The “greenhouse effect” is widely discussed in the media, and although its details are complicated, its principles are not difficult to understand. Without a greenhouse effect, radiation from the Sun (mostly in the form of visible light) would travel to Earth and be changed into heat, only to be lost to space. This scenario can be sketched as follows: Sun’s radiation → absorbed by Earth → Re-radiated to space as heat The greenhouse effect is a process where energy from the sun readily penetrates into the lower atmosphere and onto the surface of Earth and is converted to heat, but then cannot freely leave the planet. This can be sketched as follows: Sun’s Radiation → absorbed by Earth → some re-radiated to space as heat → some trapped by the atmosphere Due to the presence of certain “greenhouse gases” that trap heat, like carbon dioxide, methane, water vapour, and CFC’s, the atmosphere retains the sun’s radiation and warms up the planet. By increasing the abundance of these gases in the atmosphere, humankind is increasing the overall warming of the Earth’s surface and lower atmosphere, a process called “global warming.”
The figure below illustrates the radiation balance and the role of greenhouse effect. The Radiation Balance: Another way to think about the greenhouse effect is to consider that according to physics the radiation we receive from the Sun must be equally balanced by the heat Earth radiates out to space. If we were to give back less energy than we receive, our planet would soon be too hot for life. Likewise, if we were to give back more energy that we receive, our planet would soon be too cold for life. This can be written as a balanced equation of radiation: Solar radiation input to Earth = Earth’s output of re-radiated heat If we were to measure the temperature of the Earth from space, the Earth’s “surface” would show a temperature appropriate for this requirement of energy balance: a measurement of roughly -18 degrees Celsius (about 0 °F). At this temperature, our planet radiates a quantity of heat into space that is equivalent to the amount of energy received from the Sun.
At this point you may be asking how we can speak of “global warming” when we have just stated that the Earth (as seen from space) MUST stay at the same temperature. And how is it that the temperature of the Earth’s surface is only a chilly 0°F? The key to understanding this apparent contradiction is to remember that we live at the bottom of the atmosphere. As far as the radiation balance is concerned, the lower atmosphere and the surface of Earth form part of a “warm interior” of the planet. The apparent temperature “surface” that we would see from space is located well above the real surface of the Earth where we live. This apparent temperature “surface” is about 5000 meters up (17,000 feet) within the atmosphere. To get a better handle on this concept consider the following: the difference in elevation between 0 meters and 5,000 meters corresponds to a difference in temperature of about 60°F.
In other words, at sea level it is 60°F warmer than it would be without the atmosphere. For the last 100 years or so this apparent temperature “surface” has been moving upward in the atmosphere as a result of global warming. As the apparent “surface” rises, the bottom of the atmosphere gets warmer, a fact that can be seen in the positions of the snow line (the elevation where snow begins to form) and tree line (the elevation where it becomes too cold for trees to grow). However, despite all these changes happening in the lower atmosphere, the overall temperature of the planet as seen from space stays the same. How is it possible that the Earth exactly balances the incoming sunlight with the outgoing heat radiation? The answer is simple: the amount of heat radiation from Earth is precisely tied to the temperature of the atmosphere.
If the temperature of the apparent “surface” is too low and Earth radiates too little heat to keep the balance, Earth will warm up and radiate more heat into space. If the temperature of the apparent “surface” is too high and Earth radiates more heat than it receives, the planet will become colder and radiate less energy back to space. Overall, this “negative feedback” stabilizes the radiation balance despite all the variations of temperature from one place to another and within the vertical column of the atmosphere. It sets the temperature so that the incoming and outgoing energy is balanced. Greenhouse Gases: The three most powerful long lived greenhouse gases in the atmosphere are carbon dioxide, methane, and nitrous oxide. In this section the sources, sinks, and atmospheric concentrations of these compounds are considered. In addition we will consider the class of compounds known as halogenated organic compounds and ozone in the lower and upper atmosphere.
