots earlier than normal. Though it is common to transplant directly into the garden, you may elect to keep them in the cold frame until harvest maturity is reached. Summer: This is the ideal time for plant propagation. If the proper soil medium is utilized, cold frames can protect environment necessary for rooting of tubers, or evergreen cuttings. Shade may be required during the hottest months. To prevent temperatures from rising to extreme levels, keep lid open at all times. This allows provides for ample air circulation inside the unit. Autumn: If provided with adequate moisture and fertilization, most garden plants will continue to grow through fall in the protected environment of a cold frame. Depending on the harshness of the winter and whether or not additional heating is used, your frame may continue to provide fresh cool-season greens, herbs, and root crops throughout the coldest winter months. Backyardgardener is the Largest Gardening Store in the World.

Idealized greenhouse model

The greenhouse effect can be illustrated with an idealized planet. This is a common "textbook model" ^[1]: the planet will have a constant surface temperature T_s and an atmosphere with constant temperature T_a. For diagrammatic clarity, a gap can be depicted between the atmosphere and the surface. Alternatively, T_s could be interpreted as a temperature representative of the surface and the lower atmosphere, and T_a could be interpreted as the temperature of the upper atmosphere. In order to justify that T_a and T_s remain constant over the planet, strong ocean and atmospheric currents can be imagined to provide plentiful lateral mixing. Furthermore, any daily or seasonal cycles in temperature are assumed to be insignificant.

The model will find the values of T_s and T_a that will allow the outgoing radiative power, escaping the top of the atmosphere, to be equal to the absorbed radiative power of sunlight. When applied to a planet like Earth, the outgoing radiation will be longwave and the sunlight will be shortwave. These two streams of radiation will have distinct emission and absorption characteristics. In the idealized model, we assume the atmosphere is completely transparent to sunlight. The planetary albedo α_P is the fraction of the incoming solar flux that is reflected back to space. The flux density of the incoming solar radiation is specified by the solar constant S₀. For application to planet Earth, appropriate values are S₀=1366 W m^-2 and α_P=0.30. Accounting for the fact that the surface area of a sphere is 4 times the area of its intercept (its shadow), the average incoming radiation is S₀/4.

For short wave radiation, the surface of the Earth is assumed to have an emissivity of 1. All incident shortwave radiation is completely absorbed. The surface emits a radiative flux density F according to the Stefan-Boltzmann law:

F = σT⁴

where σ is the Stefan-Boltzmann constant. A key to understanding the greenhouse effect is Kirchoff's law of thermal radiation. At any given wavelength the absorptivity of the atmosphere will be equal to the emissivity. Radiation from the surface could be in a slightly different portion of the infrared spectrum than the radiation emitted by the atmosphere. The model assumes that the average emissivity (absorptivity) is identical for either of these streams of infrared radiation, as they interact with the atmosphere. Thus, for longwave radiation, one symbol ε denotes both the emissivity and absorptivity of the atmosphere, for any stream of infrared radiation.

Idealized greenhouse model with an isothermal atmosphere. The blue arrows denote shortwave (solar) radiative flux density and the red arrow denotes longwave (terrestrial) radiative flux density. The radiation streams are shown with lateral displacement for clarity; they are collocated in the model. The atmosphere, which interacts only with the longwave radiation, is indicated by the layer within the dashed lines. A specific solution is depicted for ε=0.78 and α_p=0.3, representing Planet Earth. The numbers in the parentheses indicate the flux densities as a percent of S₀/4.

The equilibrium solution with ε=0.82. The increase by Δε=0.04 corresponds to doubling carbon dioxide and the associated positive feedback on water vapor.

The equilibrium solution with no greenhouse effect: ε=0

The infrared flux density out of the top of the atmosphere:

In the last term, ε represents the fraction of upward longwave radiation from the surface that is absorbed, the absorptivity of the atmosphere. In the first term on the right, ε is the emissivity of the atmosphere, the adjustment of the Stefan-Boltzmann law to account for the fact that the atmosphere is not optically thick. Thus ε plays the role of neatly blending, or averaging, the two streams of radiation in the calculation of the outward flux density.

