Jacobson M.Z. Atmospheric Pollution

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dioxide and water vapor, occurred. Whereas much of the water vapor condensed to

form the oceans, most carbon dioxide remained in the air. Figure 2.12 shows that the

Earth’s atmosphere prior to the oxygen age (2.3 b.y.a.) may have contained 10–80 per-

cent carbon dioxide by volume. In terms of absolute quantities, partial pressures of

(g) during the Archean period may have been 0.01 to 0.1 atm, 30 to 300 times

their current values (Garrels and Perry, 1974; Pollack and Yung, 1980). The high par-

tial pressures of CO

(g) enhanced the greenhouse effect.

Over time, chemical weathering converted CO

(g) to carbonate-containing rocks.

Carbonate formation may have been enhanced by cyanobacteria and other autotrophic

bacteria, which use carbon dioxide as an energy source. Dead cyanobacteria pile up to

form laminated, bounded structures of trapped carbonaceous material called stromato-

lites. Stromatolites are usually found in shallow marine waters in warm regions, and

some are still being formed.

During the Proterozoic period (2.5 to 0.57 b.y.a.), two phases of continental

glaciation may have occurred (2.5 and 0.9–0.6 b.y.a., respectively) (Frakes, 1979). The

glaciations may have been due to lo

w CO

(g) partial pressure coupled with the rela-

tively low solar intensity. Between the two glaciations, the Earth was ice-free. During

the second glaciation, ice formed on continents down to low latitudes, as evidenced by

rock analysis (Williams, 1975). Because there is no corresponding evidence of sea ice

appearing at low latitudes, it is unlikely that the entire Earth was covered with ice

(Crowley and North, 1991), although many scientists have speculated that it was.

12.3.2.2. From 570 to 100 Million Years Ago

Between 570 and 100 m.y.a, which encompasses the Paleozoic era (570 to 225

m.y.a) and much of the Mesozoic era (226 to 65 m.y.a.), the Earth’s climate was rela-

tively mild, but with interruptions by tw

o important glaciations.

From 570 to 460 m.y.a., global CO

(g) mixing ratios were thirteen to fourteen

times their current value, sea levels were at an all-time high, and the Northern

Hemisphere was covered with water above 30N latitude (due to the conﬁguration of

the continents, which were concentrated in the Southern Hemisphere). The high

(g) mixing ratios and sea levels suggest that temperatures were high during this

period.

Near 440 m.y.a., the ﬁrst glaciation of the Paleozoic era occurred over parts of the

Earth, but CO

(g) partial pressures were still 10 times their current value. The reason

for this paradox is still uncertain.

During the Denovian period (395 to 345 m.y.a.), the land-plant coverage of the

Earth increased, decreasing continental albedo by up to 10 to 15 percent (Posey and

Clapp, 1964), warming air temperatures and causing glaciers to melt. Plant uptake of

(g) by photosynthesis offset some of the warming.

The second period of glaciation during the Paleozoic era began in the

Carboniferous period (345 to 280 m.y.a.). This glaciation lasted 100 million years

and decreased sea levels.

Evidence suggests warm temperatures and no glaciation during the Permian (280

to 225 m.y.a.), Triassic (225 to 190 m.y.a.), or Jurassic (190 to 136 m.y.a.) periods.

Around 220 m.y.a., during the Triassic period, the continents finally merged togeth-

er to form one supercontinent, Pangea. Since then, the continents have slowly

refractured and drifted apart, giving us our current continental distribution. The

Pangean climate was drier and warmer than is our present climate (Crowley and

North, 1991).

328 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

12.3.2.3. From 100 to 3 Million Years Ago

Between 100 and 50 m.y.a., during the Cretaceous (136 to 65 m.y.a) and early

Tertiary (65 to 2 m.y.a) periods, temperatures were again mild and the Earth was

without glaciers. In the mid-Cretaceous period (120 to 90 m.y.a.), temperatures

increased above those today. This may have been the last period of an ice-free globe.

Sea levels were high, covering about 20 percent of continental areas (Barron et al.,

1980).