At this point it is germane to note that water vapour is the single most powerful greenhouse gas in the atmosphere. Water vapour has approximately twice the effect of the second most powerful greenhouse gas—carbon dioxide. Human activities do not have any significant direct impact on the level of water vapour in the atmosphere. However, as a result of global warming it is likely that human activities will have a significant indirect impact on the level of water vapour in the atmosphere. Water vapour is the most important greenhouse gas and the development of a better understanding of the effect of global warming on atmospheric water in all its forms (solid, liquid, and gas) is of critical importance. The units for carbon dioxide concentration are parts per million (ppm) while those for methane and nitrous oxide are parts per billion (ppb), 1 ppm = 1000 ppb.
The current atmospheric concentration of carbon dioxide is 370 ppm, meaning that in samples of one million molecules of ambient air we would expect to find, on average, 370 molecules of carbon dioxide. The overall trend of carbon dioxide, methane, and nitrous oxide concentrations is the same. For the time period 1000–1800 there is little, or no, discernible change, while over the period 1800–2000 there is a substantial increase in the atmospheric levels of all three gases. It is well established that the increases in the atmospheric concentration of carbon dioxide, methane, and nitrous oxide during 1800–2000 reflect the impact of human activities in general and the industrial revolution in particular.
* Carbon Dioxide (CO2): Discussion of the human impact on the levels of carbon dioxide (CO2) in the atmosphere is complicated by two factors. First, emissions of CO2 associated with human activities, while large on a human scale, are small when compared to natural fluxes of CO2 associated with photosynthesis, respiration, uptake into ocean water, and release from ocean water. Second, there are several large reservoirs of CO2 (e.g. atmosphere, upper ocean, deep ocean, biosphere) which are continually exchanging CO2. In such a system one needs to be very careful when using the words “source”, “sink”, and “lifetime”. Instead of “source” and “sink” it is often better to refer to “net source” and “net sink”. Thus, the oceans are both a large (90 GtC yr-1) source and a large (92 GtC yr-1) sink of atmospheric CO2. Overall the oceans provide a net sink for CO2 of 2 GtC yr-1. The unit used to measure CO2 flux is GtC which stands for Giga (109) tonnes of carbon. The molecular weight of CO2 is 44 while the atomic weight of carbon is 12, hence the mass of CO2 corresponding to 1 GtC is actually (44/12) = 3.7 Gt. With continual exchange between reservoirs it is not possible to specify a single atmospheric lifetime for CO2.
The atmospheric lifetime of CO2 is typically quoted as approximately 100 years. For simplicity the discussion below will not consider the natural fluxes of carbon between the various reservoirs in the environment; instead it will focus on the human perturbation to the natural cycle. Four approaches have been used to estimate the net CO2 flux between atmosphere, oceans, and terrestrial biosphere: (i) direct local flux measurements which are extrapolated globally (ii) indirect assessment using atmospheric models calibrated, or partially validated, using various tracers (iii) monitoring of tracers coupled to the CO2cycle such as ratios of molecules containing different carbon isotopes, e.g. 13CO2/12 CO2, 14CO2/12 CO2, and (iv) measurements of the change in atmospheric CO2 concentrations as air passes over a particular region combined with “inverse” modelling to estimate the fluxes necessary to account for the observed changes.
Such techniques have been used to estimate that the oceans remove 2.0±0.8 GtC yr-1and re-growth of forests in the Northern Hemisphere removes 0.5±0.5 GtC yr-1 of CO2 from the atmosphere. Human activities are believed to lead to emission of 5.5±0.5 GtC yr-1 from fossil fuel combustion and cement production and emission of 1.6±1.0 GtC yr -1from changes in tropical land use (deforestation). The atmospheric burden of CO2 is increasing at a rate of 3.3±0.2 GtC yr-1. To balance the CO2 budget “unknown terrestrial sinks” have been invoked and have been inferred to account for 1.3±1.5 GtC yr-1of CO2. This is often known as the “missing sink”. It should be noted that zero lies within the range 1.3±1.5 and hence there may indeed be no “missing sink”. It should be noted that a highly simplified picture has been presented above. In reality the carbon cycle is very complex with a number of important feedback loops. One direct carbon cycle feedback is that certain agricultural and wild plant and tree species have shown significantly higher growth under higher (typically doubled) carbon dioxide concentrations.
