Jacobson M.Z. Atmospheric Pollution

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natural greenhouse warming. Carbon dioxide is the second most important and abundant

greenhouse gas, accounting for about 7.5 percent of the natural greenhouse effect. Black

carbon, whose major natural source is natural forest

ﬁres, is estimated to be responsible for only 0.2 per-

cent of the Earth’s natural warming above its

equilibrium temperature.

Figure 12.4 shows the percentage absorption by

different greenhouse gases at IR wavelengths. Water

vapor is a strong absorber at several wavelengths

between 0.7 and 8 m and above 12 m. Carbon

dioxide absorbs strongly at 2.7 and 4.3 m and

above 13 m. The ﬁgure indicates that little thermal-

IR absorption occurs between 8 and 12 m. This

wavelength region is called the atmospheric win-

dow. In the atmospheric window, gases are relatively

transparent to the Earth’s outgoing thermal-IR radia-

tion, allowing the radiation to escape to space. Of

the natural gases in the Earth’s atmosphere, ozone

and methyl chloride absorb radiation within the

atmospheric window. Nitrous oxide and methane

absorb at the edges of the window. An increase

in the mixing ratios of any gas that absorbs in the

window region will increase global near-surface

temperatures.

12.2.2. Historical Aspects of Global Warming

The ﬁrst scientist to consider the Earth as a green-

house was Jean Baptiste Fourier (1768–1830), a

French mathematician and physicist known for his

studies of heat conductivity and diffusion. In 1827,

Fourier suggested that the atmosphere behaved like

the glass in a hothouse, letting through “light” rays

of the sun but retaining “dark rays” from the ground.

The ﬁrst to recognize that speciﬁc gases in the

atmosphere selectively absorb thermal-IR radiation

was John Tyndall (1820–1893; Fig. 12.5), the

English experimental physicist who was also known

for studying the interactions of light with small par-

ticles (Section 7.1.5). Near 1865, Tyndall discovered

that water vapor absorbs more thermal-IR radiation

than does dry air, and postulated that water vapor

moderates the Earth’s climate.

The ﬁrst to propose the theory of global warming

was Swedish physical chemist Svante August

Arrhenius (1859–1927; Fig. 12.6). In 1896, he sug-

gested that a doubling of CO

(g) mixing ratios due

to coal combustion since the Industrial Revolution in the 18th-century United

Kingdom would lead to temperature increases of 5C (Arrhenius, 1896). This estimate

318 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 12.5. John Tyndall (1820–1893).

Figure 12.6. Svante August Arrhenius (1859–

1927).

is higher than recent estimates that a doubling of CO

(g) will result in a 2C tempera-

ture rise, but the theory is still consistent. Arrhenius also theorized that reductions in

(g) caused ice ages to occur. This theory was incorrect because changes in the

Earth’s orbit are responsible for the ice ages. The decrease in CO

(g) mixing ratios

during an ice age is an effect rather than a cause of temperature changes.

12.2.3. The Leading Causes of Global Warming

Since the time of Arrhenius, carbon dioxide has been considered the leading cause and

methane the second leading cause of global warming. Indeed, climate feedback studies

indicate that carbon dioxide does cause the most near-surface global warming. The

second leading cause of near-surface global warming may be particulate black carbon,

not methane (Jacobson, 2000, 2001b). Black carbon is emitted during coal, diesel fuel,

jet fuel, natural gas, kerosene, and biomass burning. Table 12.3 indicates that BC may

be responsible for 16 percent of near-surface global warming today. Carbon dioxide is

responsible for about 47 percent of such warming.

Although CO

(g) is the most abundant and important anthropogenically emitted

gas in terms of its effects on global warming, other gases are more efﬁcient, molecule

for molecule, at absorbing thermal-IR radiation, and the enhanced emissions of such

gases is a cause for concern. A CH

(g) molecule is approximately twenty-ﬁve times

more efﬁcient at absorbing radiation than is a CO

(g) molecule. N

O(g) and CFCl

(g)

molecules are 270 and 12,500 times more efﬁcient at absorbing, respectively, than is a

(g) molecule. Thus, controlling the emission of all greenhouse gases is important

if global warming is to be remedied (Hayhoe et al., 1999; Hansen et al., 2000).

