The Mechanism of the Greenhouse Effect The Earths Energy Source

The Earth's surface and atmosphere are kept warm almost exclusively by energy from the Sun, which radiates energy as light of many types. In its radiating characteristics, the Sun behaves much like a blackbody, i.e., an object that is 100% efficient in emitting and in absorbing light. The wavelength, Apeak, in micrometers, at which the maximum emission of energy occurs by a radiating blackbody decreases inversely with increasing Kelvin temperature, T, according to the relationship

Apeak = 2897/T

Since for the surface of the Sun, from which the star emits light, the temperature T ~ 5800 K, then from the equation it follows that Apeak is about 0.50 pun, a wavelength that lies in the visible region of the spectrum (and corresponds to green light). Indeed, the maximum observed solar output (see the dashed portion of the curve in Figure 6-1) occurs in the range of visible light, i.e., that of wavelengths between 0.40 and 0.75 /xm. Beyond the "red limit," the maximum wavelength for visible light, the Earth receives infrared light (IR) in the 0.75-4-pt,m region from the Sun. Of the energy received at the top of the Earth's atmosphere from the Sun, slightly over half the total is IR and most of the remainder is visible light. At the opposite end

FIGURE 6-1 Wavelength distributions (using different scales) for light emitted by the Sun (dashed curve) and by Earth's surface and troposphere (solid curve). ¡Source: Redrawn from J. Gribbin, "Inside Science: The Greenhouse Effect," New Scientist supplement (22 October 1988).)

UV Visible light Infrared light

Wavelength Qim)

UV Visible light Infrared light

Wavelength Qim)

FIGURE 6-1 Wavelength distributions (using different scales) for light emitted by the Sun (dashed curve) and by Earth's surface and troposphere (solid curve). ¡Source: Redrawn from J. Gribbin, "Inside Science: The Greenhouse Effect," New Scientist supplement (22 October 1988).)

of the visible wavelength spectrum from IR, beyond the "violet" limit, lies ultraviolet light (UV), which has wavelengths less than 0.4 pirn and is a minor component of sunlight, as discussed in Chapter 1.

Of the total incoming sunlight of all wavelengths that impinges upon the Earth, about 50% is absorbed at its surface by water bodies, soil, vegetation, buildings, and so on. A further 20% of the incoming light is absorbed by water droplets in air (mainly in the form of clouds) and by molecular gases— the UV component by stratospheric ozone, 03, and diatomic oxygen, 02, and the IR by carbon dioxide, C02, and especially water vapor.

The remaining 30% of incoming sunlight is reflected back into space by clouds, suspended particles, ice, snow, sand, and other reflecting bodies, without being absorbed. The fraction of sunlight reflected back into space by an object is called its albedo, which therefore is about 0.30 for the Earth overall. Clouds are good reflectors, with albedos ranging from 0.4 to 0.8, Snow and ice are also highly reflecting surfaces for visible light (high albedos), whereas bare soil and bodies of water are poor reflectors (low albedos). Thus the melting of sea ice in polar regions to produce open water greatly increases the fraction of sunlight absorbed there and decreases the Earth's overall albedo. Planting trees in snow-covered forests reduces the albedo of the surface and may actually contribute to global warming.

Historical Temperature Trends

The trends in average surface temperature for the past 2000 years, as reconstructed for most of that period from indirect evidence such as tree ring growth, is shown in Figure 6-2a. (The Medieval Warm Period early in the

FIGURE 6-2 (a) Reconstruction of changes in average global surface temperatures over the last two millennia. [Source: M. E, Mann and P. D. Jones, "Global Surface Temperatures over the ftwt Two Millennia," Geophysical Research Letters 30 (2003): 1820.] (b) Global average land-ocean temperature at the Earth's surface from 1880 to 2005. [Source: t. Brown et al., Vital Signs 2006-7 (New York: Norton, 2007).)

millennium was apparently restricted to the North Atlantic region, so it is not very evident on the global plot.) Notice the consistently downward trend in temperature until the beginnings of the Industrial Revolution.

The warming of the climate during the twentieth century stands in stark contrast to the gradual cooling trend in the previous 900 years of the millennium, producing a "hockey stick" shape to the temperature plot in Figure 6-2a. The trends in global average surface temperature over the last century and a half are illustrated in detail in Figure 6-2b. Air temperature did not increase continuously throughout the twentieth century. A significant warming trend occurred in the 1910-1940 period, due to a lack of volcanic activity and a slight increase in the intensity of sunlight. This period was followed by some cooling over the next three decades, due to aerosols resulting from increased volcanic activity. These decades were succeeded in turn by a warming period that has been sustained from about 1970 to the present and that has so far amounted to a temperature increase of about 0.6°C; this is attributed almost entirely to anthropogenic influences, as discussed in detail in this chapter. Eleven of the 12 years of the 1995-2006 period are among the 12 warmest since 1850, when instrument records began. The warmest years on record were 1998 and 2005.

Earth's Energy Emissions and the Greenhouse Effect

Like any warm body, the Earth emits energy; indeed, the amount of energy that the planet absorbs and the amount that it releases must be equal over the long term if its temperature is to remain level. (Currently, the planet is absorbing slightly more than it is emitting, thereby warming the air and the oceans.) The emitted energy (see the solid portion of the curve in Figure 6-1) is neither visible nor UV light, because the Earth is not hot enough to emit light in these regions. Since the temperature of the Earth's surface is approximately 300 K, then according to the equation above for Apeak, if the Earth acted like a blackbody, its wavelength of maximum emission would be about 10 ixm. Indeed, the Earth's emission does peak in that general region, actually at about 13 /um, and consists of infrared light having wavelengths starting at about 5 /im and extending, albeit weakly, beyond 50 /xm (Figure 6-1, solid curve). The 5-100-ju.m range is called the thermal infrared region since such energy is a form of heat, the same kind of energy a heated iron pot would radiate.