Zero net radiation leaving the top of the atmosphere requires:

Zero net radiation entering the surface requires:

Energy equilibrium of the atmosphere can be either derived from the two above equilibrium conditions, or independently deduced:

Note the important factor of 2, resulting from the fact that the atmosphere radiates both upward and downward. Thus the ratio of T_a to T_s is independent of ε:

Thus T_a can be expressed in terms of T_s, and a solution is obtained for T_s in terms of the model input parameters:

or

The solution can also be expressed in terms of the effective emission temperature T_e, which is the temperature that characterizes the outgoing infrared flux density F, as if the radiator were a perfect radiator obeying F=σT_e⁴. This is easy to conceptualize in the context of the model. T_e is also the solution for T_s, for the case of ε=0, or no atmosphere:

With the definition of T_e:

For a perfect greenhouse, with no radiation escaping from the surface, or ε=1:

Using the parameters defined above to be appropriate for Earth,

For ε=1:

For ε=0.78,

.

This value of T_s happens to be close to a very widely quoted (but unattributed) 288 K claimed for the average global "surface temperature". ε=0.78 implies 22% of the surface radiation escapes directly to space, consistent with the statement of 15% to 30% escaping in the greenhouse effect.

The radiative forcing for doubling carbon dioxide is 3.71 W m^-2, in a simple parameterization. This is also the value endorsed by the IPCC. From the equation for ,

Using the values of T_s and T_a for ε=0.78 allows for = -3.71 W m^-2 with Δε=.019. Thus a change of ε from 0.78 to 0.80 is consistent with the radiative forcing from a doubling of carbon dioxide. For ε=0.80,

Thus this model predicts a global warming of ΔT_s = 1.2 K for a doubling of carbon dioxide. A typical prediction from a GCM is 3 K surface warming, primarily because the GCM allows for positive feedback, notably from increased water vapor. A simple surrogate for including this feedback process is to posit an additional increase of Δε=.02, for a total Δε=.04, to approximate the effect of the increase in water vapor that would be associated with an increase in temperature. This idealized model then predicts a global warming of ΔT_s = 2.4 K for a doubling of carbon dioxide, roughly consistent with the IPCC.

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed.^[1]

It is thought to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human action. The switch from a "global dimming" trend to a "brightening" trend in 1990 happened just as global aerosol levels started to decline.

Global dimming has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.

Deliberate manipulation of this dimming effect is now being considered as a geoengineering technique to reduce the impact of climate change।

Causes and effects

Further information: Albedo, irradiance, and insolation

It is thought that global dimming is probably due to the increased presence of aerosol particles in the atmosphere caused by human action.^[2] Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. Water droplets in clouds coalesce around the particles.^[3] Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the earth's surface.

Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location, and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the Earth's heat loss.

[edit] Research

Further information: Climate model and pyranometer

Eastern China. Dozens of fires burning on the surface (red dots) and a thick pall of smoke and haze (greyish pixels) filling the skies overhead. Photo taken by MODIS aboard NASA's Aqua satellite.

In the late-1960s, Mikhail Ivanovich Budyko worked with simple two-dimensional energy-balance climate models to investigate the reflectivity of ice.^[4] He found that the ice-albedo feedback created a positive feedback loop in the Earth's climate system. The more snow and ice, the more solar radiation is reflected back into space and hence the colder Earth grows and the more it snows. Other studies found that pollution or a volcano eruption could snap Earth into an ice age.^[5][6]

In the mid-1980s, Atsumu Ohmura, a geography researcher at the Swiss Federal Institute of Technology, found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades. His findings are in apparent contradiction to global warming—the global temperature has steadily been going up. Less light reaching the earth would mean that it would have to cool. Ohmura published his findings "Secular variation of global radiation in Europe" in 1989.^[7] This was soon followed by others: Viivi Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia",^[8] and Beate Liepert in 1994 "Solar radiation in Germany — Observed trends and an assessment of their causes".^[9] Dimming has also been observed in sites all over the former Soviet Union.^[10] Gerry Stanhill who studied these declines worldwide in many papers (see references) coined the term "global dimming".^[11]

Smog at the Golden Gate Bridge. Smog is a likely contributor to global dimming.