12.2.4. Trends in Greenhouse Gases

Since the mid-1800s, the tropospheric mixing ratios of CO

(g), CH

(g), and N

O(g)

have increased by 30 percent, 143 percent, and 14 percent, respectively. These gases

are relatively well mixed in the lower atmosphere.

Black carbon concentrations have not been tracked so far back, but emission data

give an estimate of the change in BC atmospheric loading since prior to the Industrial

Revolution. Today, about half of BC originates from fossil fuels and half originates

from biomass burning. Since all fossil-fuel BC and 80 percent of biomass-burning BC

today are anthropogenic, 90 percent of total BC is anthropogenic. In 1850 (and pre-

sumably prior to that), biomass burning, predominantly from forest ﬁres at the time,

was about half that today (Houghton et al., 1991). As such, before fossil-fuel combus-

tion, total BC emissions may have been about 25 percent of those today, and all such

emissions were natural.

Since the beginning of the nineteenth century fossil fuel emissions of BC have

risen. Rates of coal combustion, one major source of BC, increased globally from

10 to 1,000 to 5,000 million metric tons per year from 1800 to 1900 to 1990 (McNeill,

2000). Coal production in the United States alone increased between 1984 and 1999

by 25 percent, and reﬁning (for end use and resale) of No. 2 diesel fuel and jet fuel

increased by 69 and 57 percent, respectively (EIA, 2000). Because BC is a particle

component much heavier than a gas molecule, BC atmospheric lifetimes are shorter

and its spatial distributions more variable than are those of greenhouse gases.

Figure 12.7 shows changes in the mixing ratios of CO

(g), CH

(g), and N

O(g)

from 1750, 1840, and 1988, respectively, to the present. The historical CO

(g) and

THE GREENHOUSE EFFECT AND GLOBAL WARMING 319

320 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

250

300

350

400

1750 1800 1850 1900 1950 2000

(g) mixing ratio (ppmv)

Year

0.6

0.8

1.2

1.4

1.6

1.8

1840 1880 1920 1960 2000

(g) mixing ratio (ppmv)

Year

(a) (b)

300

305

310

315

320

1988 1990 1992 1994 1996 1998 2000

O(g) mixing ratio (ppbv)

Year

(c)

Figure 12.7. Temporal changes in (a) carbon dioxide, (b) methane, and (c) nitrous oxide mixing ratios. Data

for carbon dioxide for 1744–1953 from Siple Station, Antarctica, ice core (Friedli et al., 1986), for

1958–1999 from Mauna Loa Obser

vatory (Keeling and Whor

f, 2000), and for 2000 fr

om Mauna Loa Data

Center (2001). Data for methane for 1841–1978 from Law Dome ice core, Antarctica (Ethridge et al.,

1992) and for 1983–2000 from Mauna Loa Data Center (2001). Nitrous oxide data from Mauna Loa Data

Center (2001).

(g) data originate from ice-core measurements. The more recent data in all cases

originate from ground-based ambient measurements. In 1958, Charles David Keeling

began tracking the mixing ratio of CO

(g) at Mauna Loa Observatory, Hawaii. His

record, shown in detail in Fig. 3.11 and in less detail in Fig. 12.7(a), is the longest con-

tinuous ground-based record of the gas.

The reason for the increases in tropospheric mixing ratios of CO

(g), CH

(g), and

O(g) over time is the increase in their anthropogenic emission rates. Figure 12.8

shows the anthropogenic emission rates of CO

(g) between 1750 and 1997 (Marland et

al., 2000). During this period, more than 270,000 Tg (teragrams; 1 Tg  10

metric

tons  10

g) of total carbon were released anthropogenically. Half of the emissions

have occurred since the 1970s. The 1997 emission rate was 6,593 Tg/yr, an increase of

1.2 percent over the 1996 emission rate and the largest yearly emission rate up to that

time. Liquid- and solid-fuel combustion accounted for 77.6 percent of anthropogenic

(g) emissions in 1997. Cement production and gas ﬂaring (burning of natural gas

emitted from coal mines and oil wells) contributed to less than 5 percent of the total

(g) emissions. Rates of such burning have decreased as a result of improved

technologies enabling the capture of natural gas during coal and oil mining.

