Energy and the Environment

Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

Flashbacks of Last Lecture • Air pollution • Air-quality modeling • Photo-oxidants • Acid deposition • Water pollution • Land pollution

Global Warming: Introduction • Of all environmental effects of fossil fuel usage, global warming, including its climate change, is the most perplexing, potentially most threatening, and arguably most intractable • It is caused by the ever-increasing accumulation in the atmosphere of CO2and other gases, such as CH4, N2O, chlorofluorocarbons (CFCs) (氟氯化碳), and aerosols(气溶胶), largely due to emissions of these gases from anthropogenic activities(人为活动), and reaches levels that exceed those that existed for centuries before the beginning of the industrial revolution • These substances augment the greenhouse effect of the earth’s atmosphere, which provides a warmer climate at the earth’s surface than would exist in an atmosphere-free earth

Global Warming: Introduction • Most of the emissions of greenhouse gases and aerosols are a consequence of fossil fuel usage to satisfy our growing energy needs • Methane(甲烷) emissions are in part due to fossil energy usage, as CH4 leaks from gas pipes, storage tanks, tankers, and coal mine shafts • CFCs are not directly associated with fossil fuel usage; however, they are emitted from energy-using devices, such as refrigerators, air conditioners, chillers(冷水机组), and heat pumps

Global Warming: Introduction • The prevention of global warming will require a very significant shift from our present energy use pattern toward one of lesser reliance on fossil fuels • Because of the expected socioeconomic disruptions, there is considerable opposition to implementing preventative policies • Most scientists consider preventative measures a necessity in order to avoid significant changes in global climate and the consequent impact on human habitat and ecological systems

Global Warming: Greenhouse Effect • The term greenhouse effect is derived by analogy to a garden greenhouse • A glass covered structure lets in the sun’s radiation, warming the soil and plants that grow in it, while the glass cover restricts the escape of heat into the ambient surroundings by convection (对流) and radiation • The earth’s atmosphere lets through most of the sun’s radiation, which warms the earth’s surface, but certain gases, called greenhouse gases (GHG), trap outgoing radiative heat near the surface, causing elevated surface temperatures

Global Warming: Greenhouse Effect • The warming effect on the earth’s surface by certain gases in the atmosphere was first recognized in 1827 by Jean-Baptiste Fourier, the famous French mathematician • Around 1860, the British scientist John Tyndall measured the absorption of infrared radiation by CO2 and water vapor, and he suggested that the cause of the ice ages may be due to a decrease of atmospheric concentrations of CO2 • In 1896, the Swedish scientist Svante Arrhenius estimated that doubling the concentration of CO2 in the atmosphere may lead to an increase of the earth’s surface temperature by 5–6 ◦C

Global Warming: Greenhouse Effect • The major natural GHG are water vapor and carbon dioxide • If not for these gases, the earth would be quite inhospitable, with a temperature well below freezing • Why are we concerned about adding more man-made GHG to the atmosphere? • The answer is that humans and ecological systems have adapted to present climatic patternsand perturbing those patterns may result in unpredictable climatic, ecologic, and social consequences

Global Warming: Greenhouse Effect • The sun emits electromagnetic radiation ranging from very short wavelength gamma rays, X rays, ultraviolet through visible, to infrared radiation • A section of the solar spectrum reaching the earth, which contains the near-UV, visible, and near-IR spectrum • Three spectra are presented • The uppermost curve gives the solar irradiance outside the earth’s atmosphere • The lowest curve is the spectrum of incident radiation (入射辐射) at sea level • The dashed curve is the radiance of a black body heated to a temperature of 5900 K, equalingthe total solar irradiance (太阳辐照) at the earth

Global Warming: Greenhouse Effect • The sun’s surface temperature is approximately 5900 K • A black body heated to 5900 K emits its peak radiance in the visible portion of the electromagnetic spectrum, at about 0.5 μm • The reason the solar irradiance curve does not follow the black-body radiance curve exactly is that at some wavelengths there is excess radiance, at others a deficit

Global Warming: Greenhouse Effect

Global Warming: Greenhouse Effect • The solar irradiance spectrum at sea level is much different from the spectrum at the top of the atmosphere • Gases in the earth’s atmosphere absorb or scatter(散射) radiation • On the UV side, oxygen and ozone in the stratosphere(平流层) are the major absorbing gases • In the visible portion, density fluctuations of atmospheric molecules scatter sunlight • In the infrared, molecules present in the lower atmosphere (troposphere(对流层)), such as H2O, CO2, O3, CH4, N2O, and others, absorb solar radiation