At the end of the Cretaceous period (65 m.y.a.), a mass extinction of 75 percent of

all plant and animal species, including the dinosaurs, occurred. This extinction is

called the Cretaceous–Tertiary (K-T) extinction, and may have been due to an aster-

oid 10 km wide hitting the Earth (Alvarez et al., 1980). Evidence for the asteroid

theory includes a worldwide layer of the element, iridium (Ir) in clays at depths below

the Earth’s surface corresponding to the K-T transition. Although iridium has been

measured in a Hawaiian volcanic eruption plume (Zoller et al., 1983), Hawaiian

volcanic plumes are not known to penetrate to the stratosphere,

which would be neces-

sary for the iridium to be distributed globally (Crowley and North, 1991). Currently,

the most accepted explanation for the iridium layer at the K-T boundary is an extra-

terrestrial source, such as an asteroid. The asteroid impact is thought to have created

a large dust cloud that blocked the sun for a period of weeks to months, lowering the

surface temperature by tens of degrees (Toon et al.,

1982). The reduction in surface

temperature may have been responsible for the mass extinction. Because some

extinctions occurred before the K-T transition, the asteroid theory is still open to

debate.

Temperatures continued to cool from the Cretaceous period into the Paleocene

epoch (65 to 53 m.y.a.). Temperatures abruptly increased from the late P

aleocene to

the early Eocene epoch (53 to 37 m.y.a.). From 55 to 53 m.y.a., temperatures were

higher than during any other time in the Cenozoic era, although they were lower than

during the Cretaceous period in the late Mesozoic era. At the onset of the early Eocene

warming, the carbon content of deep-sea bulk sediments decreased, indicating an

increase in CO

(g) (Berner et al., 1983; Shackleton, 1985). During the onset of warm-

ing, the Norwegian–Greenland sea opened (Talwani and Eldholm, 1977). This and

other tectonic activity may have resulted in enhanced volcanism, increasing CO

(g)

levels.

Following the early Eocene temperature maximum, a gradual period (50 to

3 m.y.a.) of global cooling occurred. During this period, continents drifted toward

higher latitudes, cooling mean temperatures over land. A sharp cooling event occurred

near 40 to 38 m.y.a., in the late Eocene epoch, resulting in one of the largest extinc-

tions during the Cenozoic era.

Cenozoic era ice may have ﬁrst appeared over the Antarctic near 40 m.y.a.,

forming the base of the present-day Antarctic Ice Sheets. An ice sheet is a broad,

thick sheet of glacier ice that covers an extensive land area for a long time. An ice

sheet can also form on top of sea ice if the sea ice is adjacent to land. A glacier is a

large mass of land ice formed by the compaction and recrystallization of snow.

Glaciers ﬂow slowly downslope or outward in all directions under their own weight.

Sea ice is ice formed by the freezing of sea water (Fig. 12.14). Today, the Antarctic is

covered by two ice sheets, the East and West Sheets, which are separated by the

Transantarctic Mountains. The West Sheet, which lies over the Ross Sea and over land

(Fig. 12.15), is an order of magnitude smaller than is the East Sheet, which lies exclu-

sively over land.

THE GREENHOUSE EFFECT AND GLOBAL WARMING 329

During the Oligocene epoch (37 to 26 m.y.a.), temperatures continued their down-

ward trend that started during the Eocene epoch. Around 31 to 29 m.y.a., Antarctic

ice cover expanded. From 26 to 15 m.y.a., during the early Miocene epoch (26 to

5 m.y.a.) temperatures temporarily stabilized. From 15 to 10 m.y.a., temperatures

dropped, increasing Antarctic ice coverage and decreasing sea levels. The cause of this

temperature decrease may have been a decrease in CO

(g) due to changes in ocean

water vertical mixing (Crowley and North, 1991).

From 5 to 4 m.y.a., in the early Pliocene epoch (5 to 2 m.y.a.), temperatures

rebounded slightly, reducing the sizes of the Antarctic ice sheets. Although much of

the Northern Hemisphere may have been warm during the early Pliocene, seasonal sea

ice over the Arctic Ocean and glaciers over Alaska may have ﬁrst formed 5 to

6 m.y.a. and persisted thereafter (Clark et al., 1980; Zubakov and Borzenkova, 1988).