Independent research in Israel and the Netherlands in the late 1980s showed an apparent reduction in the amount of sunlight,^[12] despite widespread evidence that the climate was actually becoming hotter. The rate of dimming varies around the world but is on average estimated at around 2–3% per decade. The trend reversed in the early 1990s. ^[1] It is difficult to make a precise measurement, due to the difficulty in accurately calibrating the instruments used, and the problem of spatial coverage. Nonetheless, the effect is almost certainly present.

The effect (2–3%, as above) is due to changes within the Earth's atmosphere; the value of the solar radiation at the top of the atmosphere has not changed by more than a fraction of this amount.^[13]

The effect varies greatly over the planet, but estimates of the terrestrial surface average value are:

• 5.3% (9 W/m²); over 1958–85 (Stanhill and Moreshet, 1992)^[11]
• 2%/decade over 1964–93 (Gilgen et al., 1998)^[14]
• 2.7%/decade (total 20 W/m²); up to 2000 (Stanhill and Cohen, 2001)^[15]
• 4% over 1961–90 (Liepert 2002)^[16][17]

Note that these numbers are for the terrestrial surface and not really a global average. Whether dimming (or brightening) occurred over the ocean has been a bit of an unknown though a specific measurement (see below, Causes) measured effects some 400 miles (643.7 km) from India over the Indian Ocean towards the Maldives Islands. Regional effects probably dominate but are not strictly confined to the land area, and the effects will be driven by regional air circulation.

The largest reductions are found in the northern hemisphere mid-latitudes.^[18] The region of the spectrum of light radiation most affected seems to be the visible and infrared rather than the ultraviolet part of the spectrum.^[19]

[edit] Pan evaporation data

Further information: Pan evaporation

Over the last 50 or so years, pan evaporation has been carefully monitored. For decades, nobody took much notice of the pan evaporation measurements. But in the 1990s in Europe, Israel, and North America, scientists spotted something that at the time was considered very strange: the rate of evaporation was falling although they had expected it to increase due to global warming.^[20] The same trend has been observed in China over a similar period. A decrease in solar irradiance is cited as the driving force. However, unlike in other areas of the world, in China the decrease in solar irradiance was not always accompanied by an increase in cloud cover and precipitation. It is believed that aerosols may play a critical role in the decrease of solar irradiance in China.^[21]

BBC Horizon producer David Sington believes that many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming.^[22] Pan evaporation experiments are easy to reproduce with low-cost equipment, there are many pans used for agriculture all over the world and in many instances the data has been collected for nearly a half century. However, pan evaporation depends on some additional factors besides net radiation from the sun. The other two major factors are vapor pressure deficit and wind speed.^[23] The ambient temperature turns out to be a negligible factor. The pan evaporation data corroborates the data gathered by radiometer^[20][15] and fills in the gaps in the data obtained using pyranometers. With adjustments to these factors, pan evaporation data has been compared to results of climate simulations.^[24]

[edit] Probable causes

Further information: Particulate, Black carbon, Contrail, and Volcanic ash

NASA photograph showing aircraft contrails and natural clouds. The temporary disappearance of contrails over North America due to plane groundings after the September 11, 2001 attacks, and the resulting increase in diurnal temperature range gave empirical evidence of the effect of thin ice clouds at the Earth's surface.^[25]

The incomplete combustion of fossil fuels (such as diesel) and wood releases black carbon into the air. Though black carbon, most of which is soot, is an extremely small component of air pollution at land surface levels, the phenomenon has a significant heating effect on the atmosphere at altitudes above two kilometers (6,562 ft). Also, it dims the surface of the ocean by absorbing solar radiation.^[26]

Experiments in the Maldives (comparing the atmosphere over the northern and southern islands) in the 1990s showed that the effect of macroscopic pollutants in the atmosphere at that time (blown south from India) caused about a 10% reduction in sunlight reaching the surface in the area under the pollution cloud — a much greater reduction than expected from the presence of the particles themselves.^[27] Prior to the research being undertaken, predictions were of a 0.5–1% effect from particulate matter; the variation from prediction may be explained by cloud formation with the particles acting as the focus for droplet creation. Clouds are very effective at reflecting light back out into space.