Infrared light is emitted both at the Earth's surface and by its atmosphere, though in different amounts at different altitudes since the emission rate is very temperature sensitive: In general, the warmer a body, the more energy it emits per second. The rate of release of energy as light by a blackbody increases in proportion to the fourth power of its Kelvin temperature:

rate of energy release = kT4

where k is a proportionality constant. Thus doubling its absolute temperature increases sixteen-fold (24) the rate at which a body releases energy. More realistically, for contemporary surface conditions of planet Earth, a one-degree rise in temperature would increase the rate of energy release by 1.3%.

PROBLEM 6-i

Calculate the ratio of the rates of energy release by two otherwise identical blackbodies, one of which is at 0°C and the other at 17°C. At what temperature is the rate of energy release twice that at 0°C?

FIGURE 6-3 The greenhouse effect: Outgoing IR absorbed by greenhouse gases is either re-emitted (left side of diagram) or converted to heat (right side).

FIGURE 6-3 The greenhouse effect: Outgoing IR absorbed by greenhouse gases is either re-emitted (left side of diagram) or converted to heat (right side).

Some gases in air absorb thermal infrared light—though only at characteristic wavelengths—and therefore the IR emitted from the Earth's surface and atmosphere does not all escape directly to space. Very shortly after its absorption by atmospheric gases such as C02, the IR photon may be reemitted. Alternatively, the absorbed energy may quickly be redistributed as heat among molecules that collide with the absorber molecule, and it may eventually be re-emitted as IR by them. Whether re-emitted immediately by the initial absorber molecule or later by others in the area, the direction of the photon is completely random (Figure 6-3). Consequently, some of this thermal IR is redirected back toward the Earths surface and is reabsorbed there or in the air above it.

Because the air absorbs IR photons and redistributes the energy as heat to surrounding molecules, the air temperature in the region of the absorbing molecule increases. However, this air mass does not heat up without limit as its molecules trap more and more of the outgoing infrared light because there is an opposing phenomenon that prevents such a catastrophe. As explained above, the rate of energy emission increases with temperature, so the molecules that have shared the excess energy themselves emit more and more energy as infrared light as they warm up (Figure 6-3). The water droplets and vapor in clouds are also very effective in absorbing infrared light emitted from beneath them. The temperature at the tops of clouds is quite cool relative to the air beneath them, so clouds do not radiate as much energy as they absorb. Overall, air temperatures increase only enough to re-establish the planetary equality between incoming and outgoing energy.

The phenomenon of interception of outgoing IR by atmospheric constituents and its dissipation as heat to increase the temperature of the atmosphere (as illustrated in Figure 6-3) is called the greenhouse effect. It is responsible for the average temperature at the Earth's surface and the air close to it being about + 15°C rather than about — 18°C, the temperature it would be if there were no IR-absorbing gases in the atmosphere. The surface is warmed as much by this indirect mechanism as it is by the solar energy it absorbs directly! The very fact that our planet is not entirely covered by a thick sheet of ice is due to the natural operation of the greenhouse effect, which has been in operation for billions of years.

The atmosphere operates in the same way as a blanket, retaining within the immediate region some of the heat released by a body and thereby increasing the local temperature. The phenomenon that worries environmental scientists is that increasing the concentration of the trace gases in air that absorbs thermal IR light (piling on more blankets, so to speak) would result in the conversion to heat of an even greater fraction of the outgoing thermal IR energy than occurs at present, which would thereby increase^ the average surface temperature well beyond 15°C. This phenomenon is sometimes referred to as the enhanced greenhouse effect (or artificial global warming) to distinguish its effects from the one that has been operating naturally for millennia.

The principal constituents of the atmosphere—N2, 02, and Ar—are incapable of absorbing IR light; the reasons for this will be discussed in the following section. The atmospheric gases that in the past have produced most of the greenhouse warming are water vapor (responsible for about two-thirds of the effect) and carbon dioxide (responsible for about one-quarter). Indeed, the absence of water vapor and of clouds in the dry air of desert areas leads to low nighttime temperatures there since so little of the outgoing IR is redirected back to the surface, even though the daytime temperatures are quite high on account of direct absorption of solar energy by the surface. More familiar to people living in temperate climates is the crisp chill in winter air on cloudless days and nights. Cloudy nights are usually warmer than clear ones because clouds reradiate IR that they have absorbed from surface emissions.

The greenhouse effect may be better understood by considering the following approximate model. Using physics, the temperature of an Earth that had no greenhouse gases in its air but was balanced with respect to incoming and outgoing energy would be — 18°C, or 255 K. Since, according to the equation above, the rate of energy emission from such a planet would be k (255)4, it follows that the rate of energy input from the Sun, whether or not the Earth's atmosphere contained greenhouse gases, would also be k(255)4. Overall, the real Earth acts as if about 60% of the energy it emits as infrared light is eventually transmitted into space, the remainder being the fraction that is not only absorbed by greenhouse gases but that is also reradiated downward and further heats the surface and atmosphere. Thus the rate at which the Earth loses energy to space as IR is not simply kT4, but rather 0.6 kT4. Since we know that rate of loss of energy from Earth = rate of energy input from Sun it follows for the real Earth that

Taking the fourth root of both sides, we obtain an expression for the temperature:

From this model, the Earth's calculated surface temperature is 290 K, i.e., +17°C, an increase of 35 degrees by the operation of the natural greenhouse effect.

In reality, however, very little of the IR emitted at or near the Earth's surface escapes into space. Rather it is absorbed by the air close to the ground, then re-emitted. A simple model of the atmosphere that incorporates this effect is discussed in Box 6-1. The IR from the air close to the ground that is emitted upward is mainly absorbed by the next layer of air, which is heated by it, though to a lesser extent than is the layer underneath, and is partially re-emitted. With increasing altitude, the fraction of the IR received from the air lying below a given level is less and less likely to be absorbed, since the atmosphere becomes thinner and thinner. Thus more and more of the IR is likely to pass upward into space. Indeed, very little of the IR emitted into the upper troposphere is absorbed since the air is thin at such altitudes. Because less and less IR is absorbed with increasing altitude, less and less is degraded into heat; there is therefore a natural tendency for the air to cool the higher the altitude. In reality, other factors such as convection currents in air also play an important role in determining the decline in temperature with altitude. The temperature at the top of the troposphere, from which the emitted IR reaches outer space, is only — 18°C, so overall the real Earth does radiate energy into space at the same temperature as was computed for the planet if it had no greenhouse gases. Thus the Earth emits the same amount of energy— equal to the amount absorbed from sunlight—into space with or without the operation of the greenhouse effect.

