Evs greenhouse effect - 3rd sem


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Ready Made Environmental Education Project on Greenhouse Effect

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Evs greenhouse effect - 3rd sem

  1. 1. GREEN HOUSE EFFECTACKNOWLEDGEMENT :I would like to express my special thanks of gratitude to my teacher J.N.Majumder who gave me the golden opportunity todo this wonderful project on the topic Greenhouse Effect, which also helped me in doing a lot of Research and i came toknow about so many new thingsI am really thankful to them.Secondly i would also like to thank my parents and friends who helped me a lot in finishing this project within the limitedtime.I am making this project not only for marks but to also increase my knowledge .THANKS AGAIN TO ALL WHO HELPED ME.1. INTRODUCTION :The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhousegases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, itresults in an elevation of the average surface temperature above what it would be in the absence of the gases.[1][2]Solar radiation at the frequencies of visible light largely passes through the atmosphere to warm the planetary surface, which thenemits this energy at the lower frequencies of infrared thermal radiation. Infrared radiation is absorbed by greenhouse gases, which inturn re-radiate much of the energy to the surface and lower atmosphere. The mechanism is named after the effect of solar radiationpassing through glass and warming a greenhouse, but the way it retains heat is fundamentally different as a greenhouse works by [2][3][4]reducing airflow, isolating the warm air inside the structure so that heat is not lost by convection.The existence of the greenhouse effect was argued for by Joseph Fourier in 1824. The argument and the evidence was furtherstrengthened by Claude Pouillet in 1827 and 1838, and reasoned from experimental observations by John Tyndall in 1859, and morefully quantified by Svante Arrhenius in 1896.[5][6]If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth is, it would have a temperature of about5.3 °C. However, since the Earth reflects about 30%[7] [8] of the incoming sunlight, the planets effective temperature (thetemperature of a blackbody that would emit the same amount of radiation) is about −18 °C,[9][10] about 33°C below the actual surface [11]temperature of about 14 °C. The mechanism that produces this difference between the actual surface temperature and theeffective temperature is due to the atmosphere and is known as the greenhouse effect.[12]Earth’s natural greenhouse effect makes life as we know it possible. However, human activities, primarily the burning of fossil fuelsand clearing of forests, have intensified the natural greenhouse effect, causing global warming.[13]2. PROCEDURE :The Earth receives energy from the Sun in the form UV, visible, and near IR radiation, most of which passes throughthe atmosphere without being absorbed. Of the total amount of energy available at the top of the atmosphere (TOA), about 50% isabsorbed at the Earths surface. Because it is warm, the surface radiates far IR thermal radiation that consists of wavelengths that arepredominantly much longer than the wavelengths that were absorbed (the overlap between the incident solar spectrum and theterrestrial thermal spectrum is small enough to be neglected for most purposes). Most of this thermal radiation is absorbed by theatmosphere and re-radiated both upwards and downwards; that radiated downwards is absorbed by the Earths surface. Thistrapping of long-wavelength thermal radiation leads to a higher equilibrium temperature than if the atmosphere were absent.This highly simplified picture of the basic mechanism needs to be qualified in a number of ways, none of which affect thefundamental process.The incoming radiation from the Sun is mostly in the form of visible light and nearby wavelengths, largely in the range 0.2–4 μm,corresponding to the Suns radiative temperature of 6,000 K.[16] Almost half the radiation is in the form of "visible" light, which oureyes are adapted to use.