From 2.6 to 2.4 m.y.a., a sharp decrease in temperatures caused trees to disappear in

Northern Greenland and tundra to expand in Siberia. The Greenland ice sheet may

have formed during this cooling period, although evidence for its formation between 3

to 4 m.y.a. has also been suggested (Leg 105 Shipboard Scientiﬁc Party, 1986).

12.3.2.4. From 3 Million Years Ago to 20,000 Years Ago

The past 3 million years have been characterized by advances and retreats of

Northern Hemisphere ice sheets. During the late Pliocene epoch and early Pleistocene

epoch (2 m.y.a. to 10,000 years ago, y.a.), ﬂuctuations in the advances and retreats of

330 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

(a) (b)

Figure 12.15. (a) Edge of Ross Sea Ice Sheet, December 1996. (b) Icebergs grounded on

Pennel Bank, Ross Sea, Antarctica, January 1999. Photos by Michael Van Woert, NOAA

NESDIS, ORA. available from NOAA Central Library.

Figure 12.14. (a) Spring (1950) melt of sea ice in the Beaufort Sea near Tigvariak Island,

Alaska North Slope. (b) Winter (1950) sea ice near the same location. Photos by Rear

Admiral Har

ley D. Nygren, NOAA Corps (ret.),

available from NOAA Central Librar

(a) (b)

ice sheets occurred with a period of about 40,000 years (Shackleton and Opdyke,

1976). During the past 700,000 years, such advances and retreats have occurred with

periods of about 100,000 years. The reasons for the 100,000-year cycle can be eluci-

dated from Fig. 12.16.

Figure 12.16 shows the temperature trend over the past 450,000 years, a period

covering nearly one-quarter of the Pleistocene epoch. The ﬁgure also shows carbon

dioxide and methane trends over the past 420,000 years. Data for the table were

obtained from the Vostok, Antarctica, ice core. To date, drilling of the Vostok ice core

has extended down to a depth of more than 3.6 km. Data from the core enabled the

reconstruction of 450,000 years of atmospheric history.

Figure 12.16 shows that the Earth has gone through four glacial and interglacial

periods during the past 450,000 years. Within these major periods are minor periodic

oscillations. The major and minor temperature oscillation can be explained, for the

most part, in terms of three cycles related to the Earth’s orbit, called Milankovitch

cycles. Milankovitch cycles are named after Serbian astronomer Milutin

Milankovitch (Milankovitch, 1930, 1941). They are caused by gravitational attraction

between the planets of the solar system and the Earth. Milankovitch was the ﬁrst to

calculate the effects of these cycles on incident solar radiation reaching the Earth. The

cycles are discussed brieﬂy next.

Changes in the Eccentricity of the Earth’s Orbit

Figure 12.17 shows that the Earth travels around the sun in an elliptical pattern,

with the sun at one focus. In the ﬁgure, a and b are the lengths of the major and minor

semiaxes, respectively. The distance between the center of the ellipse (point C) and

THE GREENHOUSE EFFECT AND GLOBAL WARMING 331

-10

-5

200

400

600

800

075150225300375450

Temperature deviation (°C)

(g) (ppbv) and CO

(g) (ppmv)

Thousands of years before present

(g)

Temperature

85 kyr88 kyr

108 kyr

122 kyr

Figure 12.16. Temperature, methane, and carbon dioxide variations from the Vostok ice core

ver the past 420,000 to 450,000 year

s. Sources: Jouzel et al. (1987,

1993, 1996); Petit

et al. (1999). Temperature variations are relative to a modern surface–air temperature over

the ice of 55C.

either focus is c, which is related to a and b by the Pythagorean relation, a

 b

c

The eccentricity (e) of the ellipse is

(12.7)

The eccentricity varies between 0 and 1. A circle has an eccentricity of 0. Earth’s

eccentricity is currently low, 0.017 (Example 12.2),

indicating that the earth

’s orbit is

nearly circular but noncircular enough to create a 3.3 percent difference in distance

and a 6.6 percent difference in incoming solar radiation between June and December.

e 

332 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

EXAMPLE 12.2

Calculate the current eccentricity of the Earth.