Anthropogenic sources of CO

(g) not accounted for in Fig. 12.8 are biomass burn-

ing and deforestation. Biomass burning was deﬁned in Section 5.2.1.4. Deforestation

is the clearcutting of forests for their wood and the burning of forests to make room for

farming and cattle grazing. When forests are burned, CO

(g) is emitted. Whether

forests are burned or cut for their wood, their loss prevents photosynthesis from

converting CO

(g) to organic material. Biomass burning rates are high in tropical rain

forests of South America, Africa (Fig. 12.9), and Indonesia. Clearcutting of forests for

their wood in the Paciﬁc Northwest of the United States and in parts of Canada and in

other forested regions of the world is also common. Biomass burning and deforesta-

tion result in a global emission of 1,800 to 3,700 Tg-C/yr as carbon dioxide

(Houghton, 1994; Lobert et al., 1999; Andreae and Merlet, 2001). Thus, deforestation

accounts for one-ﬁfth to one-third of the total annual anthropogenic emissions

of CO

(g).

Deforestation is one mechanism of desertiﬁcation, the conversion of viable land

into desert. Desertiﬁcation also results from overgrazing and is a problem at the border

of the Sahara Desert, which is in continuous expansion. Desertiﬁcation increases the

Earth’s albedo and decreases the speciﬁc heat of the ground. Desert sand is more

reﬂective than is forest or vegetation cover, which is a factor that would tend to cool

the climate. On the other hand, sand has a lower speciﬁc heat than do trees that it

replaces, thus, the expansion of deserts can also warm the climate. The net effect of

desertiﬁcation on temperatures is uncertain.

Figure 12.10 shows the increase in anthropogenic emissions of CH

(g) from differ-

ent sources between 1860 and 1994. Total anthropogenic methane emissions increased

from 29.3 to 371 Tg/yr during this period. The largest component of anthropogenic

methane emissions in 1994 was livestock farming, which recently overtook rice farm-

ing as the leading methane emission source.

Anthropogenic emissions of N

O(g) also increased in the nineteenth and twentieth

centuries. Nitrous oxide is emitted mostly by bacteria in fertilizers and sewage and

during biomass burning and fossil-fuel combustion.

THE GREENHOUSE EFFECT AND GLOBAL WARMING 321

100

1,000

1750 1800 1850 1900 1950 2000

(g) emissions (Tg of carbon per yr)

Year

Total

Gas fuels

Liquid fuels

Solid fuels

Cement production

Gas

flaring

Figure 12.8. Global anthropogenic emissions of carbon dioxide from 1860 to 1997. Source of

data: Marland et al. (2000). 1 Tg  10

metric tons  10

In the 1930s and 1940s, synthetic long-lived chlorinated compounds, including

CFCl

(g) (CFC-11), CF

(g) (CFC-12), CF

ClH(g) (HCFC-22), and CCl

(g),

were developed for industrial uses. These compounds not only contribute to

stratospheric ozone destruction, but are also greenhouse gases. CFC-11, CFC-12,

and HCFC-22 absorb thermal-IR radiation within the atmospheric window.

CCl

(g) absorbs at the upper edge of the window. Figure 11.19 shows the changes

in mixing ratios of these gases in recent years. Whereas the mixing ratios of CFC-

11, CFC-12, and CCl

(g) are leveling off or decreasing, those of HCFC-22, other

HCFCs, and HFCs are increasing. HFCs and other fluorine-containing compounds,

such as sulfur hexafluoride [SF

(g)] and perfluoroethane [C

(g)], are strong

absorbers in the atmospheric window; thus, the buildup of these compounds is a

cause for concern.

322 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 12.9. Fires in Africa, October 1994–March 1995. Fires are denoted by red; ﬂares from

oil and gas exploration/extraction b

y green. The more intense the color, the greater the

frequency of ﬁres. The data were collected by the U.S. Air Force Weather Agency. Image and

data processing was by NOAA’s National Geophysical Data Center.

12.3. RECENT AND HISTORICAL TEMPERATURE TRENDS

To assess whether global warming is occurring, it is necessary to investigate the

temperature record of the Earth. In the following subsections, recent and historic

changes in global temperature are examined.