Growth is also strongly influenced by nutrient and water availability as well as other stress factors (temperature, insect attacks etc.). For terrestrial systems CO2-climate feedbacks include effects of temperature, precipitation and radiation changes (changes in cloudiness) on primary production and decomposition. For marine systems feedback occurs through climatic influences on ocean circulation and chemistry. Rates of biological activity generally increase with warmer temperatures and increasing moisture. The feedbacks of changing carbon dioxide and climate on ecosystems are many and complex and will not be discussed here. The current atmospheric carbon dioxide concentration is 370 ppm and increasing at approximately 1.5 ppm per year. As shown in Figure 4 this level is 30% above that observed in pre-industrial times (280 ppm). The ice core record from Vostok in Antarctica shows that the current level of carbon dioxide in the atmosphere is substantially greater than at any time in at least the past 420 000 years. * Methane (CH4): Methane (CH4) is the most abundant well mixed greenhouse gas after carbon dioxide.
In contrast to carbon dioxide, methane is removed from the atmosphere via chemical reaction with hydroxyl (OH) radicals. Methane plays an important role in atmospheric chemistry and it can influence the levels of other important trace species via its reaction with OH. All other factors being constant, increased atmospheric levels of methane will result in decreased concentrations of OH and hence a longer lifetime for any gas whose atmospheric lifetime is influenced by reaction with OH. Also, an increase in methane will lead to the production of more tropospheric ozone which is an important greenhouse gas. Methane is involved in complex feedback loops in atmospheric chemistry (see Hydrocarbons in the Atmosphere and Gas-Phase (Photo-) Chemical Processes in the Troposphere). Methane is emitted into the atmosphere by a large number of natural and anthropogenic sources. Natural sources are believed to contribute approximately 30% of the methane flux while anthropogenic sources account for the remaining 70%. Natural sources are estimated to contribute a total of approximately 160 Tg(CH4) yr-1(1Tg= 1012 g, 1000 Tg = 1Gt).
The largest natural sources are wetlands, termites, and oceans which emit 115, 20, and 10 Tg(CH4) yr-1, respectively. Anthropogenic sources are natural gas facilities, coal mines, petroleum industry, coal combustion, enteric fermentation, rice paddies, biomass burning, landfills, animal waste, domestic sewage and are estimated to emit 40, 30, 15, 15, 85, 60, 40, 40, 25 and 25 Tg(CH4) yr-1, respectively, for a total anthropogenic contribution of 375 Tg(CH4) yr-1. The identified sources total approximately 535 Tg(CH4) yr-1. Methane is removed from the atmosphere through reaction with OH radicals in the troposphere and stratosphere with rates estimated to be 445 and 40 Tg(CH4) y-1, respectively. The tropospheric lifetime with respect to the OH radical reaction is approximately 12 years. CH4 fluxes in the terrestrial and marine systems are highly variable; the soil sink is estimated to 30 Tg(CH4) yr-1.
The sinks for methane total approximately 515 Tg(CH4) yr-1. Over the past 25 years the annual growth rate of the atmospheric concentration of methane has varied substantially between 0 and 15 ppb yr-1, corresponding to increases in the atmospheric burden of between 0 and 40 Tg yr-1. There is no quantitative understanding of the variation in rate of increase in the atmospheric methane concentration over the past few decades. It is clear that during the time period 1000 – 1750 there was little, or no, change in the global atmospheric methane concentration, but since 1750 there has been a substantial (150%) increase in the concentration of this greenhouse gas. * Nitrous Oxide (N2O): Nitrous oxide (N2O) is the third most abundant well mixed greenhouse gas after carbon dioxide and methane. N2O is a long-lived (130 years) trace constituent of the lower atmosphere present in a concentration which is currently 313 ppb and increasing at a rate of 0.5-0.9 ppb yr-1.