Global Warming: Greenhouse Effect • The earth radiates outward to space a spectrum commensurate (相称) to its surface temperature • The below figure presents the earth’s radiance • The solid curves in the figure represent black-body radiances at various temperatures • The black-body radiance curve that best represents the earth’s radiance at the spot where the spectrum was taken is that for a temperature of about 280–285 K

Global Warming: Greenhouse Effect • The radiance curves span the far-infrared region (远红外区域)of the electromagnetic spectrum, from about 5 to 50 μm, peaking at about 18 μm • But the earth’s radiance curve has several deficits • These deficits are the result of absorption, and emission at reduced levels, of the outgoing thermal radiation by greenhouse gases

Global Warming: Greenhouse Effect • The sun–earth–space radiative equilibrium is depicted in the below figure • The numbers on arrows are average annual radiation received from the sun or emitted from the earth, in watts per square meter

Global Warming: Greenhouse Effect • The solar energy input to the earth on top of the atmosphere is where • So= annual average solar energy impinging on top of the atmosphere, called the solar constant, 1367 W m−2 • RE= earth’s radius, 6371 km • α = the present average albedo (反照率) of the earth, 0.3 ± 0.03

Global Warming: Greenhouse Effect • The earth’s radiative output to space is where • σ = Stefan–Boltzmann constant, 5.67 E(−8) W m−2 K−4 • TE = equivalent black-body radiative temperature of earth, K

Global Warming: Greenhouse Effect • Calculate the earth’s radiative temperature TE where an albedo α = 0.3 • The temperature TE = 255K is the average radiative temperature of the earth and its atmosphere as it would appear to a space observer looking toward the earth • Presently, the earth’s average surface temperature is TS = 288 K (15 ◦C) • The difference TS − TE = 33 K is a consequence of the greenhouse effect

Global Warming: Greenhouse Effect • The earth’s average surface temperature has varied over past geological ages(地质时代), as evidenced by the glacial and interglacial periods(冰期和间冰期) • In part, these temperature variations may have been caused by changing GHG concentrations in the atmosphere • Other factors for global climate change may have been related to the variations of the earth’s orbit around the sun and the tilt of its axis

Global Warming: Greenhouse Effect • Modeling of global warming is based on calculations of radiative transfer between the radiating body and atmospheric molecules, together with other energy transport processes in the atmosphere • The change caused by extra amounts of GHG is called radiative forcing(辐射强迫) • Modeling requires a detailed knowledge of the spectroscopic characteristics (光谱特性) of GHG and the structure of the atmosphere, as well as very high capacity computers on which to exercise the radiative transfer models

Global Warming: Greenhouse Effect • The absorption bands of atmospheric molecules are in the far IR, between 5 and 50 μm, where the earth’s outgoing thermal radiation is prominent • Water vapor absorbs at 5–7 μm and all wavelengths above 10 μm; CO2 at 12–18 μm; CH4 and N2O at 7–8 μm; O3 at 9–10 μm; the major CFCs at 10–12 μm • Each absorption band is composed of many individual absorption lines(吸收谱线) • At some wavelengths there is overlap of absorption by individual species

Global Warming: Greenhouse Effect • Water vapor provides a quasi-continuum (准连续) absorption at the longer wavelengths • The CO2 band at 12–18 μmis so strong that it is almost saturated at the center; thus further additions of CO2 to the atmosphere will result in more absorption at the wings of the bands, but little at the center • CH4 bands are not saturated at all; thus doubling of CH4 concentrations will nearly double CH4 absorption • It is estimated that adding one molecule of CH4 is as effective in absorbing the far IR radiation as adding 20 molecules of CO2

Global Warming: Greenhouse Effect • The absorption of energy at a particular wavelength by an individual molecule depends on line intensity(谱线强度), line half-width(谱线半宽), and lower state energy • The atmosphere is modeled by many layers, in each of which the absorbing gas concentration and temperature determine the radiative transport away from and towards the earth, and from which the model predicts the surface temperature and atmospheric temperature profile

Global Warming: Greenhouse Effect • The earth’s atmosphere has a complicated temperature structure • In the troposphere the temperature decreases with altitude; in the stratosphere the temperature increases; in the mesosphere (中间层) it decreases; in the thermosphere (热层) it increases again with altitude • The GHG molecules can reduce the outgoing terrestrial radiation (地面辐射) only if they are at a colder temperature than the radiating earth

Global Warming: Greenhouse Effect • The modeling of global warming is further complicated because a rising surface temperature will cause secondary effects, called feedback effects(回馈效应) • This can be expressed by the proportionality where ΔTS is the rise of surface temperature, Δ Q is the radiative forcing (辐射强迫) due to GHG, and β is a factor accounting for the feedback effect • If β <1, the feedback is positive, which will increase the surface temperature even more than by increasing greenhouse gas concentrations alone • If β > 1, the feedback is negative, which will cause a smaller surface temperature increase