Earth's Energy Balance

The current energy inputs and outputs from the Earth—in watts (i.e., joules per second) per square meter of its surface and averaged over day and night, over all latitudes and longitudes, and over all seasons—are summarized

BOX 6-1

A Simple Model of the Greenhouse Effect

The calculation in the main text of the

Earths surface temperature assumed a specific value for the fraction of IR emitted from the surface that was transmitted through the atmosphere to outer space. However, this fraction—and the temperature—can be calculated from the physics of the situation. Consider a model Earth containing an atmosphere that consists of a single, uniform layer of air that is completely nontransparent to outgoing IR emitted from the surface, i.e., an atmosphere that absorbs all the IR and that converts it temporarily to heat. The layer of air itself is assumed to act as a blackbody that emits IR equally upward into space and downward back to the surface (Figure 1).

If a balance is to be achieved on Earth between incoming and outgoing energy, the air mass in the model must emit twice as much energy (2X) per second as is absorbed (X) from

Space

2X

X X

Surface

FIGURE 1 Energy released by the Earth's surface and absorbed and released by the atmosphere according to the model.

sunlight by the surface, since only half the air's energy escapes upward and is released into space. Define rateb as the total energy release rate from the air mass and ratea as the absorption rate from sunlight. Since we know that the rate of energy emission rises with the fourth power of the temperature, it follows that for any two Kelvin temperatures Ta and Tb involving the same type of blackbody, the ratio of rates of energy emission is given by

In our case, the rate ratio must be 2/1, and we know that Ta is 255 K. Thus

Taking square roots of both sides twice, we obtain

This simple model predicts the Earth's surface (and air) temperature to be 303 K, or 30°C, compared to the actual value of 15°C. The model is unrealistic and leads to an over-estimation of the greenhouse effect, because it assumes that all the IR escaping from the surface is absorbed and that the atmosphere is completely uniform and acts exactly as a blackbody. Earth's actual surface temperature of 15°C is more consistent with a slightly more complicated model, in which about one-third of the IR emitted from the surface passes through the atmosphere unabsorbed, and about two-thirds is absorbed by the air and subsequently re-emitted. The most accurate model consists of a sequence of several layers of air, with temperatures decreasing with altitude, each layer acting as a blackbody.

FIGURE 6-4 Globally and seasonally averaged energy fluxes to and from the Earth, in watts per square meter of surface. [Source: Data from Chapter 1 of J. T. Houghton et al., Climate Change 1995—The Science of Climate Change (Intergovernmental Panel on Climate Change) (Cambridge: Cambridge University Press, 1996).]

in Figure 6-4. A total of 342 watts/m2 (W/m2) are present in sunlight outside the Earth's atmosphere. Of this, 235 W/m2 are absorbed by the atmosphere and the surface; this much energy must be re-emitted into space if the planet is to maintain a steady temperature. Because of the presence of greenhouse gases, however, emission of only 235 W/m2 from the surface would not be sufficient to ensure this balance. Because absorption of IR by greenhouse gases heats the surface and lower atmosphere, the amount of IR released by them is increased. Given the current concentration of greenhouse gases in air, the balance is achieved and 235 W/m2 escape from the top of the atmosphere into space if 390 W/m2 are emitted from the surface, i.e., when 155 W/m2 of IR do not escape into space.

Ironically, an , increase in C02 concentration is predicted to cause a cooling of the stratosphere. This phenomenon occurs for two reasons.

• First, more outgoing thermal IR is absorbed at low altitudes (the troposphere), so less is left over to be absorbed by and warm the gases in the stratosphere.

• Second, at stratospheric temperatures, C02 emits more thermal IR upward to space and downward to the troposphere than it absorbs as photons—most of the absorption at these altitudes is due to water vapor and ozone—so increasing its concentration cools the stratosphere.

The observed cooling of the stratosphere has been taken to be a signal that the greenhouse effect is indeed undergoing enhancement.

Sun

\ 342

1 235

\ Visible and IR

Emitted by Earth

k __ Ju —---

— — —T~

___

^ V07 A

\/\235

Reflection \ (atmosphere \ Absorbed and surface) \

IR

155

Absorbed ^67 \ by air \168

390 \

^^^^ Absorbed

Emitted

^^^ by surface

by surface

Earth

FIGURE 6-4 Globally and seasonally averaged energy fluxes to and from the Earth, in watts per square meter of surface. [Source: Data from Chapter 1 of J. T. Houghton et al., Climate Change 1995—The Science of Climate Change (Intergovernmental Panel on Climate Change) (Cambridge: Cambridge University Press, 1996).]

Molecular Vibrations: Energy Absorption by Greenhouse Gases

Light is most likely to be absorbed by a molecule when its frequency almost exactly matches the frequency of an internal motion within the molecule. For frequencies in the infrared region, the relevant internal motions are the vibrations of the molecule's atoms relative to each other.

The simplest vibrational motion in a molecule is the oscillatory motion of two bonded atoms X and Y relative to each other. In this motion, called a

(a) Bond-stretching vibration

(b) Angle-bending vibration

FIGURE 6-5 The two kinds of vibrations within molecules. Bond stretching (a) is illustrated for a diatomic molecule XY. The variable R represents the average value of the X-Y distance. In (b), the angle-bending vibration is shown for a triatomic molecule XYZ. The average XYZ angle is indicated by <f>.

bond-stretching vibration, the X-to-Y distance increases beyond its average value R, then returns to R, then contracts to a lesser value, and finally returns to R, as illustrated in Figure 6-5a. Such oscillatory motion occurs in all bonds of all molecules under all temperature conditions, even at absolute zero. A huge number (about 1015) of such vibrational cycles occur each second. The exact frequency of the oscillatory motion depends primarily on the type of bond—i.e., whether it is single or double or triple—and on the identity of the two atoms involved. For many bond types, e.g., the C — H bond in methane and the O — H bond in water, the stretching frequency does not fall within the thermal infrared region. The stretching frequency of carbon-fluorine bonds does, however, occur within the thermal infrared range; thus any molecules in the atmosphere with C — F bonds will absorb outgoing thermal IR light and enhance the greenhouse effect.