[17]About 50% of the Suns energy is absorbed at the Earths surface and the rest is reflected or absorbed by the atmosphere. Thereflection of light back into space—largely by clouds—does not much affect the basic mechanism; this light, effectively, is lost to thesystem.The absorbed energy warms the surface. Simple presentations of the greenhouse effect, such as the idealized greenhouse model,show this heat being lost as thermal radiation. The reality is more complex: the atmosphere near the surface is largely opaque tothermal radiation (with important exceptions for "window" bands), and most heat loss from the surface is by sensible heat and latentheat transport. Radiative energy losses become increasingly important higher in the atmosphere largely because of the decreasingconcentration of water vapor, an important greenhouse gas. It is more realistic to think of the greenhouse effect as applying to a"surface" in the mid-troposphere, which is effectively coupled to the surface by alapse rate.The simple picture assumes a steady state. In the real world there is the diurnal cycle as well as seasonal cycles and weather. Solarheating only applies during daytime. During the night, the atmosphere cools somewhat, but not greatly, because its emissivity is low,and during the day the atmosphere warms. Diurnal temperature changes decrease with height in the atmosphere. 1
  2. 2. Within the region where radiative effects are important the description given by the idealized greenhouse model becomes realistic:The surface of the Earth, warmed to a temperature around 255 K, radiates long-wavelength, infrared heat in the range 4–100 μm.[16] At these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation are moreabsorbent.[16] Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers.It re-radiates in all directions, both upwards and downwards; in equilibrium (by definition) the same amount as it has absorbed. Thisresults in more warmth below. Increasing the concentration of the gases increases the amount of absorption and re-radiation, andthereby further warms the layers and ultimately the surface below.[10]Greenhouse gases—including most diatomic gases with two different atoms (such as carbon monoxide, CO) and all gases with threeor more atoms—are able to absorb and emit infrared radiation. Though more than 99% of the dry atmosphere is IR transparent(because the main constituents—N2, O2, and Ar—are not able to directly absorb or emit infrared radiation), intermolecular collisionscause the energy absorbed and emitted by the greenhouse gases to be shared with the other, non-IR-active, gases.3. RESULTS :3.1 Greenhouse gasesBy their percentage contribution to the greenhouse effect on Earth the four major gases are:water vapor, 36–70%carbon dioxide, 9–26%methane, 4–9%ozone, 3–7%The major non-gas contributor to the Earths greenhouse effect, clouds, also absorb and emit infrared radiation and thus have aneffect on radiative properties of the atmosphere.3.2 RADIATIVE FORCING OF CLIMATE CHANGE :Net radiation is defined as the difference between the solar radiation absorbed by theEarth-atmosphere system and the longwave radiation emitted by the Earth-atmospheresystem to space. Net radiation influences the Earths climate because it determines theenergy available for heating the atmosphere, ocean and land. Hence net radiationinfluences the seasonal variation of rainfall and the strength of the global circulationpatterns. When greenhouse gases increase in the atmosphere on account of humanactivities, the radiative balance of the Earth is altered. The greenhouse gases absorb thelongwave radiation emitted by the Earth but are transparent to the radiation comingfrom the sun. Hence the increase in greenhouse gases causes an increase in the netradiation at the top of the atmosphere. The change in net radiation at the tropopausecaused by changes in greenhouse gas or aerosol concentrations is called radiativeforcing. Radiative forcing can be calculated accurately if the temperature profile and theconcentration of the greenhouse gases in the atmosphere is known. Radiative forcingdepends upon how strongly a greenhouse gas absorbs radiation and the location of itsabsorption bands. As discussed in section 2 and illustrated in Figure 3, the radiationemitted by the Earth-atmosphere system is at its maximum around in the wavelengthregion 10-12 microns (1 micron = 10-6 meter). If the absorption band of a greenhousegas is located in this region then it will tend to have a high radiative forcing. On theother hand, if the absorption lies in a region in which there is already strong absorptionby greenhouse gases presently in the atmosphere (for example at around 15 μm whereCO2 absorbs, see Figure 3) then it will have a low radiative forcing. This later effectexplains why increases in atmospheric CO2 levels have a radiative forcing effect whichare small (on a molecule for molecule basis) relative to the effect of other greenhousegases. During the calculation of radiative forcing one must account for the overlapbetween the absorption bands of different greenhouse gases. Some gases, such as CFC-12 (CCl2F2), can cause direct as well as indirect