12.3.1. The Recent Temperature Record

Three types of datasets are currently used to assess global and regional air temperature

changes during the twentieth century. All three show evidence of global warming.

The ﬁrst type of dataset is one that uses temperature measurements taken between

2 and 10 m above the surface at land-based meteorological stations and ﬁxed-position

weather ships. Two datasets using these types of measurements include the Global

Historical Climate Network dataset (GHCN, Peterson and Vose, 1997) and the dataset

of Jones et al. (2000). Both datasets include measurements back to the 1850s. The

number of measurement stations included in each dataset changes yearly. The GHCN

dataset includes data from less than 500 stations prior to 1880, a high of 5,464 stations

in 1966, and about 2,000 stations today. The Jones et al. dataset includes data from

less than 250 stations prior to 1880, up to 1,800 stations in the 1950s, and less than

1,000 stations today.

Figure 12.11 shows changes in globally averaged near-surface air temperatures

between 1856 and 1999, relative to the mean temperature between 1961 and 1990,

from the Jones et al. dataset. The trend from the GHCN dataset is similar.

Temperatures in both datasets were relatively stable between 1860 and 1910 but

steadily increased from 1910 to 1940. Temperatures were stable or slightly decreased

between 1940 and the mid-1970s, but they increased rapidly from the mid-1970s

through 1998. The 1998 globally averaged temperature was the highest on record

through that date. The six warmest years during the 143-year record were in the 1990s.

Since 1856, the average global near-surface air temperature has warmed by about 0.5

THE GREENHOUSE EFFECT AND GLOBAL WARMING 323

100

1860 1880 1900 1920 1940 1960 1980 2000

(g) emissions (Tg of methane per yr)

Year

Total

Rice farming

Livestock

Landfills

Gas flaring

Gas supply

Coal mining

Biomass burning

Figure 12.10. Global anthropogenic emissions of methane from different sources from 1860

to 1994. Source of data: Stern and Kaufman (1998). 1 Tg  10

metric tons  10

to 0.8C. The linear ﬁt through the data in Fig. 12.11 indicates that the rate of increase

in the globally averaged near-surface temperature between 1856 and 1999 was

0.00425C per year; between 1958 and 1999, the rate of increase in temperature w

0.011C/yr. Additional statistics indicate that the three warmest winters in the contigu-

ous 48 states in the United States from 1895 to 2000 were in 1999–2000, 1998–1999,

and 1997–1998, respectively. The warmest nine-month period on record in the United

States was January–October, 2000.

A second type of dataset is one that uses temperature measurements at different

altitudes taken from balloons (radiosondes). One example is the 63-station, globally

distributed radiosonde dataset compiled by Angell, (1999, 2000). Temperatures for this

dataset are divided into near-surface (0 to 1.5 km), 850 to 300 mb (about 1.5 to 9 km),

300 to 100 mb (about 9 to 16 km), and 100 to 30 mb (about 16 to 24 km) tempera-

tures. Temperatures in each pressure division are considered an average for the

division. Figure 12.12 shows the vertical variation in the change in globally averaged

temperature between 1958 and 1999 from the dataset. The ﬁgure shows that tempera-

tures near the surface have increased and temperatures in the upper troposphere (300

to 100 mb) and stratosphere (100 to 30 mb) have decreased from 1958 to 1999. Both

trends (near-surface warming and stratospheric cooling) are consistent with the theory

of global warming.

The reason for the upper tropospherestratospheric cooling relative to the surface

warming in Fig. 12.12 is that absorption of thermal-IR radiation by greenhouse gases,

particularly carbon dioxide, in the lower and midtroposphere reduces transfer of that

radiation to the upper troposphere and stratosphere, where some of it would otherwise

be absorbed by greenhouse gases. In other words, the addition of greenhouse gases to

the air lowers the altitude of heating that occurs in the atmosphere, warming air near

the surface and cooling the stratosphere. Surface warming and stratospheric cooling,

observed in the radiosonde data, are simulated in models when carbon dioxide and

other greenhouse gases are added to the air. The near-surface globally distributed

324 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

-0.6

-0.4

-0.2

0.2

0.4

0.6

1850 1875 1900 1925 1950 1975 2000

Temperature deviation (°C)

Year

Figure 12.11. Globally averaged annual temperature variations between 1856 and 1999

relative to the 1961–1990 mean temperature. Source of data: Jones et al. (2000).