Global Warming: Greenhouse Effect • There are several feedback effects possible: water vapor, cloud, aerosol, ice-albedo(冰反照率), and ocean circulation feedbacks • Water vapor may be the most important feedback effect • It is a strong infrared absorbing gas at wavelengths between 5 and 7 μm and above 10 μm • As the average earth’s surface temperature rises because of increasing GHG concentrations, more evaporation will occur from the vast ocean surfaces, hence the atmosphere will become laden with more water vapor, which is a positive feedback • Radiative models predict that the water feedback effect may increase the GHG caused global warming by about 60%

Global Warming: Greenhouse Effect • The feedback effect of cloud radiation is complicated • Clouds can have both a negative and positive feedback effect • The negative effect is due to reflection by clouds of the incoming solar radiation, contributing to an increase of the earth’s albedo and thus reducing the earth’s surface temperature • The positive effect is due to the clouds reflecting the outgoing earth’s thermal radiation • The balance of the two effects is dependent on cloud characteristics, their altitude, and droplet or crystal size • The cloud feedback effect will add another 20% to the surface temperature increase due to increasing GHG concentrations

Global Warming: Greenhouse Effect • Natural and man-made small particles suspended(悬浮) in the air, called aerosols, can interfere with the incoming solar and outgoing terrestrial radiation • The average composition of aerosols is about one-third crustal material (地壳物质), one-third sulfate(硫酸), and one third carbonaceous matter (碳物质)and nitrate (硝酸盐) • The diameter range of the aerosols is much less than a micrometer, which is more effective in scattering incoming solar radiation than in reflecting outgoing terrestrial radiation • The aerosol feedback effect is thought to be negative, reducing the GHG effect by 10–15%

Global Warming: Greenhouse Effect • As the earth’s surface warms due to increased GHG concentrations, the fringes of the Arctic and Antarctic ice caps(南北两极冰边缘) may melt • Glaciers (冰川), which already are receding in this interglacial period, may recede even faster • Because ice has a higher albedo than water and land, the disappearance of ice will lead to a decrease in the earth’s albedo • This will cause the earth’s radiative temperature TE , and the surface temperature TS, to increase slightly • The ice–albedo effect may add 20% to the GHG-caused surface warming

Global Warming: Greenhouse Effect • Another possible feedback mechanism is the alteration of the ocean circulation and currents (海洋环流) • In general, cold and highly saline water(高盐度海水) sinks to greater depths, and warm, less saline water rises • These waters sink to the ocean bottom and move toward the equator(赤道), where warmer and less saline waters rise and move toward the poles, completing a loop • With melting ice caps, along with an increase of precipitation(降水) at high latitudes, the normal ocean circulation pattern may be altered, with possible changes in the average global surface temperature • This feedback effect on average surface temperature is very difficult to predict, which may be negative, positive, or neutral

Global Warming: Greenhouse Effect • Based on radiative forcing models, the Intergovernmental Panel on Climate Change (IPCC) (联合国政府间气候变化专门委员会) projects the average earth’s surface temperature increase in the twenty-first century • This projection is based on estimated increase of CO2 and other GHG emissions, as well on various feedback effects

Global Warming: Greenhouse Effect • As can be seen, there are wide fluctuations from year to year • In spite of the fluctuations, there is an unmistakable upward trend • Averaging the fluctuations, the mean global temperature appears to have risen by about 0.5 ◦C to 1◦C

Global Warming: Greenhouse Effect • As a consequence of increased GHG concentrations in the atmosphere, the earth’s surface temperature may rise • The surface temperature rise may cause several ancillary effects on global climate and hydrogeology(水文), which in turn will affect human habitat, welfare, and the ecology

Global Warming: Greenhouse Effect • With increasing surface temperatures the average sea level will rise because of three factors: melting of polar ice caps (极地冰盖融化), receding of glaciers (冰川退缩), and thermal expansion of the ocean surface waters(海洋表层海水的热膨胀) • The largest contribution to sea level rise will come from the thermal expansion of the surface layer of the oceans • The estimation of this contribution is quite complicated, because the coefficient of thermal expansion of water is a function of temperature, and the depth of the mixed surface layer which varies over the globe

Global Warming: Greenhouse Effect • Predicting global and regional climatic changes as a consequence of average surface temperature rise is extremely difficult with uncertainties • Regional temperatures, prevailing winds(盛行风), and storm and precipitation patterns will change, but where and when, and to what extent changes will occur, is a subject of intensive investigation and high computing efforts • Climate is influenced not only by surface temperature changes, but also by biological and hydrological processes and by the response of ocean circulation, which are all coupled to temperature changes

Energy and the Environment