Solution

Figure 12.1 shows that the Earth-sun distance during the winter solstice (when the Earth is closest to

the sun) is currently a  c  147 million km and that during the summer solstice (when the Earth is

furthest from the sun) is a  c  152 million km. Solving these two equations gives c  2.5 million

km and a  149.5 million km. Substituting these numbers into Equation 12.7 gives the current eccen-

tricity of the Earth’s orbit around the sun as e  0.017.

Sun

Earth

High eccentricity orbit

Sun

Earth

Low eccentricity orbit

Figure 12.17. Earth’s orbit around the sun during periods of high and low orbital eccentricity.

The eccentricities are exaggerated in comparison with real eccentricities of the Earth’s orbit.

The eccentricity of the Earth varies sinusoidally with a period of roughly 100,000

years. The minimum and maximum eccentricities during each 100,000-year period

vary as well. The minimum eccentricity is usually greater than 0.01 and the maximum

is usually less than 0.05. Whereas today the Earth is in an orbit of relatively low eccen-

tricity, in 50,000 years it will be in an orbit of high eccentricity. During orbits of high

eccentricity, the Earth–sun distance can be about 10 percent greater in June than in

December, and the incident solar radiation can be about 21 percent less in June than in

December. Today, the Earth–sun distance is about 3 percent greater in June than

in December, and incident solar radiation is about 7 percent less in June than

in December.

Because the yearly averaged distance of the Earth from the sun is less in a period

of low eccentricity than in a period of high eccentricity, yearly averaged temperatures

are higher in periods of low eccentricity than in periods of high eccentricity. This can be

seen in Fig. 12.16, which shows that natural interglacial temperature maxima occurred

122,000, 230,000, 315,000, and 403,000 years ago, all times of low eccentricity.

Changes in the Obliquity of the Earth’s Axis

The obliquity of the Earth’s axis of rotation is the angle of the axis relative to a line

perpendicular to the plane of the Earth’s orbit around the sun. Figure 12.18(a) shows

the variation in the Earth’s obliquity. Every 41,000 years, the Earth goes through a com-

plete cycle where the obliquity changes from 22 to 24.5 and back to 22. Currently,

the obliquity is 23.5, in the middle of a cycle. When the obliquity is low (22), more

sunlight hits the equator, increasing south–north temperature contrasts. When the obliq-

uity is high (24.5), more sunlight reaches higher latitudes, reducing temperature

contrasts. Thus, the obliquity affects the seasons and temperatures at each latitude on

Earth. Superimposed on the large temperature variations seen in Fig. 12.16 are smaller

variations, some resulting from changes in obliquity.

Precession of the Earth’s Axis of Rotation

Precession is the angular motion (wobble) of the axis of rotation of the Earth

about an axis ﬁxed in space [Fig. 12.18(b)]. It is caused by the gravitational attraction

between the Earth and other bodies in the solar system. Currently, the Northern

Hemisphere is further from the sun in summer than in winter. In 11,000 years, angular

motion of the Earth’s axis of rotation will cause the Northern Hemisphere to be closer

to the sun in summer than in winter. The complete cycle of the precession of the

Earth’s axis is 22,000 years. Precession of the Earth’s axis does not change the yearly

or globally averaged incident solar radiation at the top of the Earth’s atmosphere.

Instead, it changes the quantity of incident radiation at each latitude during a season.

In 11,000 years, for example, Northern Hemisphere summers will be warmer and

Southern Hemisphere summers will be cooler than they are today. Because seasonal

changes in temperature result in yearly averaged changes in temperature at a given lat-

itude, some of the cyclical changes in temperatures seen in the Antarctic data shown in

Fig. 12.16 are due to changes in precession.

Effects of the Milankovitch Cycles

The Milankovitch cycles appear to be responsible for the cyclical changes in the

Earth’s temperature, seen in Figure 12.16, and the corresponding advances and retreats

of glaciers during the Pleistocene epoch. As such, the Milankovitch cycles must also

be responsible for changes in CO

(g), and CH

(g), which are correlated with changes

in temperature. Increases in temperature decrease the solubility of CO

(g) in seawater,

increasing the atmospheric loading of CO

(g). Changes in temperature also change

THE GREENHOUSE EFFECT AND GLOBAL WARMING 333

Figure 12.18. Changes in the (a) obliquity of the Earth’s axis of rotation and (b) precession of the Earth’s

axis relative to a point in space.