The other relevant type of vibration is an oscillation in the distance between two atoms X and Z bonded to a common atom Y but not bonded to each other. Such motion, called a bending vibration, alters the XYZ bond angle from its average value <fi. All molecules containing three or more atoms possess bending vibrations. The oscillatory cycle of bond angle increase, followed by a decrease, and then another increase, etc., is illustrated in Figure 6-5b. The frequencies of many types of bending vibrations in most organic molecules occur within the thermal infrared region.

If infrared light is to be absorbed by a vibrating molecule, there must be a difference in the relative positions of the molecule's center of positive charge (its nuclei) and its center of negative charge (its electron "cloud") at some point during the motion. More compactly stated, in order to absorb IR light, the molecule must have a dipole moment during some stage of the vibration. Technically, there must be a change in the magnitude of the dipole moment during the vibration, but this is more or less guaranteed to be the case if there is a nonzero dipole moment at any point in the vibration. The positive and negative centers of charge coincide in free atoms and (by definition) in homonuclear diatomic molecules like Oz and N2, and the molecules have dipole moments of zero at all times in their stretching vibration. Thus argon gas, Ar, diatomic nitrogen gas, N2, and diatomic oxygen, 02, do not absorb IR light.

For carbon dioxide, during the vibratory motion in which both C—O bonds lengthen and shorten simultaneously, i.e., synchronously, there is at no time any difference in position between the centers of positive and negative charges, since both lie precisely at the central nucleus. Consequently, during this vibration, called the symmetric stretch, the molecule cannot absorb IR light. However, in the antisymmetric stretch vibration in C02, the contraction of one C—O bond occurs when the other is lengthening, or vice versa, so that during the motion the centers of charge no longer necessarily coincide. Therefore, IR light at this vibration's frequency can be absorbed since, at some points in the vibration, the molecule does have a dipole moment.

Similarly, the bending vibration in a C02 molecule, in which the three atoms depart from a colinear geometry, is a vibration that can absorb IR light since the centers of positive and negative charge do not coincide when the molecule is nonlinear.

Molecules with three or more atoms generally have some vibrations that absorb IR, since even if their average shape is highly symmetric with a zero dipole moment, they undergo some vibrations that reduce this symmetry and produce a nonzero dipole moment. For example, CH4 molecules have an average structure that is exactly tetrahedral, and hence a zero average dipole moment, because the polarities of the C—H bonds exactly cancel each other in this geometry. The zero dipole is maintained during the vibration in which all four bonds simultaneously stretch or contract. However, during the vibrational motions in which some of the bonds stretch while others contract, and those in which some H—C—H bond angles become greater than tetrahedral while others become less, the molecule has a nonzero dipole moment. Molecules of CH4 undergoing such unsymmetrical vibrations can absorb infrared light.

PROBLEM 6-2

Deduce whether the following molecules will absorb infrared light due to internal vibrational motions:

symmetric stretch antisymmetric stretch

PROBLEM 6-3

None of the four diatomic molecules listed in Problem 6-2 actually absorb much, if any, of the Earth's outbound light in the thermal infrared region. What does this imply about the frequencies of the bond-stretching vibrational motion of those molecules that can, in principle, absorb IR light?

The Major Greenhouse Gases Carbon Dioxide: Absorption of Infrared Light

As stated previously, the absorption of light by a molecule occurs most efficiently when the frequencies of the light and of one of the molecule's vibrations match almost exactly. However, light of somewhat lower or higher frequency than that of the vibration is absorbed by a collection of molecules. This ability of molecules to absorb infrared light over a short range of frequencies rather than at just a single frequency occurs because it is not only the energy associated with vibration that changes when an infrared photon is absorbed; there is also a change in the energy associated with the rotation (tumbling) of the molecule about its internal axes. This rotational energy of a molecule can be either slightly increased or slightly decreased when IR light is absorbed to increase its vibrational energy. Consequently, photon absorption occurs at a slightly higher or lower frequency than that corresponding to the frequency of the vibration. Generally, the absorption tendency of a gas falls off rapidly for light frequency that lies farther and farther in either direction from the vibrational frequency.

The absorption spectrum for carbon dioxide in a portion of the infrared range is shown in Figure 6-6. For C02, the maximum absorption of light in the thermal infrared range occurs at a wavelength of 15.0 fim, which corresponds to a frequency of 2 X 1013 cycles per second (hertz). The absorption occurs at this particular frequency because it matches that of one of the vibrations in a C02 molecule, namely the OCO angle-bending vibration. Carbon dioxide also strongly absorbs IR light having a wavelength of 4.26 fjtm, which corresponds to the 7 X 1013 cycles per second (hertz) frequency of the antisymmetric OCO stretching vibration.

PROBLEM 6-4

Calculate the energy absorbed per mole and per molecule of carbon dioxide when it absorbs infrared light (a) at 15.0 /xm and (b) at 4.26 ju.m. Express the per mole energies as fractions of that required to dissociate CO? into CO and atomic oxygen, given that the enthalpies of formation of the three gaseous species are —393.5, —110.5, and +249.2 kj/mol, respectively. [Hint: Recall the relationship between wavelength and energy in Chapter 1. Avogadro's constant = 6.02 X 1023.]

FIGURE 6-6 The infrared absorption spectrum for carbon dioxide. The scale for wavelength is linear when expressed in wavenumbers, which have units of cm"'; waven umber = 10,000/ wavelength in nm. [Source: Redrawn from A.T. Schwartz et at, Chemistry in Context: Applying Chemistry to Society, American Chemical Society (Dubuque, IA: Wm. C. Brown, 1994).]