radiative forcing. The direct radiativeforcing of CFC-12 is positive since it has an absorption band in the infrared region. Theindirect radiative forcing associated with CFC-12 is negative because emission of CFC-12 leads to the loss of stratospheric ozone with a resulting cooling effectHow do we compare the radiative impact of different greenhouse gases released byhuman beings? Chlorofluorocarbons absorb much more longwave radiation than carbondioxide on a per molecule basis. The change in atmospheric abundance ofchlorofluorocarbons has, however, been much smaller than that of carbon dioxide. Tocompare the effect of different gases with different abundance and different capabilityto absorb longwave radiation it is most appropriate to compare their impact on netradiation at the tropopause (boundary between the troposphere and stratosphere, locatedat approximately 10-15 km). Since pre-industrial times the amount of carbon dioxide in 2
  3. 3. the atmosphere has increased from 278 ppm to 370 ppm and resulted in a radiativeforcing change of approximately 1.46 W m-2. The amount of methane in the atmospherehas increased from 700 ppb (in pre-industrial times) to 1700 ppb at present and hasresulted in a radiative forcing change of 0.47 W m-2. On the other hand, the amount ofCFC-12 has increased from 0 to 0.5 ppb since pre-industrial times and resulted in aradiative forcing change of 0.14 W m-2. Radiative forcing is a measure of the effect ofdifferent greenhouse gases or aerosols on radiative balance at the tropopause. Note thatthe radiative forcing due to increased carbon dioxide is three times as large as thatcaused by the increase in methane and eleven times as large as that due to CFC-12.Hence, it can be inferred that carbon dioxide is more important than methane or CFC-12as regards its impact on global warming. It is reasonable to conclude that attempts tosignificantly reduce global warming should include a reduction of carbon dioxideemissions from human activities. To evaluate the long-term impact of emission ofgreenhouse gases we need information about the residence times of greenhouse gasesemitted on account of human activities. A gas with a long residence time will have agreater impact on future climate than a gas with a short residence time even if theradiative forcing of the two gases is the same. The atmospheric lifetime of carbondioxide is around 100 years while that of methane is around 12 years.Table 1. Direct Global Warming Potential (GWP) values for selected well mixedgreenhouse gases for three different time horizonsFinally, it should be noted that the values in Table 1 are direct GWPs and do not includeany indirect effects caused by secondary processes. Some compounds have significantindirect GWPs. For example CFCs deplete stratospheric ozone and hence have asignificant indirect impact. The indirect GWP (due to ozone depletion) caused by CFC-12 over a 20 year time horizon is estimated to be in the range -3100 to -600. Hence, thenet GWP due to CFC-12 is estimated to be in the range 7100 to 9600.4. CONCLUSION :At the beginning of this chapter the following questions were posed: "Is global climatechanging?”, "To what degree are human activities responsible for climate change?","What is the state of the science?", "What will future global climate be like?" and "Whatshould we do?". In the light of the discussion in the previous sections we are now in aposition to provide answers to these questions."Is global climate changing?" The answer to this question is "Yes". There is a largebody of scientific data that shows beyond any reasonable doubt that the global climatehas warmed significantly over the past 150 years. Probably the most compellingevidence for global warming comes from the temperature record shown in Figures 1 and2. However, this data is by no means the only indication of a change in climate. Inaddition, observed retreat of glaciers around the world, decreased snow cover in theNorthern hemisphere, decreased tropical precipitation, increased mid-to-high latitudeprecipitation, sea level rise, increased ocean temperature, decreased extent of Arctic ice,and decreased thickness of Arctic ice all point to a warming global climate. Thecombined data prove beyond any reasonable doubt that there has been a definitewarming trend in global climate over the past century. It is important to stress that thewarming trend is in "global climate", not "local climate" or "global weather". Thus,neither a long term cooler climate in restricted geographical areas, nor a decrease inaverage global temperature from one year to the next should be viewed as inconsistentwith