Measurements were from land-based meteorological stations and ﬁxed-position weather

ships. The slope of the linear ﬁt through the curve is 0.00429C per year. The slope of the

data from 1958 to 1999 is 0.011C per year.

radiosonde temperature trend of 0.014C/yr, seen in Fig. 12.12, is close to the surface-

station trend of 0.011C/yr from 1958 to 1999 (Fig. 12.11).

The third type of temperature dataset is the microwave sounding unit (MSU)

satellite dataset, compiled since 1979 (Spencer and Christy, 1990). Temperatures for

this dataset are derived from a comparison of the brightness or intensity of microwaves

(with a wavelength of 5,000 m) emitted by molecular oxygen in the air. Reported

temperatures from this dataset are averages over a horizontal footprint of 110 km in

diameter and a vertical thickness at 0 to 8 km (1,000 to 350 mb). Thus, the satellite

dataset represents temperatures high above the boundary layer (e.g., on the order of

4 km), not at the surface of the Earth. Some error in the MSU measurements arises

because a portion of the radiation measured by the MSU originates from the Earth’s

surface, not from the atmosphere (Hurrell and Trenberth, 1997). In addition, raindrops

and large ice crystals can cause the MSU instrument to slightly underpredict tempera-

tures in rain-forming clouds. The MSU dataset shows that, between 1979 and February

2001, global temperatures at 0 to 8 km increased by 0.0035C/yr. The MSU satellite

dataset also shows that stratospheric temperatures (14 to 22 km) cooled between 1979

and 1999 by 0.054C/yr, a trend consistent with the 0.045C/yr stratospheric cool-

ing trend between 1958 and 1999 found in the radiosonde data and a trend consistent

with the theory of global warming.

In sum, three types of global temperature measurements all show global warming.

The two types that measure near-surface temperatures (surface and radiosonde) both

show strong global warming. The two types that measure stratospheric temperatures

(radiosonde and satellite) both show cooling there, which is consistent with the theory

of global warming. The two that measure temperatures between the surface and strato-

sphere (radiosonde and satellite) both show global warming, but lower than at the

surface. Thus, global near-surface warming and stratospheric cooling is evident in

all temperature records. The underlying reason for the surface warming and simultaneous

THE GREENHOUSE EFFECT AND GLOBAL WARMING 325

-2

-1.5

-1

-0.5

0.5

1960 1970 1980 1990 2000

Temperature deviation (°C)

Year

100–30 mb

300–100 mb

850–300 mb

Near-surface

Figure 12.12. Globally and annually averaged air-temperature variations at the surface,

at 850 to 300 mb, at 300 to 100 mb, and at 100 to 30 mb, derived from radiosonde

records from 1958 to 1999. Source of data: Angell (1999, 2000). Deviations are rela-

tive to means derived from 1958–1977 data. Linear fits through the curves indicate

that the air temperature changes between 1958 and 1999 were 0.014C/yr at the sur-

face, 0.01C/yr at 850–300 mb, 0.024C/yr at 300–100 mb, and 0.045C/yr at

100–30 mb.

upper-atmospheric cooling is the increase in human-emitted pollutants. This physical

phenomenon has been demonstrated repeatedly by computer models since the early

1970s.

12.3.2. The Historical Temperature Record

Whereas Figs. 12.11 and 12.12 indicate that recent increases in the Earth’s near-

surface temperature have occurred, it is important to put the temperature changes in

perspective by examining past climates of the Earth. This is done next.

Historical temperatures are determined in many ways. Two techniques are ice-core

analysis and oceanﬂoor sediment analysis. Other methods include analyses of lake

levels, lake-bed sediments, tree rings, rocks, pollens in soil deposits,

and pollens in

deep-sea sediments.