"O

—"H

rv —

f

/

r

Ranges of maximum absorption ^

Wavelength (fim) of transmitted light

9 10 12 1416

Wavelength (fim) of transmitted light

9 10 12 1416

The carbon dioxide molecules that are now present in air collectively absorb about half of the outgoing thermal infrared light with wavelengths in the 14-16-jU,m region, together with a sizable portion of that in the 12-14-and 16-18-/xm regions. It is because of CC^'s absorption that the solid curve in Figure 6-7, representing the amount of IR light that actually escapes from our atmosphere, falls so steeply around 15 jam; the vertical separation between the curves is proportional to the amount of IR of a given wavelength that is being absorbed rather than escaping. Further increases in the C02 concentration in the atmosphere will prevent more of the remaining IR from escaping, especially in the "shoulder" regions around 15 jam, and will further warm the air. (Although carbon dioxide also absorbs IR light at 4-3 /xm due to the antisymmetric stretching vibration, there is little energy emitted from the Earth at this wavelength—see Figure 6-1—so this potential absorption is not very important.)

Carbon Dioxide: Past Concentration and Emission Trends

Measurements of air trapped in ice-core samples from Antarctica indicate that the atmospheric concentration of carbon dioxide in preindustrial times (i.e., before about 1750) was about 280 ppm. The concentration had increased by one-third, to 382 ppm, by 2006. A plot of the increase in the annual atmospheric C02 concentration over time is shown in Figure 6-8.

Wavelength (jum)

FIGURE 6-7 Experimentally measured intensity (black curve) of thermal IR light leaving the Earth's surface and lower atmosphere {above the Sahara desert) compared with the theoretical intensity (green curve) that would be expected without absorption by atmospheric greenhouse gases. The regions in which the various gases have their greatest absorption are indicated.

[Source: E. S. Nesbit, Leaving Eden (Cambridge: Cambridge University Press, 1991).]

FIGURE 6-8 The historic variation in the atmospheric concentration of carbon dioxide. The insert shows the trend, with seasonal fluctuations, in recent times.

(Source: Main graph: Adapted from J. L. Sarmiento and N. Gruber, "Sinks for Anthropogenic Carbon," Physics Today 55 (August 2002): 30; Insert: NOAA.]

Year

Year

FIGURE 6-8 The historic variation in the atmospheric concentration of carbon dioxide. The insert shows the trend, with seasonal fluctuations, in recent times.

(Source: Main graph: Adapted from J. L. Sarmiento and N. Gruber, "Sinks for Anthropogenic Carbon," Physics Today 55 (August 2002): 30; Insert: NOAA.]

The insert to the figure shows details of the increase in recent times. In the period from 1975 to 2000, the concentration grew at an average annual rate of about 0.4%, or 1.6 ppm—almost double that of the 1960s, The rate of increase in the first half-decade of the twenty-first century rose to about 2.0 ppm annually.

The seasonal fluctuations in the C02 concentrations are due to the spurt in the growth of vegetation in the spring and summer, which removes C02 from air, and the vegetation decay cycle in fall and winter, which increases it. In particular, huge quantities of C02 are extracted from the air each spring and summer by the process of plant photosynthesis:

sunlight

The term polymeric CH20 used for the product in this equation is an umbrella word for plant fiber, typically the cellulose that gives wood its mass and bulk. The C02 "captured" by the photosynthetic process is no longer free to function as a greenhouse gas—or as any gas—while it is packed away in this polymeric form. The carbon that is trapped in this way is called fixed carbon. However, the biological decay of this plant material, the reverse of the reaction, which occurs mainly in the fall and winter, frees the withdrawn carbon dioxide. Notice that the global carbon dioxide fluctuations follow the seasons of the Northern Hemisphere, since there is so much more land mass—and hence much more vegetation—there compared to the Southern Hemisphere.

Much of the considerable increase in anthropogenic contributions to the increase in carbon dioxide concentration in air is due to the combustion of fossil fuels—chiefly coal, oil, and natural gas—that were formed eons ago when plant and animal matter was covered by geological deposits before it could be broken down by air oxidation.

On average, each person in the industrial countries is responsible for the release of about 5 metric tons (a metric ton is 1000 kg, i.e., 2200 lb, whereas a conventional ton is 2000 lb) of C02 from carbon -containing fuels each year! There is considerable variation in the per capita releases among different industrialized countries; this is discussed in Chapter 7. Some of the per capita carbon dioxide output is direct, e.g., that released as gases when vehicles are driven and homes are warmed by burning a fossil fuel. The remainder is indirect, arising when energy is used to produce and transport goods; heat and cool factories, classrooms, and offices; produce and refine oil—in fact, to accomplish virtually any constructive economic purpose in an industrialized society. This topic is also discussed in greater depth in Chapter 7.

A significant amount of carbon dioxide is added to the atmosphere when forests are cleared and the wood burned to provide land for agricultural use. This sort of activity occurred on a massive scale in temperate climate zones in past centuries {consider the immense deforestation that accompanied the settlement of the United States and southern Canada) but has now shifted largely to the tropics. The greatest single amount of current deforestation occurs in Brazil and involves both rain forest and moist deciduous forest, but the annual rate of deforestation on a percentage basis is actually greater in Southeast Asia and Central America than in South America. Overall, deforestation accounts for about one-quarter of the annual anthropogenic release of C02, the other three-quarters originating mainly in the combustion of fossil fuels. Notwithstanding forestry harvesting operations, the total amount of carbon contained in the forests of the Northern Hemisphere (including their soils) is increasing, and in recent decades the annual increment approximately equaled the decreases in stored carbon cited above in Asia and South and Central America.