The atmospheric concentration of N2O has increased by approximately 16% since pre-industrial times. In addition to its importance as a greenhouse gas, N2O is transported through the troposphere into the stratosphere where it reacts with O (1D) atoms and is the source of stratospheric NOx (O(1D) atoms are electronically excited oxygen atoms). Natural sources of N2O associated with emission from soils and the oceans are estimated to deliver 10.2 TgN y-1 to the atmosphere. Anthropogenic emissions of N2O are associated with biomass burning, fossil fuel combustion, industrial production of nitric acids, and the use of nitrogen fertilizer, and are believed to total 3.2 TgN yr-1. Photo dissociation in the stratosphere is the major (90%) loss mechanism for N2O in the atmosphere. Reaction with O (1D) atoms is a minor (10%) loss of N2O. * Halogenated Organic Compounds: Halogenated organic compounds are organic compounds containing one or more halogen atoms.
Halogenated organic compounds can be fully substituted where all of the hydrogen in the molecule has been replaced by halogen atoms or partially substituted where some hydrogen remains. Chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs) are two subsets of halogenated organic compounds in which all hydrogen atoms have been substituted by fluorine and chlorine atoms, or solely by fluorine atoms. CFC-12 (CCl2F2) and CFC-11 (CCl3F) are the two most abundant CFCs in the atmosphere and are present at levels of 0.5 and 0.25 ppb, respectively. CF4 is the most abundant PFC and is found at a concentration of 0.08 ppb in the atmosphere. Hydrofluorocarbons (HFCs) and hydrofluorochlorocarbons (HCFCs) are compounds in which some, but not all, of the hydrogens have been replaced with fluorine (HFCs) or fluorine and chlorine atoms (HCFCs). Halons are a class of compounds containing bromine and chlorine (but no hydrogen).
There are no significant natural sources of CFCs, PFCs, HFCs, HCFCs, or Halons. These compounds were not present in the preindustrial atmosphere, and their presence in contemporary air reflects emissions associated with industrial activities * Sulfur Hexaflouride (SF6): On a per molecule basis, sulfur hexafluoride (SF6) is one of the most potent greenhouse gases known. Its potency stems from its intense absorption at 10.3 μm (969 cm-1) in the atmospheric window region and its extremely long atmospheric lifetime of 3200 years. SF6 is present in small amounts in fluorites and degassing from these minerals provides a small natural source which results in a natural background concentration of 0.01 ppt. SF6 is a useful industrial chemical used as an insulating gas in electrical switching equipment. As a result of anthropogenic emissions the current level of SF6 in the atmosphere is approximately 400 times that of the natural background and increasing at a rate of approximately 0.2 ppt yr-1.Very recently a new SF6-like greenhouse gas was detected in the atmosphere: SF5CF3.
The level of SF5CF3 in the global atmosphere increased from essentially zero in 1960 to 0.12 ppt in 1999. While the concentration of SF5CF3 is very low and this compound does not play any significant role in global warming it is of interest because on a per molecule basis it is the most potent greenhouse gas yet identified in the atmosphere. The discovery of SF5CF3 illustrates that there is much that is still to be learnt concerning such greenhouse gases. While it is unlikely that any single major greenhouse gas awaits discovery, the possibility that many compounds such as SF5CF3 are present, with each making a small contribution that when summed represent a non-negligible contribution cannot be ruled out at present. * Ozone (O3): Prior to discussing the relationship between ozone and global climate change it is useful to provide a brief background on the atmospheric chemistry of ozone. In contrast to all other greenhouse gases, ozone is not emitted into the atmosphere.