the overall warming trend."To what degree are human activities responsible for climate change?" The answer tothis somewhat controversial question is "It is very likely that human activities areresponsible for a substantial fraction of the observed climate change". The naturalgreenhouse effect is a well known phenomenon. It is well established that humanactivities have led to large increases in the atmospheric concentrations of the three mostimportant long lived greenhouse gases. State-of-the-art computational climate modelsare only able to reproduce the historical climate record when forcings associated withhuman activities are included. Models which ignore forcings associated with humanactivities are not able to reproduce the historical climate record. Natural climate forcingagents such as the sun and volcanoes play an important role in the climate system but donot explain the observed increase in global temperature over the past century."What is the state of the science?" It has been known for a very long time that gasessuch as water vapor, carbon dioxide, and methane trap infrared radiation in the 3
  4. 4. atmosphere and warm the Earth. In 1827 Fourier compared this effect to that of theglass in a greenhouse from which the term "greenhouse effect" is derived. In 1896Arrhenius calculated that a doubling of atmospheric carbon dioxide would cause a 5-6oC increase in global temperature. In 2001, state-of-the-art climate models run on thelatest super computers predict that a doubling of carbon dioxide would lead to a 1.5 –4.5 oC warming. While it is remarkable how little change there has been over the past105 years in the prediction of the global warming caused by a doubling of carbondioxide, this fact should not be taken as an indication that the prediction of futureclimate is a mature scientific discipline. Global climate models are in an early stage ofdevelopment. There is no doubt that in the coming decades the sophistication and levelof detail in the models, and hence their predictive capability, will increase dramatically.Current areas of significant uncertainty in the models are: (i) future levels of emission ofgreenhouse gases (ii) future levels of atmospheric aerosols (iii) magnitude of feedbackmechanisms which amplify (or damp) the climate response to increased greenhouse gaslevels (iv) treatment of clouds and (v) the possible existence and importance ofmetastable climate states. It seems likely that significant surprises lie in store in thefuture scientific exploration of this fascinating field."What will future global climate be like?" As with all questions involving a predictionof future events, this is a difficult question to answer. The best answer to this question is"While the details are unclear, it appears likely that the future global climate will besignificantly warmer than that at present (during the next century the global climate willmost probably warm by an amount which is approximately 10 times that of the longterm natural variability of the climate system indicated by the black line in Figure 1)".There are two fundamental problems associated with prediction of future global climate.The first and most serious problem is that the future emission rates of greenhouse gasesare unknown. Will the world continue to rely heavily on fossil fuel for the next century,or will non-fossil fuel energy sources such as nuclear and renewable options gain inimportance? It is clear that when looking 10 or 20 years into the future that fossil fuelwill continue as the dominant energy source. However, it is very difficult to predict thedominant energy source 50 to 100 years into the future. Global population growth andthe development path chosen by the developing nations are important factors affectingfuture greenhouse gas emissions. The second problem is that even if the future emissionrate of greenhouse gases was known exactly, the uncertainties inherent in the scientificunderstanding of the link between greenhouse gas emissions and global climate change(see previous paragraph) would still introduce substantial uncertainty in the predictionsof future climate. The Intergovernmental Panel on Climate Change considered manydifferent possible emission scenarios for greenhouse gases with a range of differentassumptions concerning economic growth, population growth, and reliance on fossilfuels. The projected annual carbon dioxide emissions in the year 2100 from thesescenarios varied widely from a high of 30 GtC to a low of 4 GtC. The projected increasein average global temperature for the scenarios is between 1.4 and 5.8 oC with awarming of 2.5 – 3.2 oC being most