PROBLEM 6-5

Carbon dioxide is also released into the atmosphere when calcium carbonate rock (limestone) is heated to produce the quicklime, i.e., calcium oxide, used in the manufacture of cement:

Calculate the mass, in metric tons, of C02 released per metric ton of limestone used in this process. What is the mass of carbon that the air gains for each gram of carbon dioxide that enters the atmosphere? Note that at least as much carbon dioxide is released from combustion of the fossil fuel needed to heat the limestone as is released from the limestone itself.

The growth of the total annual emissions, in terms of the mass of carbon, of carbon dioxide from fossil-fuel combustion and cement production since the start of the Industrial Revolution is shown by the uppermost (black) curve in Figure 6-9. The contributions to this total are shown by the other curves. Historically, the emission rate in the second half of the twentieth century grew rapidly, the rate of increase being about five times greater than that in the first half. The annual growth rate in emissions from 2000 to 2005 rose to 3%, compared to 1% in the 1990s, due mainly to a rebound in coal production (gray curve) and continuing growth in oil (green curve) and natural gas (dashed curve) usage.

Carbon Dioxide: Atmospheric Lifetime and Fate of Its Emissions

The lifetime of a carbon dioxide molecule emitted into the atmosphere is a complicated measurement since, in contrast to most gases, it is not decomposed chemically or photochemically. On average, within a few years of its release into the air, a C02 molecule will likely dissolve in surface seawater or be absorbed by a growing plant. However, many such carbon dioxide molecules are released back into the air a few years later on average, so this disposal is only a temporary

FIGURE 6-9 Annual global C02 emissions since the Industrial Revolution. The black line gives total emissions from fossil-fuel combustion and cement manufacture. The contributions from solids (mainly coal) are shown by the gray line, from liquids (mainly petroleum) by the green line, and from gases (mainly natural gas) by the dashed black line. [Source: U.S. Department of Energy Carbon Dioxide Information Analysis Center, cdiac.ornl.gov/trends/ emis/glo.htm]

8000

7000

6000

5000

4000

3000

2000

1000

1863 1877 1891 1905 1919 1933 1947 1961 1975 1989 2003 Year sink for the gas. The only permanent sink for it is deposition in the deep waters of the ocean and/or precipitation there as insoluble calcium carbonate. However, the top few hundred meters of seawater mix slowly with deeper waters; thus carbon dioxide that is newly dissolved in surface water requires hundreds of years to penetrate to the ocean depths. Consequently, although the oceans will ultimately dissolve much of the increased C02 now in the air, the time scale associated with this permanent sink is very long, hundreds of years.

Because the processes involving the interchange of carbon dioxide among the air, the biomass, and shallow ocean waters and between shallow and deep seawater are complicated, it is not possible to cite a meaningful average lifetime for the gas in air alone. Rather we should think of new C02 fossil-fuel emissions as being rather quickly allocated among air, the shallow ocean waters, and biomass, with interchange among these three compartments occurring continuously. Then slowly, over a period of many decades and even centuries, almost all this new carbon dioxide will eventually enter its final sink, the deep ocean. In effect, the atmosphere rids itself of almost half of any new carbon dioxide within a decade or two but requires a much longer period of time to dispose of the rest. It is commonly quoted as taking 50 to 200 years for the carbon dioxide level to adjust to its new equilibrium concentration if a source of it increases. In summary, the effective lifetime of

Atmosphere (total stored -800 Gt)

Net loss of 0.9

Net loss of 2.2

r

\

/

\

7.2

1.5

2.4

22.2

20.0

Fossil-fuel

Forests

combustion

+

Cement production

Oceans (total stored -38,000 Gt)

Land

(total

fossil

fuel stored -6000 Gt)

1

Sediments

FIGURE 6-10 Annual fluxes of C02 to and from the atmosphere, in units of megatonnes of carbon. The total amounts stored in various locations are shown in bold. Note that the values for the air/ocean interchange include natural as well as anthropogenic carbon. |Data source: UNESCO SCOPE Policy Briefs 2006 H2, The Global Carbon Cycle.)

additional C02 in the atmosphere should be considered to be long, on the order of many decades or centuries, rather than the few years required for its initial dissolution in seawater or absorption by biomass.

The annual inputs and outputs of anthropogenic carbon dioxide to and from our atmosphere, as of the early 2000s, are summarized in Figure 6-10. (Releases and absorption by natural processes are overall in balance and are not included in the diagram.) Fossil-fuel combustion and cement production released 7.2 gigatonnes (Gt, i.e., billions of tonnes, equivalent to petagrams, 1015 g) of the carbon component (only) of C02 into the air, of which 4-4 Gt (about 60%) did not find a sink. The upper layers of the ocean absorbed about 22 Gt of carbon but released only about 20 Gt, giving a net absorption by this principal sink of almost 2 Gt. The carbon released by slash-and-burn tropical deforestation and other land-use changes amounted to about 1 Gt less than that absorbed by forest growth and soil storage. Because overall more than half the anthropogenic C02 emissions are quickly removed, over the short and medium term the gas continues to accumulate in the atmosphere.

The variations over the last century and a half in the various annual sources and sinks for C02 are summarized in Figure 6-11. Notice the year-to-year variations in the amount of the gas absorbed by the oceans and especially in that absorbed by land sinks (biomass). Although the fraction of the new emissions that remains in the atmosphere undergoes substantial variations from year to year, its average increment is increasing with time.

FIGURE 6-11 Annual fluxes of anthropogenic C02 to and from various sources and sinks from 1850 to 2005. Note that the unit of picograms (1 0t2 g) is equivalent to the megatonne unit used in Figure 6-10, since 1 megatonne = 106 tonnes, 1 tonne = 1000 kg, and 1 kg = 1000 g.) (Source: M. Raupach (Global Carbon Project), Carbon in the Earth System: Dynamics and Vulnerabilities (Beijing, November, 2006).|

The increase in growth rate of certain types of trees due to the increased concentration of carbon dioxide in the air is called C02 fertilization. Some scientists suspect that the rate of photosynthesis is speeding up as the level of CO2 and the air temperature increase and that the formation of greater amounts of fixed carbon represents an important sink for the gas. Indeed, an increase in the biomass of northern temperate forests is the most likely sink to account for the annual atmospheric C02 loss for which scientists had previously been unable to assign a cause. This increased activity in photosynthesis has been confirmed by satellite data for the region between 45°N and 70°N. Boreal (evergreen) forests of the Northern Hemisphere currently store almost 1 Gt of carbon into standing biomass alone. Much of the increase in the biomass of temperate forests at high latitudes occurs in the soil, especially as peat. The anthropogenic releases of C02 amount only to about 4% of the enormous amounts produced by nature, so a very small variation in the rate at which carbon is absorbed into biomass could have a large effect on the residual amount of C02 that accumulates in the atmosphere. Unfortunately, scientists still do not completely understand the global carbon cycle. As Figure 6-8 indicates, however, there is no doubt that the atmospheric C02 concentration is increasing.