Ozone is generated in-situ in the atmosphere from two processes: (i) photolysis of molecular oxygen (O2) which gives oxygen atoms (O) which then add to molecular oxygen to give ozone (O3) and (ii) oxidation of organic compounds (from natural and man-made sources) in the presence of nitrogen oxides (NOx). The first process only occurs in the upper atmosphere where there is sufficient short wavelength sunlight to photo dissociate molecular oxygen and this process gives rise to the stratospheric ozone layer at altitudes of 20-50 km. The second process occurs throughout the atmosphere but, because of the much greater availability of organic compounds and NOx near the Earth’s surface, is much more important in the lower atmosphere (troposphere).
Emission of large amounts of organic compounds and NOx in urban areas leads to the formation of substantial amounts of ozone in, and downwind of, large metropolitan centres around the world. In discussions of the climatic impact of human perturbations of atmospheric ozone levels a clear distinction must be made between ozone in the upper atmosphere (stratospheric ozone), which has decreased as a result of human activities, and ozone in the lower atmosphere (tropospheric ozone) which has increased as a result of human activities.
Finally, in contrast to all other greenhouse gases considered here, the atmospheric lifetime of ozone is short (of the order of days or weeks, depending on local conditions) and hence its concentration responds quickly to changes in atmospheric conditions. Ozone concentrations in the lower atmosphere are typically 10-100 ppb with levels at the low end of the range being characteristic of remote pristine environments and levels at the high end of the range being typical of polluted urban air masses. It is believed that the levels of ozone in the troposphere have increased by 30-40% since 1750 due to increased emission of organic compounds and NOx. This increased concentration of tropospheric ozone has contributed a positive radioactive forcing of 0.35 Wm-2. The forcing associated with tropospheric ozone varies substantially by region and season and will respond quickly to changes in emissions of ozone forming compounds.
Average Temperatures on the Moon: We can get another idea about what the temperature on Earth would be like without a greenhouse atmosphere by contemplating the Moon. The Earth’s satellite has no atmosphere because its gravitational force is not strong enough to retain gas for long. It has the same distance from the Sun as the Earth, but its temperature varies enormously: where the Sun is shining, the Moon’s temperature rises to 230°F and where it is dark falls to negative 290°F. The average surface temperature of the moon, about the same distance as the Earth from the Sun, is also near 0°F, but of course, the moon has no atmosphere. By contrast, the average surface temperature of the Earth is 60°F at sea level. On Earth, the contrast between maximum and minimum temperatures would not be as great as on the Moon, even without an atmosphere, because the Earth rotates once in a day, while the Moon only rotates once in a month.
However, without an atmosphere the Earth’s contrast between day and night and the contrast between summer and winter would be very large indeed. Not all the gases in the atmosphere are equally active in keeping Earth warm. In fact, the atmosphere’s most abundant gas, molecular nitrogen, does very little in this regard, and the same is true for the second most abundant gas, molecular oxygen. The most important ingredient of the air for producing the greenhouse effect is water vapour. However, its abundance depends on the air’s temperature. The warmer the air, the more water vapour it can hold. (As air cools, the vapour condenses into rain or snow.) It is carbon dioxide that moves the air toward higher temperature, so that water vapour can take over and warm it some more. Carbon dioxide molecules intercept infrared radiation, warming the air and increasing water vapour through evaporation from the sea surface and from plants and soil moisture.
Water vapour then increases the temperature even more. The process is checked by a rise in infrared radiation to space and by formation of clouds. Unfortunately, the role of clouds in the radiation balance is as yet poorly understood. Different types of clouds have different effects, and this makes the calculations complicated and the results uncertain. Solar Variations: One last point to consider when discussing the greenhouse effect is the amount of sunlight coming in to Earth. The quantity of sunlight we receive depends on the size and the brightness of the Sun and the distance between it and the Earth. As far as we know, the size of the Sun does not change much over the time spans we are considering; we can assume it is constant. The sun’s brightness varies only a little, about one-thousandth over an eleven-year sunspot cycle, but perhaps more over longer time spans. We can take that as constant too, calling the incoming energy flux “the solar constant.”