probable. To place this prediction into context it isuseful to refer to Figure 1 and to extrapolate an additional 2.5 – 3.2 oC rise from 2000 to2100. It is clear that by comparison to the natural variability observed over the past1000 years, the climate change expected over the next 100 years will be substantial."What should we do?" This is of course a value question and as such it is not possible toprovide a purely scientific answer. However, science can provide some guidance inaddressing this question. Over the past century the demand for energy has increasedsignificantly. This demand has been satisfied primarily by fossil fuel. There was asubstantial (12 fold) and sustained (during each and every decade) increase in fossil fuelusage over the past century and it is projected that fossil fuel use will continue toincrease for at least the next couple of decades. Carbon dioxide has a long atmosphericlifetime and there is a delay of approximately 100 years between changes in carbondioxide emissions and equilibration of its atmospheric concentration. In the absence ofpolicies to address the accumulation of greenhouse gases in the atmosphere it appearsvery likely that over the next century the concentration of atmospheric carbon dioxidewill reach a level which is substantially (a factor of 2-3) greater than that of the preindustrialera. It would seem prudent at this point in time to pursue the research anddevelopment of alternative global energy strategies which either decrease our long termreliance on fossil fuels, or reduce carbon dioxide emission associated with fossil fueluse (e.g. geologic sequestering of carbon dioxide). It would also seem prudent to devoteadditional resources to improve the scientific understanding of the climate system and 4
  5. 5. hence reduce the uncertainties associated with projections of future climate change.Climate change is a long term global issue which requires a long term global solution.5. REFERENCE :Arrhenius, S., (1896). On the influence of carbonic acid in the air on the temperature of the ground, Phil. Mag., S. 41, 237-277. [Firstassessment of the likely climatic impact of doubling atmospheric CO levels due to fossil fuel combustion] 2Fourier, J., (1827). Memoire sur les Temperatures du Globe Terrestre et des Escapes Planetaires, Mem. Aca. Sci. Inst. Fr., 7, 569-604.[First account of the greenhouse effect]Houghton, J., (1997). Global Warming: The Complete Briefing, Cambridge University Press (1997). [Overview of global warming andits impacts]http://www.epa.gov/globalwarming// [United States Environmental Protection Agency site with information on climate change andits impacts]http://www.giss.nasa.gov/ [National Aeronautics and Space Administration site with a wealth of global warming datasets andimages.http:/www.ipcc.ch/ [Home page for Intergovernmental Panel on Climate Change]http://www.met-office.gov.uk/research/hadleycenter/ [General information about climate modeling and results (including animatedmovies) from climate models at Hadley Center]http://www.pewclimate.org/ [Non governmental organization site for global warming information]Intergovernmental Panel on Climate Change, (1996). Climate Change 1995: The Science of Climate Change, Cambridge UniversityPress (1996). [Comprehensive account of the state of scientific understanding of climate change as of 1996 with numerousreferences]Intergovernmental Panel on Climate Change, (2001). Climate Change 2001: The Scientific Basis, Cambridge University Press.[Comprehensive account of the current state of scientific understanding of climate change with numerous references] 5
  6. 6. IntroProcedureThe solar radiation spectrum for direct light at both the top ofthe Earths atmosphere and at sea level 6
  7. 7. Synthetic stick absorption spectrum of a simple gas mixturecorresponding to theEarths atmosphere composition basedonHITRAN data created using Hitran on the Web system. Green color -water vapor, red - carbon dioxide, WN - wavenumber(caution:lower wavelengths on the right, higher on the left).Mechanism Of Greenhouse Effect 7
  8. 8. ResultsRadiative Forcing of Climate ChangeGlobal WarmingConclusion 8
  9. 9. CONTENTS :SERIAL No. TITLE PAGE No. 1 Introduction 2 Procedure 3 Results 3.1 Greenhouse Gases 3.2 Radiative Forcing of Climate Change 4 Conclusion 5 Reference 9
  10. 10. GREENHOUSE EFFECT RCC INSTITUTE OF INFORMATION TECHNOLOGY-----------------------------------------------------------------------------------------------------------------------NAME : ANIRBAN DASSTREAM : CSE 2ND YEAR 3RD SEMESTERROLL No. : CSE/2011/039SUBJECT : BASIC ENVIRONMENTAL ENGINEERING & ELEMENTARY BIOLOGY 10