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Energy and the Environment
Fall 2012
Instructor: Xiaodong Chu
Email：[email protected]
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 CO2 and 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 patterns and 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, equaling the 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
I S  S oRE2 (1   )
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
I E  4RE2TE4
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
 S (1   ) 
TE   o

 4 
1/ 4
 255K(18o C)
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
Global Warming: Greenhouse Effect
• Water vapor provides a quasi-continuum (准连续) absorption
at the longer wavelengths
• The CO2 band at 12–18 μm is 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
Global Warming: Greenhouse Effect
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
TS  Q / 
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