Current N.H. summer

(June)

N.H. winter in 11,000 yr

(December)

N.H. summer in 11,000 yr

(June)

Current N.H. winter

(December)

Sun

22°

24.5

Line perpendicular to

plane of Earth’s orbit

around the sun

Axis of

rotation

Earth

(a) (b)

vertical mixing rates of ocean water, nutrient uptake rates by phytoplankton, and rates

of erosion of continental shelves (which affect biomass loadings), thereby affecting

mixing ratios of CO

(g) (e.g., Crowley and North, 1999). Changes in microbiological

activity resulting from changes in temperature may explain the correlation between

temperatures and CH

(g).

Figure 12.16 shows that a temperature minimum occurred about 150,000 y.a. Near

that time, glaciers extended down to Wisconsin in the United States, and possibly fur-

ther south in Europe (Kukla, 1977). Temperatures increased about 130,000 y.a., causing

deglaciation. Over the Antarctic, temperatures rose 2 to 3C above what they are today

(Fig. 12.16). As the eccentricity of the Earth’s orbit increased, temperatures cooled

again, causing a renewed period of glaciation. During this period (the last glacial peri-

od), two major stages of glaciation occurred, the ﬁrst starting 115,000 y.a. and the

second starting 75,000 y.a. The second stage continued until about 6,000 years ago.

12.3.2.5. From 20,000 to 9,000 Years Ago

The last glacial maximum (last ice age) occurred 22,000 to 14,000 y

.a. (Fig. 12.16).

Depending on whether glaciation over eastern North America, western Europe, or the

Alps is considered, this maximum is called the Wisconsin, Weichselian, or Würm.

During the maximum, an ice sheet called the Laurentide Ice Sheet covered North

America, and another called the Fennoscandian Ice Sheet covered much of Northern

Europe. These ice sheets were about 3,500 to 4,000 m thick and dre

w up enough ocean

water to decrease the sea level by about 120 m (CLIMAP, 1981; Fairbanks, 1989). The

decrease in sea level was sufﬁcient to expose land connecting Siberia to Alaska, creat-

ing the Bering land bridge. This land bridge allowed humans to migrate from Asia to

North America and, ultimately, to Central and South America. The Laurentide sheet

extended from the Rocky Mountains in the west to the

Atlantic Ocean in the east, but

only as far south as the Missouri and Ohio Valleys.

Temperatures during the last ice age were about 4C less than they are today

over the Northern Hemisphere and 8C less than they are today over the Antarctic

(Fig. 12.19). During the last ice age, Antarctic ice coverage expanded as did Arctic sea

ice coverage. In the tropics, precipitation decreased, resulting in lower inland lake and

river levels. Globally, near-surface winds may have been 20 to 50 percent higher than

those today. CO

(g) mixing ratios were about 200 ppmv, as seen in Fig. 12.16, almost

half their current value. CH

(g) mixing ratios were about 0.35 ppmv, 20 percent of

their current value.

Figure 12.19 shows temperature change estimates in the Northern Hemisphere and

from the Vostok ice core in the Antarctic during the last 20,000 years. The ice core

data indicate that temperatures over the Antarctic increased between 17,000 and

11,000 y.a., with a hiatus between 13,500 and 12,000 y.a. The increases in tempera-

tures were caused by Milankovitch cycle variations and were responsible for the

melting of ice over the Antarctic starting 16,000 to 17,000 y.a. (Labeyrie et al., 1986;

Jones and Keigwin, 1988).

In the Northern Hemisphere, temperature increases and deglaciation started around

the same time as they did over the Antarctic, near 17,000 y.a. At ﬁrst, Northern

Hemisphere deglaciation was slow. From 13,000 to 12,000 y.a., an abrupt increase in

temperatures hastened deglaciation. Around 12,000 y.a., temperatures dropped slightly,

then plunged 10,900 y.a. This strong cooling, which lasted until 10,100 y.a., is called

the Younger Dryas period. Dryas is the name of an Arctic ﬂower. The Younger Dryas

cooling period followed a shorter Older Dryas cooling period, which occurred

334 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

12,100–11,950 y.a. The Younger Dryas cooling may have been due to changes in

atmospheric circulation resulting from large releases of meltw

ater from the glaciers.