One of the contentious issues in the discussions about the global warming of the last 100 years that has not been fully resolved is the question of whether a brighter Sun may have contributed to the recently observed temperature rise. Consequences of Global Warming: A whole host of consequences will result. Some are probably already occurring. Temperature measurements of the sea surface and Deep Ocean indicate that the oceans are warming. Rising ocean temperature causes rising sea level from thermal expansion of the water. Rising temperature also means melting glaciers and rising sea level through addition of melt water to the oceans. Sea level rose about 1 foot during the last century, mostly from thermal expansion of the oceans. Sea level is expected to rise closer to 3 feet during the coming century.
Rising sea level will cause increasing coastal erosion, flooding, and property damage during coastal storms on top of the potential for major loss of life from storms in low-lying coastal countries like Bangladesh and island nations in the Indian and Pacific Oceans. Warmer sea surface temperatures will result in more and stronger tropical storms (hurricanes and typhoons). Coastlines already ravaged by these storms will expect to see more strong storms than before, increasing the loss of life and damage to infrastructure. It is much more difficult to predict how regional and local weather patterns will change but there will certainly be changes. While higher temperatures will produce more rainfall across the globe, the regional rainfall patterns will likely change. Some areas will get more, some areas will get less.
The timing of wet and dry periods may change. But higher temperatures will also mean more evaporation. Higher temperatures may also mean stronger storms with damaging winds. All of these mean new risks and changing conditions for agriculture. Centuries old farming practices will have to change. Some areas may go from being marginal to becoming a breadbasket region, while other regions may go from major agricultural production to marginal. Higher CO2 allows plants to grow faster (more CO2 enhances photosynthesis). That would sound good for agriculture. However, weed species tend to grow even better than crop plants under enhanced CO2 conditions so improved crop growth may be nullified by weed competition. Natural ecosystems will be hard pressed to keep up with the changing climate because the rate of change will be faster than typical long-term natural climate change.
Many species, especially plant species, will not be able to migrate to cooler areas fast enough to keep up with the warming of their habitats. And arctic species will have no place to go and may not be able to adapt to the new conditions. Severe summer heat in areas not used to it can lead to deaths. Higher heat and expansion of tropical areas may lead to increased incidence of malaria. What Can We Do About Global Warming: We can’t realistically stop the rise of CO2 in the near term, but we can slow it and therefore reduce the consequences that will occur, more fuel-efficient cars, less frivolous driving, more use of mass transit, improved insulation to decrease the fuel burned to heat and cool our homes, more efficient appliances, use of fluorescent rather than incandescent light bulbs, and careful monitoring of home electricity usage reduce our energy needs.
Planting large areas with trees will consume CO2 as the trees grow, until the forests mature. Stopping deforestation in the tropical forests around the world, especially in the Amazon and Indonesian rain forests, will keep that carbon in the forest rather than sending it back into the atmosphere as the trees are burned or decay and are not replaced by more.
Other techniques have also been proposed such as the chemical removal of CO2 from smokestacks and burial in deep underground reservoirs, though only certain areas can benefit from this, or disposal in the deep ocean where they will form a semi-stable compound under the cold temperatures and high pressures, though the CO2 could too easily come bubbling back up. These latter solutions are not well studied and wouldn’t be especially cheap. Moreover, leaders, societies, communities, local planners, farmers, health organizations, need to recognize the changing climate and rising sea level as they make plans for the future. Our citizens need to be educated as to likely changes and how best to deal with the changing conditions.
http://www.lenntech.com/greenhouse-effect/greenhouse-effect-mechanism.htm http://www.met.ie/education/pdfs_eng/Lesson%20Plan%20Greenhouse%20Effect.pdf http://www.google.co.in/search?hl=en&q=green+house+effect&bav=on.2,or.r_gc.r_pw.r_qf.&biw=1366&bih=667&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi&ei=JdlvUK6BJserrAel8oHICw
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