The phenomenon underlying global dimming may also have regional effects. While most of the earth has warmed, the regions that are downwind from major sources of air pollution (specifically sulfur dioxide emissions) have generally cooled. This may explain the cooling of the eastern United States relative to the warming western part.^[28]

However some research shows that black carbon will actually increase global warming being second only to CO₂. They believe that soot will absorb solar energy and transport it to other areas such as the Himalayas where glacial melt occurs. It can also darken Arctic ice reducing reflectivity and increasing absorption of solar radiation.^[29]

Some climate scientists have theorized that aircraft contrails (also called vapor trails) are implicated in global dimming, but the constant flow of air traffic previously meant that this could not be tested. The near-total shutdown of civil air traffic during the three days following the September 11, 2001 attacks afforded a rare opportunity in which to observe the climate of the United States absent from the effect of contrails. During this period, an increase in diurnal temperature variation of over 1 °C (1.8 °F) was observed in some parts of the U.S., i.e. aircraft contrails may have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought.^[25]

Airborne volcanic ash can reflect the Sun's rays back out into space and cool the planet. Dips in earth temperatures have been observed from large volcano eruptions such as Mount Agung in Bali that erupted in 1963, El Chichon (Mexico) 1983, Ruiz (Colombia) 1985, and Pinatubo (Philippines) 1991. But even for major eruptions, the ash clouds remain only for relatively short periods.^[30]

[edit] Recent reversal of the trend

Further information: Clean Air Act

Sun-blocking aerosols around the world steadily declined (red line) since the 1991 eruption of Mount Pinatubo, according to satellite estimates. Credit: Michael Mishchenko, NASA

Wild et al., using measurements over land, report brightening since 1990,^[12][31][32] and Pinker et al.^[33] found that slight dimming continued over land while brightening occurred over the ocean.^[34] Hence, over the land surface, Wild et al. and Pinker et al. disagree. A 2007 NASA sponsored satellite-based study sheds light on the puzzling observations by other scientists that the amount of sunlight reaching Earth's surface had been steadily declining in recent decades, suddenly started to rebound around 1990. This switch from a "global dimming" trend to a "brightening" trend happened just as global aerosol levels started to decline.^[30][35]

It is likely that at least some of this change, particularly over Europe, is due to decreases in pollution. Most governments of developed nations have done more to reduce aerosols released into the atmosphere, which helps reduce global dimming, than to reduce CO₂ emissions.

Sulfate aerosols have declined significantly since 1970 with the Clean Air Act in the United States and similar policies in Europe. The Clean Air Act was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including PM’s, dropped by 53% in the US. In 1975, the masked effects of trapped greenhouse gases finally started to emerge and have dominated ever since.^[36]

The Baseline Surface Radiation Network (BSRN) has been collecting surface measurements. BSRN was started in the early 1990s and updated the archives in this time. Analysis of recent data reveals that the surface of the planet has brightened by about 4% in the past decade. The brightening trend is corroborated by other data, including satellite analyses.

[edit] Relationship to hydrological cycle

This figure shows the level of agreement between a climate model driven by five factors and the historical temperature record. The negative component identified as "sulfate" is associated with the aerosol emissions blamed for global dimming.

Further information: Hydrological cycle

Pollution produced by humans may be seriously weakening the Earth's water cycle — reducing rainfall and threatening fresh water supplies. A 2001 study by researchers at the Scripps Institution of Oceanography suggests that tiny particles of soot and other pollutants have a significant effect on the hydrological cycle. According to Professor V. Ramanathan: "The energy for the hydrological cycle comes from sunlight. As sunlight heats the ocean, water escapes into the atmosphere and falls out as rain. So as aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet."^[37]

Large scale changes in weather patterns may also have been caused by global dimming. Climate models speculatively suggest that this reduction in sunshine at the surface may have led to the failure of the monsoon in sub-Saharan Africa during the 1970s and 1980s, together with the associated famines such as the Sahel drought, caused by Northern hemisphere pollution cooling the Atlantic.^[38] Because of this, the Tropical rain belt may not have risen to its northern latitudes, thus causing an absence of seasonal rains. This claim is not universally accepted and is very difficult to test.