During glacial retreat, bursts of meltwater ﬂooded the Columbia Plateau in eastern

Washington, the largest ﬂood in known history. Up to forty similar bursts may have

occurred during this period (Waitt, 1985). Some of the bursts may have poured water

into the North Atlantic Ocean.

Following the Younger Dryas, temperatures increased again. Most of the Northern

Hemisphere ice sheets disappeared by 9,000 y.a., although remnants of the Laurentide

sheet remained until 6,000 y.a. In sum, the bulk of the two major ice sheets

over the Northern Hemisphere disappeared in a period of 5,000 years, from 14,000 to

9,000 y.a.

12.3.2.6. The Holocene Epoch

The period from 10,000 y.a. to the present is the Holocene epoch. Temperatures

during the ﬁrst 5,500 years of the Holocene were warmer than were those during the

second 4,500 years. Humans developed agriculture about 8,000 y.a. Temperatures in

the Northern Hemisphere 6,000 to 5,000 y.a. were about 1C warmer than they are

today, as seen in Fig. 12.19. This warm period is referred to as the mid-Holocene

maximum. Because plants ﬂourished, the period is also known as the climatic opti-

mum. Corresponding temperature changes over the Antarctic are less obvious.

After the mid-Holocene, temperatures cooled. Between 1100 and 1300

A.D., tem-

peratures in the Northern Hemisphere, particularly in Europe, warmed again,

becoming warmer than at any time since the mid-Holocene. During this warm period,

called the medieval climatic optimum, Arctic ice retreated, Iceland and Greenland

were colonized by the Vikings, Alpine passes between Germany and Italy became free

of ice, and grapes were harvested for wine production in England (Le Roy Ladurie,

1971; Crowley and North, 1991).

THE GREENHOUSE EFFECT AND GLOBAL WARMING 335

Figure 12.19. Temperature variation in the Northern Hemisphere (top line) and in the Antarctic

(bottom line) during the last 20,000 years. Antarctic data were from the Vostok ice core

(Jouzel et al., 1987, 1993, 1996; Petit et al., 1999). The deviations for the ice core data

are relative to a modern surface-air temperature over the ice of 55C. The Northern

Hemisphere ﬁgure was modiﬁed from Ahrens (1994). The deviations for these data are rela-

tive to current surface-air temperatures.

-10

-5

-6

-4

-2

05101520

Temperature deviation (°C)

Thousands of years before present

Younger Dryas

Holocene maximum

Medieval climatic optimum

Little ice age

Antarctic

Northern

Hemisphere

Between 1450 and 1890 A.D., temperatures in North

America, particularly in

Europe, cooled, and mild glaciation reappeared. This period is called the Little Ice

Age (Matthes, 1939). The coldest temperatures during this period were during the

mid-to late 1600s, early 1800s, and late 1800s (Crowley and North, 1991). During the

Little Ice Age, glaciers advanced in the European Alps, Sierra Nevada Mountains,

Rocky Mountains, Himalayas, southern Andes Mountains, and mountains in eastern

Africa, New Guinea, and New Zealand. The area of sea ice around Iceland increased

(Bergthorsson, 1969). Although temperatures decreased over much of the world,

temperatures over the Antarctic warmed during part of the Little Ice Age, as seen in

Fig. 12.20. Temperature decreases during the Little Ice Age were smaller in magnitude

than those during the Younger Dryas period.

From April 5–12, 1815, during the Little Ice Age, the most deadly volcano in

human history, Tambora, Indonesia, erupted, killing an estimated 92,000 islanders

directly and through disease and famine. The volcano emitted an estimated 1.7 million

tons of ash and aerosol particles that traveled globally, cooling temperatures in North

America and Europe during the following 2 years (Stommel and Stommel, 1981;

Stothers, 1984). In 1816, frost in the spring and summer killed crops in the United

States, Canada, and Europe, sparking famine in some areas of Europe. In the northeast

United States and Canada, 1816 was known as the year without a summer. Colorful

sunsets caused by the volcanic particles were captured in paintings by J. M. W. Turner.

In the dark summer of 1816, George Gordon (Lord) Byron (1788–1824) met and

became friends with Percy Bysshe Shelley (1792–1822) in Geneva, Switzerland.

Because of the depressing nature of the weather, Lord Byron and Shelley’s wife, Mary

Wollstonecraft Shelley (1797–1851) entered into a competition to write the most

depressing work. Lord Byron wrote the poem Darkness, and Mary Shelley started the

novel, Frankenstein: The Modern Prometheus, which she ﬁnished in 1818.

336 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

-2

-0.5

0.5

200018001600140012001000

Temperature deviation (°C)

Year a.d.

Year without a summer (1816)

Medieval climatic optimum Little Ice Age

Northern

Hemisphere

Antarctic

Figure 12.20. Temperature variation in the Northern Hemisphere summer (top line) and in

the Antarctic (bottom line) during the last 1,000 years. Antarctic data were from the Vostok

ice core (Jouzel et al., 1987, 1993, 1996; Petit et al., 1999). The deviations for the ice core

data are relative to a modern surface-air temperature over the ice of 55C. The slope of

the linear ﬁt through the curve (solid ﬂat line) is 0.000519C per year. The Northern

Hemisphere ﬁgure was modiﬁed from Bradley and Jones (1993). The deviations for these

data are relative to 1961–1990 sur face-air temperatures.

12.3.2.7. Comparison of Temperatures Today with Historical Temperatures

Temperatures today are not unprecedented in the Earth’s history, but the rate of

temperature increase is unusual in comparison with historical rates of temperature

increase. Table 12.4 shows that the rate of temperature increase from 1856 to 1999 was

about 0.43C per 100 years, and that from 1958 to 1999 was about 1.1 to 1.4C per

100 years. For comparison, the rate of temperature increase during the last millennium

was about 0.052C per 100 years, that during the deglaciation after the Younger Dryas

period was 0.3C per 100 years, and that during the deglaciation leading to the last

interglacial period was 0.13C per 100 years. In all cases, rates of temperature increase

were less than are those today. Yet, a careful examination of the Vostok core data indi-

cates that during deglaciations, temperature increases over short time periods (as

opposed to averaged over the entire deglaciation period) could be quite rapid. For

example, during the last years of de

glaciation after the Younger Dryas, the rate of tem-

perature increase was 2.2C per 100 years. The difference today, is that the Earth is in

an interglacial, not a deglaciation period. The current rapid rate of increase in tem-

perature is unusually high for an interglacial period.

12.4. FEEDBACKS AND OTHER FACTORS THAT MAY AFFECT

GLOBAL TEMPERATURES

Whereas recent near-surface air temperatures appear to have increased at unprecedent-

ed rates in comparison with the known climate record and levels of CO

(g), BC(s),

(g), and N

O(g) have increased since the mid-1800s, questions are still asked as to

whether any factor aside from pollutants could cause global warming. In addition, the

full effect of continued emission of greenhouse gases and particles on climate is still

uncertain. These topics are discussed next.

12.4.1. Alternative Arguments Used to Explain Global Warming

A common alternative explanation for global wa

rming is that increased temperatures

are due to natural Milankovitch cycle variations. One argument is that because the

current eccentricity of the Earth’s orbit (0.017) is in a declining stage, temperatures

should naturally increase over the next several hundred years, and such increases

might explain global warming. Because the eccentricity has also been declining for the

last 1,000 years, and the rates of temperature increase today are much higher than

THE GREENHOUSE EFFECT AND GLOBAL WARMING 337

Table 12.4. Rates of Temperature Change during Different Historical Periods

From 1958 to 1999 (radiosonde records, Fig. 12.12) 42–2 1.4

From 1958 to 1999 (land and ship measurements, Fig. 12.11) 42–2 1.1

From 1856 to 1999 (land and ship measurements, Fig. 12.11) 144–2 0.43

During the last 1,000 years (Vostok core, Fig. 12.20) 1,000–0 0.052

Deglaciation after Younger Dryas (Vostok core, Fig. 12.19) 12,632–11, 191 0.3

Last years of deglaciation after Younger Dryas (Vostok core, Fig. 12.19) 11,237–11,191 2.2

Deglaciation leading to last interglacial period (Vostok core, Fig. 12.16) 138,000–128,000 0.13

Temperature

Change (K)

Period Years ago per 100 Years