Ice-Core Analysis

When snow accumulates over the Antarctic, it slowly compacts and recrystallizes

into ice by a process called sintering, which is the chemical bonding of a material by

atomic or molecular diffusion. During the ice-formation process, air becomes isolated

in bubbles or pores within the ice. The trapped air contains roughly the composition of

the air at the time the ice was formed. Among the constituents of the air trapped in the

ice bubbles are carbon dioxide, methane, and molecular oxygen. When vertical ice

cores are extracted, the composition of their air bubbles can be analyzed. Whereas

most oxygen atoms in molecular oxygen contain 8 protons and 8 neutrons in their

nucleus, giving them an atomic mass of 16, about 1 in every 1,000 such atoms con-

tains 10 neutrons, giving it an atomic mass of 18. Such atoms are called isotopically

enriched atomic oxygen (

O), which is heavier than is standard atomic oxygen (

O).

Generally, the warmer the snow or air temperature, the greater the relative ratio of

O in molecular oxygen trapped in snow. In fact, a relatively linear relationship has

been found between the annual average snow surface temperature and the

O to

ratio. Thus, from ice-core measurements of

O, past air temperatures can be estimated

(Jouzel et al., 1987, 1993).

Ocean-Floor Sediment Analysis

A related method of obtaining temperature data is by drilling deep into the

sediment of ocean floors and analyzing the composition and distribution of calcium

carbonate shells. When water, which contains hydrogen and oxygen, evaporates

from the ocean surface, the ratio of

O to

O in the ocean increases because

O is

lighter than is

O, and water containing

O is more likely to evaporate than is water

containing

O. Because calcium carbonate in shells contains oxygen from seawater,

and because higher water temperatures correlate with lower

O to

O ratios in

water, greater

O to

O ratios in shells correlate with higher water temperatures.

Because certain shelled organism live within specified temperature ranges, the distri-

bution of shells in a vertical core sample also gives insight into historical water

temperatures.

12.3.2.1. From the Origin of the Earth to 570 Million Years Ago

Figure 12.13 shows the geologic time scale on the Earth. Since the Earth’s forma-

tion 4.6 billion years ago (b.y.a.), near-surface air temperatures have gone through

great swings. Between 4.5 and 4.0 b.y.a., surface temperatures increased due to

326 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

conduction of energy from the Earth’s core to its surface,

creating magma oceans

(Section 2.3.1) that resulted in air temperatures possibly 300 to 400C greater than

those today (Crowley and North, 1991). During that time, energy released by meteorite

impacts also contributed to high temperatures on the surface.

Between 4.5 and 2.5 b.y.a. (Archean period), the intensity of solar radiation was

about 20 to 30 percent lower than it is today. Yet, even after the magma oceans solidi-

ﬁed to form the Earth’s crust 3.8 to 4 b.y.a., air temperatures were as high as 58C, in

comparison with 15C today (Crowley and North, 1991). The low sun intensity

coupled with the high air temperatures following crustal formation is referred to as the

Faint–Young Sun paradox (Ulrich, 1975; Newman and Rood, 1977; Gilliland, 1989).

The reason for the high temperatures may be an enhanced greenhouse effect. During

solidiﬁcation of the Earth’s crust and mantle, outgassing, particularly of carbon

THE GREENHOUSE EFFECT AND GLOBAL WARMING 327

Figure 12.13. The geologic time scale and important events since the formation of the Earth.

Adapted from Crowley (1983). Age is in units of million years ago (m.y.a.).

Phanerozoic

Paleozoic Mesozoic CenozoicPrecambrian

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Cretaceous

Jurassic

Triassic

Tertiary

Quaternary

Devonian

Silurian

Ordovician

Cambrian

Permian

Carbonif-

erous

Pennsyl-

vanian

Missis-

sippian

Proterozoic

Archean

0.1

136

190

225

280

320

345

395

430

500

570

2,300–2,800

4,600–4,700

Age of mammals

Age of dinosaurs

Oldest recorded rocks (3,800)

Earliest recorded life (3,500)

First shelled invertebrates (570)

Primitive fish (430–500)

First land plants (395–430)

Coal formation (280–320)

Oxygen-producing photosynthesis (2,300)

Formation of Pangea (220)

Meteorite bombardment (4,600–4,100)

Earliest Eukaryotes (1,400)

Era Period Epoch Age (m.y.a)

Antarctic ice (40)

Greenland ice sheet (2.4–4)

Arctic sea ice and Alaskan glaciers (5–6)

Homo habilis (2.0)