It is also concluded that the imbalance between global dimming and global warming at the surface leads to weaker turbulent heat fluxes to the atmosphere. This means globally reduced evaporation and hence precipitation occur in a dimmer and warmer world, which could ultimately lead to a more humid atmosphere in which it rains less.^[39]

A natural form of large scale environmental shading/dimming has been identified that affected the 2006 northern hemisphere hurricane season. The NASA study found that several major dust storms in June and July in the Sahara desert sent dust drifting over the Atlantic Ocean and through several effects caused cooling of the waters — and thus dampening the development of hurricanes.^[40][41]

[edit] Relationship to global warming

Further information: Global warming

Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise.^[17] According to Beate Liepert, "We lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter."^[42] The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.^[43]

Interactions between the two theories for climate modification have also been studied, as global warming and global dimming are not mutually exclusive or contradictory. In a paper published on March 8, 2005 in the American Geophysical Union's Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and global dimming can occur at the same time.^[44] Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is one of the greenhouse gases. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies.

Brown clouds have been found to amplify global warming according to V. Ramanathan, an atmospheric chemist at the Scripps Institution of Oceanography in La Jolla, CA. "The conventional thinking is that brown clouds have masked as much as 50 percent of global warming by greenhouse gases through so-called global dimming... While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are in fact amplifying the atmospheric warming trend caused by greenhouse gases by as much as 50 percent."^[45]

[edit] Possible use to mitigate global warming

Further information: Mitigation of global warming and Albedo

Some scientists have suggested using aerosols to stave off the effects of global warming as an emergency geoengineering measure.^[46] In 1974, Mikhail Budyko suggested that if global warming became a problem, the planet could be cooled by burning sulfur in the stratosphere, which would create a haze.^[47][48] An increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO₂ doubling.^[49]

However, Earth would still face many problems, such as:

• Using sulfates causes environmental problems such as acid rain^[50]
• Using carbon black causes human health problems^[50]
• Dimming causes ecological problems such as changes in evaporation and rainfall patterns^[50]
• Droughts and/or increased rainfall cause problems for agriculture^[50]
• Aerosol has a relatively short lifetime

In a weblog posting, Gavin Schmidt stated that "Ideas that we should increase aerosol emissions to counteract global warming have been described as a 'Faustian bargain' because that would imply an ever increasing amount of emissions in order to match the accumulated greenhouse gas in the atmosphere, with ever increasing monetary and health costs."^[51]

Earth's radiation balance

Earth's radiation balance is the equation of the incoming and outgoing thermal radiation.

An instrument for measuring the net radiation balance and albedo. Model shown CNR 1. Courtesy of Kipp & Zonen

The incoming solar radiation is short wave, therefore the equation below is called the short wave radiation balance Qs:

Qs = G - R = D + H - R or depending on the albedo (back-reflection to space): = G (1 - a)

• G = global radiation
• D = direct radiation
• H = diffuse radiation
• R = reflected portion of global radiation (ca. 4%)
• a = albedo

The Earth's surface and atmosphere emits heat radiation (in the infrared spectrum). There is little overlap between this and the solar spectrum. Since this is long wave radiation, this formula also is known as the long wave radiation balance(Ql):

Ql = AE = AO - AG

• AE = effective radiation
• AO = radiation of the Earth's surface
• AG = trapped radiation (radiation forcing, also known as the so called greenhouse effect)

From those two equations for incoming and outgoing radiation, the total amount of energy now can be calculated (total radiation balance (Qt), net radiation):

Qt = Qs - Ql = G - R - AE

The difficulty is to precisely quantify the various internal and external factors influencing the radiation balance. Internal factors are all mechanisms affecting atmospheric composition (volcanism, biological activity, land use change, human activities etc.). The main external factor is solar radiation. It is interesting to note in this context that over its lifetime the sun's average luminosity to date has increased by ca. 25%.

External and internal factors are also closely interconnected. Increased solar radiation for example results in higher average temperatures and higher water vapour content of the atmosphere. Water vapor, a heat trapping gas absorbing infrared radiation emitted by the Earth's surface, can lead to either higher temperatures through radiation forces or lower temperatures as a result of increased cloud formation and hence increased albedo. The situation is complicated since countless factors are involved, and therefore the concept of radiation balances is - albeit physically correct - of limited value.

From Wikipedia, the free encyclopedia Jump to: navigation, search A schematic representation of the energy exchanges between the Earth's surface, the

The Earth can be considered as a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The planet is approximately in equilibrium, so the sum of the gains is approximately equal to the sum of the losses.

Note on accompanying images: These graphics depict only net energy transfer. There is no attempt to depict the role of greenhouse gases and the exchange that occurs between the Earth's surface and the atmosphere or any other exchanges।

The energy budget

Incoming energy

The total flux of energy entering the Earth's atmosphere is estimated at 174 petawatts. This flux consists of:

• solar radiation (99.978%, or nearly 174 petawatts; or about 340 W m^-2)
  • This is equal to the product of the solar constant, about 1,366 watts per square metre, and the area of the Earth's disc as seen from the Sun, about 1.28 × 10¹⁴ square metres, averaged over the Earth's surface, which is four times larger. The solar flux averaged over just the sunlit half of the earth's surface is about 680 W m^-2
  • Note that the solar constant varies (by approximately 0.1% over a solar cycle); and is not known absolutely to within better than about one watt per square metre. Hence the geothermal and tidal contributions are less than the uncertainty in the solar power.
• geothermal energy (0.013%, or about 23 terawatts; or about 0.045 W m^-2)
  • This is produced by stored heat and heat produced by radioactive decay leaking out of the Earth's interior.
• tidal energy (0.002%, or about 3 terawatts; or about 0.0059 W m^-2)
  • This is produced by the interaction of the Earth's mass with the gravitational fields of other bodies such as the Moon and Sun.
• waste heat from fossil fuel consumption (about 0.007%, or about 13 terawatts; or about 0.025 W m^-2) [1].

There are other minor sources of energy that are usually ignored in these calculations: accretion of interplanetary dust and solar wind, light from distant stars, the thermal radiation of space. Although these are now known to be negligibly small, this was not always obvious: Joseph Fourier initially thought radiation from deep space was significant when he discussed the earth's energy budget in a paper often cited as the first on the greenhouse effect [2].

[edit] Outgoing energy

The average albedo (reflectivity) of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected back into space, while 70% is absorbed by the Earth and reradiated as infrared. The planet's albedo varies from month to month, but 0.3 is the average figure. It also varies very strongly spatially: ice sheets have a high albedo, oceans low. The contributions from geothermal and tidal power sources are so small that they are omitted from the following calculations.

So 30% of the incident energy is reflected, consisting of:

• 6% reflected from the atmosphere
• 20% reflected from clouds
• 4% reflected from the ground (including land, water and ice)

Earth's longwave thermal radiation intensity, from clouds, atmosphere and ground

The remaining 70% of the incident energy is absorbed:

• 51% absorbed by land and water, then emerging in the following ways:
  • 23% transferred back into the atmosphere as latent heat by the evaporation of water, called latent heat flux
  • 7% transferred back into the atmosphere by heated rising air, called Sensible heat flux
  • 6% radiated directly into space
  • 15% transferred into the atmosphere by radiation, then reradiated into space
• 19% absorbed by the atmosphere and clouds, including:
  • 16% reradiated back into space
  • 3% transferred to clouds, from where it is radiated back into space

When the Earth is at thermal equilibrium, the same 70% that is absorbed is reradiated:

• 64% by the clouds and atmosphere
• 6% by the ground

Anthropogenic modification

Emissions of greenhouse gases, and other factors such as land-use changes, modify the energy budget slightly but significantly. The Intergovernmental Panel on Climate Change (IPCC) provides an estimate of this forcing, insofar as it is known [3]. The largest and best-known are from the well-mixed greenhouse gases (CO₂, CH₄, halocarbons, etc.), totalling an increase in forcing of 2.4 W m^-2 relative to 1750. This is less than 1% of the solar input, but contributes to the observed increase in atmospheric and oceanic temperature.

Radiative forcing

In climate science, radiative forcing is (loosely) defined as the change in net irradiance at the tropopause. "Net irradiance" is the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system and is thus measured in Watts per square meter. The change is computed based on "unperturbed" values, as defined by the Intergovernmental Panel on Climate Change (IPCC) as the measured difference relative to a base period. For radiative forcings for the industrial era, it is customary to take the year 1750 as the defined starting point. A positive forcing (more incoming energy) tends to warm the system, while a negative forcing (more outgoing energy) tends to cool it. Possible sources of radiative forcing are changes in insolation (incident solar radiation), or the effects of variations in the amount of radiatively active gases and aerosols present. Because the IPCC regularly assesses the radiative forcing, it also has a more specific technical definition - see "IPCC usage" section.

Contents [hide] 1 Radiation balance 2 IPCC usage 3 Examples of radiative forcing calculations 4 Related measures 5 References 6 See also 7 External links

[edit] Radiation balance

The vast majority of the energy which affects Earth's weather comes from the Sun. The planet and its atmosphere absorb and reflect some of the energy, while long-wave energy is radiated back into space. The balance between absorbed and radiated energy determines the average temperature. The planet is warmer than it would be in the absence of the atmosphere: see greenhouse effect.

The radiation balance can be altered by factors such as intensity of solar energy, reflection by clouds or gases, absorption by various gases or surfaces, and emission of heat by various materials. Any such alteration is a radiative forcing, and causes a new balance to be reached. In the real world this happens continuously as sunlight hits the surface, clouds and aerosols form, the concentrations of atmospheric gases vary, and seasons alter the ground cover.

[edit] IPCC usage

2005 radiative forcings as estimated by the IPCC.

The term “radiative forcing” has been employed in the IPCC Assessments with a specific technical meaning to denote an externally imposed perturbation in the radiative energy budget of the Earth’s climate system, which may lead to changes in climate parameters [1]. The exact definition used is:

The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm^-2) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values. [2]

In the context of climate change, the term forcing is restricted to changes in the radiation balance of the surface-troposphere system imposed by external factors, with no changes in stratospheric dynamics, without any surface and tropospheric feedbacks in operation (i.e., no secondary effects induced because of changes in tropospheric motions or its thermodynamic state), and with no dynamically-induced changes in the amount and distribution of atmospheric water (vapour, liquid, and solid forms).

Radiative forcing can be used to estimate a subsequent change in equilibrium surface temperature ΔT_s change arising from that radiative forcing via the equation:

where λ is the climate sensitivity, usually with units in K/(W/m²), and ΔF is the radiative forcing. A typical value is 0.8 K/(W/m²), which gives a warming of 3K for doubling of CO₂.

[edit] Examples of radiative forcing calculations

Radiative forcing for doubling CO2, as calculated by radiative transfer code Modtran. Red lines are Planck curves.

Radiative forcing for eight times increase of CH4, as calculated by radiative transfer code Modtran.

Radiative forcing (often measured in watts per square meter) can be estimated in different ways for different components. For the case of a change in solar irradiance, the radiative forcing is the change in the solar constant divided by 4 and multiplied by 0.7 to take into account the geometry of the sphere and the amount of reflected sunlight. For a greenhouse gas, such as carbon dioxide, radiative transfer codes that examine each spectral line for atmospheric conditions can be used to calculate the change ΔF as a function of changing concentration. These calculations can often be simplified into an algebraic formulation that is specific to that gas.

For instance, the simplified first-order approximation expression for carbon dioxide is:

where C is the CO₂ concentration in parts per million by volume and C₀ is the reference concentration^[1]. The relationship between carbon dioxide and radiative forcing is logarithmic so that increased concentrations have a progressively smaller warming effect.

Formulas for other greenhouse gases such as methane, N2O or CFCs are given in the IPCC reports [3].

[edit] Related measures

Radiative forcing is intended as a useful way to compare different causes of perturbations in a climate system. Other possible tools can be constructed for the same purpose: for example Shine et al. ^[2] say "...recent experiments indicate that for changes in absorbing aerosols and ozone, the predictive ability of radiative forcing is much worse... we propose an alternative, the 'adjusted troposphere and stratosphere forcing'. We present GCM calculations showing that it is a significantly more reliable predictor of this GCM's surface temperature change than radiative forcing. It is a candidate to supplement radiative forcing as a metric for comparing different mechanisms...". In this quote, the word "predictive" may be confusing: it refers to the ability of the tool to help explain the response, not to the ability of GCMs to forecast climate change.