Land use

Other emissions -

Fossil-fuel emissions

1850 1900 1950 2000

Land

1—1—i—1—1—i—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—|-

Land use

Other emissions -

Fossil-fuel emissions

1850 1900 1950 2000

Land

PROBLEM 6-6

(a) Given that the atmospheric burden of carbon (as C02) increases by about 4-7 Gt annually, calculate the increase in the ppm concentration of carbon dioxide that this brings about, (b) Given that its total concentration was 382 ppm in 2006, calculate the total mass of CO2 that was present in the air. After converting to mass of carbon, does your answer agree with the value listed in Figure 6-10? Note that the mass of the atmosphere = 5.1 X 1021 g and that air's average molar mass = 29.0 g/mol. [Hint: Express the amounts of CO, and air as moles, and recall the definition of ppm units in moles.]

Green Chemistry: Supercritical Carbon Dioxide in the Production of Computer Chips

In this example of green chemistry, we see how waste C02—which would normally be vented to the atmosphere—can be put to good use as a solvent. We will also see how using C02 as a solvent pays additional environmental dividends in terms of both energy and resource conservation and reduction of wastes.

As technology relentlessly pervades our planet, the demand for integrated circuits (ICs) and computer chips increases dramatically each year. Computer chips are used in almost any electronic device one can imagine, including telephones, televisions, radios, automobiles, trucks, computers, airplanes, rockets, smart bombs, calculators, and cameras. It is estimated that the combination of the average personal computer, keyboard, monitor, and printer has a mass of about 25 kg and contains about 9 g of silicon and metal in the ICs that are the heart of each computer.

The manufacture of computers, other electronic devices, and chips involves high-tech, high-paying, highly skilled, and highly sought-after jobs. Facilities involved in these activities are considered by most people to be "clean" industries, especially when compared to the automobile and chemical industries. It is a little-known fact, except to those who work in the field or study the chip-manufacturing process, that chip making actually creates more waste than any other process involved in the manufacture of computers and is very energy intensive! By some measures, chip manufacturing is orders of magnitude more wasteful and polluting than the production of automobiles. It is estimated that the fabrication of the chips in your computer generated about 196 kg of waste (4500 times the mass of the average chip) and used about 10,600 L of water. The ratio of the mass of the materials (chemicals and fossil fuels) needed to produce a chip to the mass of the chip is estimated at 630:1, while the analogous ratio for the production of an automobile is approximately 2:1. Consequently, there are ongoing efforts to find less resource-intensive and less wasteful methods of chip production.

Photolithography

Clean wafer -> Deposit Si02

Si02 Si

Coat with photoresist

Photoresist Si02 Si

Soft bake Photoresist Si02 Si

Photolithography continued

Expose (,h\) photoresist

W\/W\/ V V V V

Photoresist

Photoresist

SiO Si

2

Develop resist-> Hard bake

Photo

Photo

resist

resist

SiO

2

Si

Photo

Photo

resist

resist

SiO

2

Si

Photoresist

SiO?

Photoresist

SiO?

Photo

resist

Si02

Si

Si02 Si02 Si

FIGURE 6-12 The fabrication of integrated circuits. [Source: L. Rothman, C. Jacobson, and C.Taylor, "Supercritical C02 Resist Remover-SCORR/' a proposal submitted to the Presidential Green Chemistry Challenge Awards Program, 2002.1

The process of producing a 2-g computer chip entails many steps and requires 72 g of chemicals, 32 L of water (mostly for rinsing), and 700 g of process gases. For production and use over a four-year lifetime, a 2-g chip requires 1.6 kg of fossil fuels. A typical chip-manufacturing facility uses millions of liters of highly purified water per month. A few of these steps are outlined in Figure 6-12. The process begins with the mechanical or chemical cleaning of the surface of highly purified silicon, followed by deposition of silicon dioxide; then a process known as photolithography defines the shape and pattern of individual components on an IC.

Photolithography begins with the deposition of a photoresist polymer, followed by baking and exposure of selected areas of the polymer to light. The light causes the polymer to cross-link, i.e., form bonds that link the polymer chains to one another at many positions along each chain (see Figure 6-13). The chip is then developed (a process that removes the photoresist polymer from the unexposed areas) and hard baked, the Si02 is etched, and the

FIGURE 6-13 The cross-linking of polymer chains, remaining photoresist polymer is removed, creating a pattern on the surface of the chip. The removal of the photoresist is accomplished with large amounts of aqueous solutions of strong acid (sulfuric or hydrochloric) or base, or by use of organic solvents (halogenated or polycyclic aromatics). The chip is then rinsed several times with copious amounts of highly purified water and dried with alcohol. The removal of the photoresist is very resource- and energy-intensive and creates large amounts of waste. The layering, developing, and etching process is repeated several times for each chip.

Scientists at Los Alamos National Laboratories in New Mexico and SC Liquids in Nashua, New Hampshire, were awarded a Presidential Green Chemistry Challenge Award in 2002 for their development of a new process for removing photoresist in chip manufacturing, known as SCORR (supercritical carbon dioxide resist remover). The process employs supercritical carbon dioxide (see Box 6-2) as the solvent for removal of the photoresist (the last step of Figure 6-12). The use of SCORR offers several environmental benefits over traditional methods, including the following:

• The rinse step is no longer necessary, thus eliminating the need for millions of liters of highly purified water, the energy required to produce this water, and the associated wastewater. This also reduces the amount of fossil fuel required to produce the highly purified water and the accompanying formation of carbon dioxide.

• The need for (and waste from) hazardous and toxic chemicals such as s txong ac ids or bases or organic solvents in the photoresist removal step is eliminated or reduced. This also enhances worker safety.

• The need for alcohols used for drying after the aqueous rinse step is eliminated.

• The carbon dioxide is recovered after each use and reused.

• The only waste left after evaporation (and recovery) of the carbon dioxide is the spent photoresist, which is unregulated.

BOX 6-2

Supercritical Carbon Dioxide

The supercritical fluid state of matter is produced when gases or liquids are subjected to very high pressures and, in some cases, to elevated temperatures. At pressures and temperatures at or beyond the critical point, separate gaseous and liquid phases of a substance no longer exist. Under these conditions, only the supercritical state, with properties that lie between those of a gas and those of a liquid, exists. For carbon dioxide, the critical pressure is 72.9 atm and the critical temperature is only

31.3°C, as illustrated in the phase diagram in Figure 1. Depending upon exactly how much pressure is applied, the physical properties of the supercritical fluid vary between those of a gas (relatively lower pressures) and those of a liquid (higher pressures); the variation of properties with P or T is particularly acute near the critical point. Thus the density of supercritical carbon dioxide varies over a considerable range, depending upon how much pressure (beyond 73 atm) is applied to it.

10,000 1000 100 10 1

- Solid

1

1 Supercritical

-

1 fluid

-

/ Liquid

Critical point

/

31°C at 72.8 atm

/

Gas

/ \ , 1

1 , 1

Temperature (°C)

Temperature (°C)

FIGURE 1 Phase diagram for carbon dioxide.

The carbon dioxide used in this process can be obtained as a waste byproduct of other processes (as indicated in Chapter 1 during the discussion of using carbon dioxide as a blowing agent). Processes such as producing ammonia and drilling for natural gas yield large amounts of this gas, which would normally be released to the atmosphere and add to its concentration of carbon dioxide. If we can capture this "unwanted" by-product and find constructive (and environmentally sound) uses for carbon dioxide, then we have not only prevented its release into the atmosphere; we may also have reduced our reliance on valuable resources and prevented the formation of other pollutants. SCORR, the use of carbon dioxide as a blowing agent (green chemistry section, Chapter 2), and the use of carbon dioxide as a solvent for various cleaning purposes (green chemistry section, Chapter 3) are all examples of this. In general, chemists are seeking ways to find beneficial uses for byproducts of other processes and reactions that would normally be considered waste and would add to the environmental burden of the planet. The use of carbon dioxide as a blowing agent and solvent for cleaning and photoresist removal offers three significant examples of this paradigm.

Employing the SCORR process yields several additional advantages. As the architecture of computer chips continually becomes smaller, water (because of its high surface tension) will no longer be able to penetrate these small spaces. Supercritical fluids have low viscosity, low surface tension, and high diffusivity. Because of these properties, they are ideal for cleaning rough, irregular surfaces with small openings. Supercritical carbon dioxide offers the answer to the cleaning problems associated with the smaller features of the new generations of chips. Other advantages of the SCORR process include the facts that (1) cleaning times are cut in half, (2) eliminating the rinse step allows for greater throughput (more chips in less time), and (3) carbon dioxide is less costly than conventional solvents.

Water Vapor: Its Infrared Absorption and Role in Feedback

Water molecules, always abundant in air, absorb thermal 1R light through their H — O — H bending vibration; the peak in the spectrum for this absorption occurs at about 6.3 jxm. As a consequence, almost all the relatively small amount of outgoing IR in the 5,5- to 7.5-/luti region is intercepted by water vapor (see Figure 6-7). (The symmetric and antisymmetric stretching vibrations for water occur near 2.7 fxm, outside the thermal IR region. The symmetric stretch in a symmetric but nonlinear molecule like H20 does absorb IR.) Absorption of light leading to increases in the rotational energy of water molecules, without any change in vibrational energy, removes thermal infrared light of 18-/u,m and longer wavelengths. In fact, water is the most important greenhouse gas in the Earth's atmosphere, in the sense that it produces more greenhouse warming than does any other gas, although on a per molecule basis it is a less efficient IR absorber than is C02.

Although human activities, such as the burning of fossil fuels, produce water as a product, the concentration of water vapor in air is determined primarily by temperature and by other aspects of the weather. Virtually all the H,0 in the troposphere arises from the evaporation of liquid and solid water on the Earth's surface and in clouds. The rate at which water evaporates and the maximum amount of water vapor that an air mass can hold both increase sharply with increasing temperature. Indeed, the equilibrium vapor pressure of liquid water increases exponentially with temperature. The rise in air temperature that is caused by increases in the concentration of the other greenhouse gases, and by other global warming factors, heats the surface water and ice, thereby causing more evaporation to occur. Indeed, the average atmospheric content of water vapor has increased since at least the 1980s.

The consequent increase in the water vapor concentration from global warming due to increased C02, etc. produces an additional amount of global warming due to H20(g) that is comparable in magnitude to the original amount due to the other greenhouse gases, because water vapor is a greenhouse gas! This behavior of water is an example of the general phenomenon called positive feedback: The operation of a phenomenon produces a result that itself further amplifies the result. Feedback is a reaction to change; with positive feedback, the reaction accelerates the pace of future change. On the other hand, a system whose output reduces the subsequent level of output displays negative feedback. An example of negative feedback from daily life is the attempt by a business to raise its profits by increasing its prices. However, the rise in price often results in a reduction in demand for the item of concern, and the rise in profits is less than anticipated. (No value judgment as to the desirability of the effect is implied by the terms positive and negative; only the increase or decrease in the pace of change is meant.)

Since it comes about as an indirect effect of increasing the levels of other gases, and since it is not within our control, the warming increment due to water is usually apportioned without further comment into the direct warm

Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment